Crystalline rechargeable battery :
850-1040 Watt-Hours/Kg, & Thermoelectric Capacitor Cooling
Device
David YURTH : “Seeing Past The
Edge”
[ Excerpt ]
At the Institute for Problems of Materials Science located in Kiev, Republic of Ukraine, a scientific team led by Academicians Trefilov, Tovschuk [ Tovstjuk ] and Kovalyuk created a solid-state energy cell which produces 850-1040 watt-hours/kilogram, in laboratory prototypes. This is at least 35-50 times the energy density of any known conventional energy storage devices developed in the West. The reliability of their claims regarding this technology has been verified by INEEL, DARPA and the AMTL. A key element of their crystalline lattice deposition method relies on the effects of a torsion field beam. Scientists working at Sandia Laboratories in Los Alamos, New Mexico, have reported the successful development of a thin-film solid-state energy storage device which reportedly demonstrates energy density in the range of 250-400 watt-hours/kilogram.
xxvi -- David Yurth, The Anthropos Files, loc. cit. IPMS has also perfected the use of a specially modulated torsion field device to manufacture mono-molecular powders of strategic metals. Using this revolutionary manufacturing method, metals can be stored in conventional glass containers without involuntarily generating static electricity...
...In 1992, I watched a demonstration conducted by two technicians from IPMS in the board room of the law firm of O'Melveny and Meiers in Newport Beach, California. They attached a piece of flat black material to a small clip suspended from a conventional chemistry lab test tube stand [single metal pole extending vertically from a cast iron base]. To the corners of this material they attached the leads from a conventional 9-volt battery, using small alligator clips, one attached at each corner. Within 20 seconds, the top surface of the tarot-card sized flat black card became covered with a layer of ice crystals. Within 30 seconds, a continuous cloud of frozen ice crystals [looking for all the world like the vapor which rolls out of a bucket of water when a piece of dry ice is dropped into it] began to pour off the upper surface of the suspended card and onto the top of the 20-foot long board room table. Within a minute, the cloud entirely covered the board table and was pouring off the edge of the table onto the laps of the people who were seated around the table watching this demonstration.
When the technician offered to allow someone to actually hold the "card" in their hands, everyone who had seen a demonstration of the Peltier Effect refused – in our materials science lexicon, Peltier materials get very cold on one surface but demonstrate compensating heat on the opposing side. So when the demonstrator disconnected the card and held it in his hand, everyone who thought they knew what was happening gasped. With a digital thermometer, the demonstrator measured the surface temperature of the opposing side of the card – it was 72 degrees F. The top surface of the card was -68 degrees F. It was extracting heat from the local address and dissipating it non-locally in the presence of a very small activating DC voltage field with sufficient efficiency to freeze free-standing CO2 from the atmosphere.
In a different demonstration, IPMS scientists provided us with a material they referred to as an "energy accumulator." In their frame of reference, this material operated by capturing free electrons and warehousing them in energy wells defined by the upper and lower surfaces of a material they referred to as a "one-dimensional" crystalline lattice. The lattice was made of atomically engineered carbon, intercalated with atoms of other materials, in a way that created a series of virtually mono-molecular carbon films which were folded back and forth on top of each other. When viewed under our electron microscope, this material looked like an endless field of egg-carton peaks and valleys. When the peaks from the lower layer were brought into contact with the peaks from the upper layer, a virtual energy well was created, within which electrons were reposed while waiting to be directed elsewhere. This solid-state energy accumulator device demonstrated an energy density 1.4 times greater than gasoline. INEEL's report showed output results varying from 840-1024 watt-hours per
kilogram over extended tests.
When we submitted the six energy accumulator devices to INEEL and DARPA for examination and analysis, an important discovery was made. Scientists at INEEL weighed the battery containers [we sent six Diehard battery containers to IPMS because they had no containers to put their materials in] fully charged, they were significantly more massive than when they were fully discharged. Think about this carefully – this created a tremendous reaction from DARPA. The US government attempted to classify and confiscate these devices and would have succeeded except for the fact that our attorneys succeeded in disabusing them from this course of action by pointing out that this stuff belonged to another government-owned laboratory in a foreign country. The implications of this discovery are extremely important because this material, manufactured in the presence of precisely modulated torsion fields, proves that E does not equal MC squared. Never did. In this instance, energy [as represented by nothing other than electrons sitting still within the crystalline lattice of a carbonized film] manifests itself as measurably massive while sitting at rest. Again, Einstein was mistaken, and the Standard Model (of physics) is wrong. In this case, E equals M with no velocity at all...
LAMELLAR CRYSTALLINE MATERIAL AND A METHOD
OF ITS PREPARING
LAYERED CRYSTALLINE MATERIAL CAPABLE OF HIGH GUEST LOADING
RU94039535
WO9308981
EP0662251
LAYERED CRYSTALLINE MATERIAL CAPABLE OF HIGH GUEST LOADING
RU94039535
WO9308981
EP0662251
Inventor: TREFILOV V I // TOVSTJUK K D (+4)
FIELD: monocrystalline materials of high quality. SUBSTANCE: lamellar monocrystalline material showing low defect level and the corresponding distribution of impurities that allows to intercalate at least 3 mole lithium into van der Waals channel per one mole of crystalline material without significant crystalline lattice distortion. Also, change of Gibbs free energy does not depend of lithium concentration incorporation. Material is prepared by the sequence stages. Ampoule is filled with stoichiometric amount of chalcogenid element and bismuth under nonoxidizing atmosphere and sealed. Then ampoule is heated to temperature 5-10 C above of bismuth chalcogenid melting point and cooled to the room temperature. Obtained polycrystalline bismuth chalcogenid is contacted with seeding crystal with specific structure of crystalline lattice. Obtained harmless nondefected monocrystal of bismuth chalcogenid is cooled to the room temperature. Some admixtures can be added to the prepared crystalline material to obtain highly intercalated crystalline material. Obtained crystalline material is used as active battery member, thermoelectric device and capacitor. EFFECT: enhanced quality of material.
Background of the Invention
The present invention relates to highly defect-free monocrystalline materials capable of intercalating high levels of guest species and a method of making the same. The present invention further relates to a highly intercalated layered crystalline material.
Layered crystalline materials (hosts) can accommodate foreign species (guests) in the channels between the layers thereby forming intercalated materials. Intercalation compounds, such as graphite and transition metal chalcogenides, have been intensively investigated over the last two decades and have found utility in a number of fields, including batteries, photovoltaic cells, superconductivity and hydrogen accumulation.
The hosts of interest to this application have anisotropic lamellar structures separated by van der Waals channels which are capable of accommodating small guest particles, such a lithium or hydrogen.
Because of the weakness of a van der Waals attractive force, the van der Waals channels can readily accept guest species, which can be neutral or charged. The guest species fill the channel until no more sites are energetically available. The amount of guest species which occupy sites in the van der Waals channel is the loading capacity of the guest. "Loading" or 'loading capacity" is defined herein as moles guest species per mole host material; species" is defined as ions or uncharged atoms of elements. It is recognized that for many applications, increased loading capacity of the guest into the layered material would significantly improve device performance.
Distortion of a layered crystalline material has been used to increase the channel width and thereby increase guest loading. U.S.
4,288,508 reports a cathode active material for a battery having the preferred formula This TiS2, where y is in the range of 0.15-0.20 and z can be as high as 3.25. However, increased loading in these materials is accomplished by using the larger sodium atom to pry open the van der Waals channels. This results in significant distortion of the TiS2 lattice.
U.S. 4,309,491 reports a solid solution containing a bismuth halcogenide for use as a cathode active material. The guest loading process in the bismuth chalcogenide is reported to require up to six
Faradays of electrons, suggesting that six moles of guest are intercalated. However, it is also reported that there is substantial dependence of the discharge voltage on the guest concentration.
Additionally, there is no indication that the bismuth chalcogenide has been prepared in a highly defect-free form with controlled lattice parameters.
Accordingly, it is the object of the present invention to provide a mono-crystalline material capable of intercalating extremely high loads of a guest species in which a change in the Gibbs free energy (AG) of the material is substantially independent of intercalant concentration.
Under these conditions, properties dependent upon AG, such as working potential of an electrochemical cell, remain unchanged.
It is a further object of the invention, to provide a highly intercalated crystalline material having controlled lattice parameters.
It is yet a further object of the invention to provide a method of making a highly defect-free mono-crystalline material with controlled lattice parameters capable of high guest loading.
Summary of the Invention
The present invention overcomes the limitations of the prior art by providing a highly defect-free material with controlled lattice characteristics. When used in devices such as a battery it provides superior performance over prior art devices.
In one aspect of the invention, a layered mono-crystalline material is provided having a defect density which is sufficiently low to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of the material without significant distortion of the lattice. The material is further characterized in that its change in Gibbs free energy is substantially independent of the lithium intercalation concentration.
In another aspect of the present invention, a mono-crystalline material having the formula, MeyChz, is provided where Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, y is 1 or 2 and z is 1, 2, or 3. The material has a defect density which is sufficiently low to permit intercalation of at least 3 moles of lithium within van der Waals channels per one mole of the material without significant distortion of the lattice. The material is further characterized in that AG is substantially independent of the lithium intercalation concentration.Further detail concerning intercalation compounds and devices using these compounds are given in the following applications, filed on equal date as "Layered Crystalline Material Capable of High Guest Loading", and incorporated herein by reference: "Energy Storage Device", "Electrolytic Double Layer Capacitor", and "Capacitive Thermoelectric Device.
In preferred embodiments, the material has a hexagonal crystal lattice structure, a defect density of less than 10'2/elm3 and a gradient of impurity defect distribution in the van der Waals channel inversely proportional to the direction of lithium intercalation. The monocrystalline materials can be further characterized in that a condensation of lithium occurs at a loading capacity of approximately three. The condensation is from a lattice gas to a quasi-liquid. The lattice structure may be rhombohedral or hexagonal. The monocrystalline material is preferably a single crystal. In preferred embodiments, the defect density is minimized and perfection of the lattice crystal is accomplished by using careful processing controls.In other preferred embodiments, the mono-crystalline material is used in a battery, a capacitor or a thermal electric device; although the monocrystalline material may be ground into a powder for some device applications.
In another aspect of the invention, a method of preparing a highly purified bismuth chalcogenide consists of the following steps: an ampoule is charged with stoichiometric quantities of a chalcogenide element and bismuth; the ampoule is provided with an atmosphere selected to prevent oxidation and sealed; the sealed ampoule is heated at a temperature in the range of 5"C to 10 C above Tijq of the bismuth chalcogenide, the temperature is controlled to within a range of +0.5 along the length of the ampoule and for a time sufficient to melt the component materials and react to form the bismuth chalcogenide, whereby the ampoule is agitated during heating to ensure homogeneous mixing of the component materials; the material is cooled to room temperature at a controlled rate to form a homogeneous polycrystalline bismuth chalcogenide; the polycrystalline bismuth chalcogenide is placed in surface contact with a seed crystal of a specific crystal lattice structure; the seed crystal and polycrystalline bismuth chalcogenide are heated to a temperature which is in the range of 30"C to 400C below This of the bismuth chalcogenide; a zone of the polycrystalline bismuth chalcogenide adjacent to the seed crystal is heated to a temperature which is in the range of 0 C to 150C above Tljq of the bismuth chalcogenide; the zone is moved along the length of the polycrystalline chalcogenide at rate in the range of 2 to 10 mm/Er, whereby a single crystal highly defect-free bismuth chalcogenide is formed; and the single crystal bismuth chalcogenide is cooled to room temperature at the controlled cooling rate.
In another aspect of the present invention, a highly intercalated crystalline material is provided having a guest capacity in the range of three to ten moles within van der Waals channels per one mole of said intercalated material without significant distortion of the lattice. The guest selected from the group consisting of Group IA and Group IIA metals. The intercalated material is further characterized in that AG is substantially independent of the guest concentration. The intercalated material may have the formula, GxMeyChz, where G is selected from the group consisting of Group IA and Group IIA metals, Me is selected from the group consisting of Bi and Sb, Ch is selected from the group consisting of Te, Se and S, xis in the range of 3 to 10, yis 1 or 2 and z is in the range of 1 to 3.
Brief Description of the Drawing
In the Drawing;
Figure 1 is a schematic illustration of the localized sites in the van der Waals channels for the crystal lattice structures used in the present invention;
Figure 2 is a cross-sectional schematic illustration of the zonerefinement apparatus used in preparing single crystals of the present invention; and
Figure 3 is a discharge curve of a battery using the crystalline material of the present invention.
Description of the Preferred Embodiment
The present invention has identified that highly defect-free crystalline layered materials with appropriate impurity distribution are capable of intercalating high loads of guest species into van der Waals channels of the material.
Structure. A family of highly defect-free compounds of the formula, BiyChz, where Ch is Te, Se or S, y is 1 or 2 and z is in the range of 1 to 3, have been identified which permit extremely high loading capacity of the guest species, well beyond the loading capacity conventionally predicted by the lattice structure of the crystal and the model of the lattice gas. Solid solutions of these compounds, i.e., Bi2(Tel.xSex)3, are also within the scope of the invention. Importantly, high loading is achieved without significant distortion of the crystal lattice and without significant dependence of AG of the material on the intercalant concentration. AG has been correlated with the operating performance of the crystalline material when used in devices such as a cathode material in a galvanic cell.
Although discussions are directed to a bismuth chalcogenide, it is contemplated that any layered material of requisite crystalline lattice parameters, guest loading capacity and thermodynamic behavior is within the scope of the present invention.
The bismuth chalcogenide family is known to crystallize in rhombohedral and hexagonal lattices. The hexagonal and rhombohedral crystal lattices possess two types of energetically accessible sites which permit Iocalization of the guest species within the van der Waals channel. The basis for this observation has been presented in a co-pending application U.S.S.N. 07/784,525 of which this application is a continuation-in-part and which is herein incorporated by reference.
Figure 1 is a schematic representation of the two types of guest sites. A first site 22 is in the plane of the center of the channel, while a remaining site 24 localizes the guest species along walls 26 of the channel Total loading of the channel by guest is predicted to be three.
Occupation of sites 22 is more energetically favorable at the beginning of intercalation than of sites 24. However, relative energy levels change during the intercalation process. All sites are sufficiently close in energy that "hopping" of guests from one to another site of differing energy is possible. The guest species behave as a 'lattice gas".
The conventional model would seem to suggest, therefore, that the upper limit to guest loading without distortion of the lattice is three. However, we have discovered that much higher loading is possible For the specified lattice types shown in Fig. 1, orbital interaction in a filled van der Waals channel results in increased guestguest interaction and a decrease in the average guest-guest interatomic distance. This conversion from occupation of localized energy minima to free movement throughout the van der Waals channel is equivalent to a phase change. The lattice gas condenses into a high density state, which is defined herein as a "quasi-liquid phase".
Because the new phase has a smaller average interatomic distance, additional guests can be introduced without distortion of the crystal lattice. Hence, a loading capacity of three is no longer a limitation to the system and rapid and high levels of guest loading is now possible. Loading capacity of lithium of up to eight and nine have been observed in the bismuth chalcogenide compounds of this invention.
We estimate that the loading capacity in this system can be even higher, in particular, capacity of up to ten is considered possible.
The class of compounds of the present invention has stable crystalline phases of hexagonal or rhombohedral symmetry which can be prepared with minimal defect densities and the appropriate impurity distribution. It is known in the prior art that the Bi2Ch3 class of compounds may crystallize in a rhombohedral unit cell of space group D%s (R3m, at=9.83 ; a=24.4" for bismuth selenide) containing five atoms. This crystal structure consists of layers formed by equal atoms in hexagonal arrangement. It is also given in prior art that a hexagonal unit cell for bismuth selenide (at=4.14 ; cho=28.55 ) also has been identified.
In a battery, the discharge curve is directly correlated to AG of the cathode material (a bismuth chalcogenide). The following thermodynamic parameters which are related to AG must be considered when evaluating a guest/host combination: the entropy of distribution of the host/guest atoms, the energy of guest-guest and guest-host interactions, the change in the Fermi energy (AFe), and the lattice distortion (LD)-
The lattice gas to quasi-liquid condensation of the system prevents significant distortion of the lattice. In the present case, loading over the range of intercalation from 0 to 8 or 9 results in distortion only in the range of 2-3%. Such distortion does not contribute significantly to the Gibbs free energy of the system. In the invention, distortion is held not exceed 10%.In contrast, in prior art TiS2 intercalation of Lithium, the c-axis of the intercalated LiXTiS2, i.e., the axis perpendicular to the intercalated van der Waals channel, has been shown to increase by 10% in response to as little intercalation as x=O to s=1. The change in entropy (AS) is significant only in the early stages of the intercalation process. AS is therefore very small over the course of the process and need not be considered in the Gibbs free energy equation.
The characteristics of the crystal lattice have a great effect on the remaining two thermodynamic parameters, however. The energy of interaction, SDt7 is a measurement of guest-guest and guest-host interactions. Both of these are greatly affected by lattice crystalline characteristics. If the crystal lattice contains significant levels of defects and/or dislocations or has sufficiently uneven distribution of deflects, the energy minima associated with the localized sites will be disrupted and filling is not uniform along the length of the channel.
Fermi energy level of the crystal is also effected by the interstitial and lattice site impurities and the lattice structure. The defect or impurity distribution is important in identifying acceptable crystalline purity. If all defects are clustered near the entrance to van der Waals channels, no guests can enter and guest capacity is low even though crystalline lattice purity is high. It is apparent therefore, that careful crystal growth is important to preparing layered crystalline materials capable of the high loading of the present invention.
Process
The following detailed description is presented to provide details of the crystal growth process providing highly defect-free layered crystalline materials with the specific lattice characteristics, such as defect distribution, of the present invention. The description which follows is for the bismuth chalcogenide family; however, it is contemplated that any layered material of requisite crystalline lattice parameters, guest loading capacity and thermodynamic behavior is within the scope of the present invention.
Stoichiometric quantities of highly purified (99.9999% pure) bismuth and chalcogenide are charged into a quartz ampoule. If necessary, the materials are zone refined before use. Off-stoichiometry results in an n- or p-doped material with characteristic degradation of the lattice structure and the associated performance. The ampoule is evacuated to 10 7 mmHg and backfilled to a pressure of 10 mmHg with a small amount of inert gas, such as argon, or a reducing gas, such as hydrogen (3-10 cycles), and then sealed. Hydrogen is particularly preferred because it reacts with oxygen during processing to prevent oxidation and decrease the segregation of chalcogenide by reducing its vapor pressure.
A highly homogeneous polycrystalline material is prepared in a first processing step. The sealed ampoule is placed in a furnace at room temperature and heated to a temperature 5-100C above its melting point. The ramp rate, temperature and reaction time are selected for the final compound. The reaction conditions are listed in Table I for the preparation of polycrystalline Bi2S3, Bi2Se3, and Bi2Te3. The temperature of the furnace over the entire length of the ampoule is controlled to within +0.5 C. Careful and accurate control of the temperature is important because of the high volatility of chalcogenides.
Temperature variation along the ampoule length causes segregation of chalcogenide which leads to off-stoichiometry. To optimize the temperature control along the length of the ampoule, a long furnace can be used. Additional heating coils can be used at furnace ends to reduce the temperature gradient at the furnace exits.
Table I -- Processing conditions for polycrystalline material.
During the last hour of reaction time, the ampoule is agitated or vibrated to insure complete mixing of the ampoule components. The ampoule vibration preferably is in the range of 25-100 Hz and is accomplished by fixing one end of the ampoule to an oscillation source.
Any conventional vibration means is contemplated by the present invention. After reaction is complete, the ampoule is cooled at a slow controlled rate.
Once a homogeneous polycrystalline material is obtained, it can be further processed into a highly defect-free bismuth chalcogenide single crystal. Any known method of growing single crystals can be used, such as Bridgeman techniques, Czolchralski process and zone refinement techniques (recrystallization). In particular zone refinement has proved to be highly effective in obtaining high purity single crystals.
Referring to Fig. 2, zone refinement is carried out in a quartz boat 40 containing a seed crystal 42 of the desired lattice structure. It is recommended that clean rooms levels of Class 100 be maintained.
The seed crystal 42 is oriented in the boat such that crystal layers 43 are horizontal. The entire apparatus should be shock-mounted to insulate against environmental vibrations. The boule 44 of polycrystalline material is positioned in surface contact with the seed crystal.
The furnace comprise two parts, an outer furnace 46 for maintaining an elevated temperature along the entire boule length and a narrow zone 47 movable in the direction of arrow 48 for heating a small portion of the polycrystalline material. For production of hexagonal structure, the outer furnace 46 is maintained at 35"C below the melting point, and the zone 47, which is 2-3 cm in length, is held at 10 C above the melting point of the polycrystalline material. Unlike for the preparation of the polycrystalline material int he first processing step, the boule can in this step be rapidly heated to the operating temperature. The zone is initially positioned at the seed crystal/boule interface and this region is heated to the melting point of the material.
The zone 47 is then moved slowly down the length of the boule. Zone travel rate is selected according to the particular composition and recommended rates are shown, along with other processing parameters, in Table II. Zone travel rate is an important processing parameter. If the rate is too great, crystallization is incomplete and defects are formed. If the rate is too slow, layer distortions result. The lower portion of the heat-treated boule in contact with the quartz boat is preferably removed before use. The process produces a single crystalline material having less than 1012/cm3 defect density and an impurity distribution which is inversely proportional to the intended direction of intercalation. The single crystal typically contains 106 layers/mm with a spacing of 3-4 A/layer.
Table II. Processing conditions for hexagonal single crystal growth
processing conditions Bi2Te3 Bi2Se3 Bi2S3 r boule temperature Mp - 350C Mp - 350C Mp 35"C zone temperature Mp + 10 C Mp + 10 C Mp + 10 C zone travel rate 8 mm/hr 6 mm/hr 3 mm/hr cooling rate 50 "C/hr 400C/hr 35 C/hr
The above process can be modified slightly to produce crystals of rhombohedral structure, in which case a rhombohedral seed crystal is employed in the zone refinement process.In addition, to produce rhombohedral crystals, the furnace temperature is held at 300C below the melting point and the zone is maintained at the melting point of the p olycrystalline material.
Depending on the composition of the material, there is a preference for either hexagonal or rhombohedral lattice structure. This is summarized below in Table III.
Table III -- Preferred lattice structure for bismuth chalcogenide.
lattice structure Bi2Te3 Bi2Se3 Bi2S3 rhombohedral x hexagonal X X
The above process provides a highly defect-free single crystalline material. The crystal can be further ground into particles for use in devices and each such particle is a mono-crystal. A grinding technique is selected so as not to introduce many defects and dislocations into the crystal. However, because of the weakness of the van der Waals attractive force, the crystal cleaves readily along the length of the channel without much danger of lattice distortion.
Once formed, the material is tested by introducing it as the cathode-active material into a galvanic cell. The use of highly intercalated crystalline materials in an energy storage cell is disclosed in a co-pending U.S. application entitled "Energy Storage System" which is being filed this date. A standard battery is constructed using a lithium anode, a non-aqueous Lilo4 electrolyte solution and the test material as the cathode. The amount of lithium that can be introduced into the van der Waals layer is determined by monitoring the moles of Faraday electrons passed through an external circuit during intercalation. A typical discharge curve 50 using a bismuth chalcogenide cathode prepared according to the invention is shown in Fig. 3.An acceptable crystalline material is capable of intercalating at least three mole of lithium per mole of bismuth chalcogenide and has discharge curve that is essentially flat, that is, a change of no more than 0.1-0.3 V is observed over an intercalation capacity range of 0.4 to 8 moles lithium. The flatness of the curve is an indication of the substantial independence of Gibbs free energy change on guest concentration.
Condensation of a lattice gas to a quasi-liquid manifests itself in the discharge curve as a sudden change in the discharge voltage. The actual change in voltage is quite small, however, and is not significant to operation of the cell. Fig. 3 illustrates the smooth, flat discharge curve. In an enlarged portion 52 of the curve at approximately a guest load of three it is possible to observe a "blip" in the curve. This is observable under carefully controlled conditions.
Once a highly defect free crystalline material is prepared as described above, it can be intercalated to obtain the highly intercalated crystalline material of the present invention. Intercalation can be carried out using conventional methods, such as exposing the crystalline material to the vapor phase of the intercalant or placing the crystalline material in a liquid that contains the intercalant or passing a current through an electrochemical cell where the crystalline material is an electroactive material in one of the electrodes. The preferred method for achieving high load of intercalant is the electrochemical method.
WO9309570
CAPACITIVE THERMOELECTRIC DEVICE
CAPACITIVE THERMOELECTRIC DEVICE
Also published as: MX9206260 (A) EP0610370 (A1) CA2122448 (A1) AU2908192 (A)
TOVSTJUK KORNEI D [UA]
GRIGORTCHAK IVAN I
A thermoelectric device for cooling a mass; the device comprising a capacitor having two electrodes and being in thermal contact with the mass to be cooled. The capacitor produces a cooling effect when it is repeatedly isothermally charged and adiabatically discharged.
Background of the Invention
This invention relates to solid state thermoelectric devices, and particularly relates to thermoelectric device materials.
Conventional solid state thermoelectric devices operate based on the Peltier effect, which is achieved by passing an electric current through a junction of dissimilar metals. Such a device is typically constructed from two semiconductor blocks, one being a heavily doped n-type material and the other being a heavily doped ptype material. The blocks are connected electrically in series and thermally in parallel. In this arrangement, heat is absorbed at a socalled cold junction of the semiconductor blocks and is transferred to a so-called hot junction at a rate proportional to the current passed through the semiconductor blocks. When the current is applied in series with the n-type and p-type blocks, electrons pass from a low energy level in the p-type material to a higher energy level in the n-type material.This diode-like action causes heat to be removed from the cold junction of the two blocks and pumped through the blocks (in parallel) to the hot junction.
Bismuth telluride is currently the most popular semiconducting material used for Peltier effect thermoelectric devices. It exhibits high mechanical strength, high electrical conductivity, and low thermal conductivity, three desirable characteristics for a thermoelectric device. Commercially manufactured bismuth telluride Peltier devices can achieve temperature differentials of approximately 650C, and can pump tens of watts of heat.
Summary of the Invention
In general, in one aspect, the invention features a thermoelectric device comprising two electrodes separated by an electrolyte. Each of the electrodes comprises a crystalline material characterized by the presence of van der Waals channels, the van der Waals channels being capable of accommodating the electrolyte within the channels to form a double layer of charge at interfaces of the van der Waals channels and the electrolyte when a voltage is applied between the two electrodes. The thermoelectric device exhibits a temperature decrease in a two-stage cooling process comprising application of a voltage between the electrodes to thereby form the double layer and replacement of the voltage with a load through which the double layer is discharged.Using selected materials, the thermoelectric device of the invention exhibits a 0.5 C temperature decrease for each two-stage cooling process repetition, and thus provides a significant thermoelectric effect. Further detailed descriptions of compounds having van der Waals channels and devices which utilize these compounds are provided in the following applications, all filed of equal date as "Electrolytic Double Layer Capacitor", and hereby incorporated by reference: "Capacitive Thermoelectric Device" and "Energy Storage Device".
In preferred embodiments, the voltage application is controlled to charge said double layer isothermally and discharge the double layer adiabatically.
Preferably, the crystalline material is a bismuth chalcogenide, and most preferably, each of the two electrodes comprises a monocrystal of the bismuth chalcogenide. In other preferred embodiments, the two electrodes each comprise monocrystalline powder particles of the crystalline material. The van der Waals channels of the crystalline material are adapted to accommodate the electrolyte by a training process comprising intercalation of electrolyte into the channels.
Preferably the intercalation is produced by a training voltage sufficiently high as to achieve electrolyte penetration of the van der Waals channels, and most preferably, the training voltage is periodically reversed in polarity between the electrodes.
In general, in another aspe ct, the invention provides a thermoelectric device for cooling a mass, the device comprising a capacitor having two electrodes and being thermally coupled to the mass, the capacitor providing a cooling effect when it is repeatedly isothemlly charged and adiabatically discharged. Preferably, the capacitor is a double layer capacitor, and more preferably, the capacitor is an electrolytic capacitor.
In general, in another aspect, the invention provides a thermoelectric device comprising two electrodes separated by an electrolyte, each electrode comprising a porous material capable of forming a double layer of charge at interfaces of the porous material and the electrolyte when a voltage is applied between the two electrodes. The device exhibits a temperature decrease in a two stage cooling process comprising application of a voltage between the electrodes to thereby form the double layer and replacement of the voltage with a load through which the double layer is discharged.
In preferred embodiments, the porous material comprises a compound of bismuth chalcogenide, activated carbon, and heat-treated graphite, and preferably, the electrolyte comprises sulphuric acid. Preferably, the voltage application is controlled to charge said double layer isothermally and discharge the double layer adiabatically.
The thermoelectric device of the invention exhibits an accumulated temperature decrease produced by repetitions of a two-stage cooling process, namely charging and discharging. Its compact structure and elegantly simple operation provide significant advantages over conventional thermoelectric devices. Other features and advantages of the invention will become apparent from the following description, and from the claims.
Brief Descrintion of the Drawing
Fig. 1 is a schematic illustration of one embodiment of the thermoelectric device of the invention;
Fig. 2A is a schematic illustration of the device of Fig. 1 at a first stage of training;
Fig. 2B is a schematic illustration of the device of Fig. 2A at a later stage of training;
Fig. 2C is a schematic illustration of the device of Fig. 2A at a final stage of training;
Fig. 2D is a schematic illustration of the device of Fig. 2A including the formation of a double layer of charge; and
Fig 3 is a diagram of an electrical circuit for operating the thermoelectric device of the invention.
Description of the Preferred Embodiment
We first present a discussion of the operational theory of the thermoelectric device of the invention. The thermoelectric device of the invention is a capacitive device characterized by the ability to accommodate a very high charge density, and correspondingly high energy storage, and very low internal resistance. The capacitor thereby achieves very high discharge power, and in fact, a discharge rate which is so fast as to be energetically adiabatic.
Furthermore, very little resistive, or joule, heating occurs during the discharge process. As will be explained further below, these characteristics of the discharge process are so pronounced as to provide a cooling effect via the following device operation. The capacitive device is first isothermally "charged" to a very high charge density, thus in an isothermal manner, whereby the entropy of the device is decreased The capacitive device is then adiabatically discharged, whereby the entropy of the device is increased, necessarily resulting in a temperature decrease, or cooling, of the device. The charge/discharge cycle may thus be understood as a classical Carnot cycle, in which the cycle temperature is directly related to the cycle kinetics and changes in entropy.
The inventors herein have recognized that while theoretically, any capacitive structure is capable of thermoelectric cooling, particular capacitive schemes and particular device materials are best-suited for achieving thermoelectric cooling. Based on the operational theory explained above, a capacitive cooling device ideally comprises a capacitive structure capable of very high charge density accumulation and very low internal resistance; in particular, the resistance of the capacitor electrodes ideally is held low. Given these criteria, electrolytic double layer capacitors, known to be capable of quite high charge accommodation, have been identified as providing a superior capacitive structure for a cooling device. Accordingly, the cooling device of the invention is based on a double layer capacitive structure.
As an example of the charge density accumulation limitations of capacitors, the capacitance of a typical parallel plate capacitor is given by: analyze the operational electrical properties of this structure, consider first
C = where vg is a constant, 6 is the dielectric constant of the medium between the capacitor electrodes, S is the surface area of the capacitor electrodes, and d is the width of the medium separating the electrodes. The charge density accommodation, i.e., the capacitance, of a given capacitor is thus limited by the geometry, i.e., the surface area, the electrode spacing, the material properties of the electrodes, and the medium separating them.
The definition of capacitance for a double layer capacitor is further specified by the structure of the charged double layer and its geometry. This double layer comprises charge accumulation on the electrode surface and accumulation of ions at the electrode surface-electrolyte interface. Thus, for double layer electrolytic capacitors, the width d in the capacitance equation is given by the distance between the centers of the two regions constituting the double layer. This distance is on the order of angstroms, thereby providing a very large capacitance value for a given capacitor surface area.
The capacitive cooling device of the invention provides an even greater increase in capacitance (and charge density accommodation) by providing an increase in surface area of the capacitor electrodes over conventional double layer electrodes, and through proper selection of the electrode material and the electrolyte. Furthermore, as explained in detail below, the selected electrode materials and electrolytes provide a dramatically low internal resistance, a parameter which is critical to the cooling device operation.
In the capacitive cooling device of the invention, increased electrode surface area is obtained using a particular class of materials, namely intercalation compounds, which are characterized by a layered crystalline structure. The crystal layers of intercalation compounds comprise planes of molecules or atoms which are weakly bound together and separated from each other by van der
Waals regions. These van der Waals regions form anisotropic channels in the crystal lattice between the planes of molecules or atoms, resulting, in effect, in a atwo dimensional" crystal structure. Intercalation materials typically exhibit on the order of 106-107 layers per millimeter of material thickness.Due to the weak van der Waals force between the crystal layers, the lattice channels can accommodate the physical introduction, or so-called intercalation, of a guest intercalant species into them.
The inventors herein have recognized that a particular type of intercalation compound, namely bismuth chalcongenides, including Bi2Te, and Bi2Se3, are particularly well-suited for providing van der Waals channels as an extension of electrode surface area. Electrodes composed of these materials, when used in combination with a suitable electrolyte, generate a highly uniform double layer of a desirable structure.As is well-known to those skilled in the art, bismuth chalcongenides exhibit a layered crystalline lattice which is layered at the molecular level, each layer being separated by a van der Waals channel having a width on the order of 3-4 A. Further material properties of bismuth chalcogenides are given in the copending United States Patent Application entitled layered Crystalline Material Capable of High Guest Loading," herein incorporated by reference, being filed on the same day as the present application.Of the materials surveyed, the inventors have found that of the bismuth chalcogenides, Bi2Te3 exhibits the best electrical conductivity, and correspondingly, the lowest resistance, and is thus most preferable as an electrode material, while Bi2Se3 exhibits a lower conductivity, and thus is less preferable as an electrode material.
It must be emphasized that while the bismuth chalcogenides are understood to provide superior electrical properties, other intercalation materials may be employed in the capacitive cooling device of the invention, and materials characterized by other crystal structures which provide high charge density may also be utilized as an electrode material. For example, activated carbon, a porous material, may also be utilized as an electrode material; the porous nature of activated carbon enhances the material's effective surface area.
In one embodiment of the capacitive cooling device of the invention employing intercalation electrode materials, the van der Waals regions of the electrode material are employed such that the surfaces of the crystal lattice channels, although internal to the electrode material, contribute to the overall electrode surface area, and thereby dramatically increase the effective electrode surface area beyond that of its macroscopic surface. As described in detail below, surfaces of the van der Waals channels of the electrode material are capable of forming a double layer with an electrolyte in the same manner as the electrode macroscopic surface forms a double layer.Recognition and exploitation of this physical process has enabled the inventors herein to achieve the significant cooling capability in the cooling device of the invention as a result of increased charge density accommodation.
Depending on the intercalation electrode material and the electrolyte selected for the cooling device, the van der Waals channels of the electrodes may need to be manipulated, or "trained" to effectively accommodate the electrolyte within them. The condition for this requirement is based on the structural dimensions of the van der Waals channels and the solvated complex radius of the electrolyte's solvated ions. If the complex radius of the solvated ions is larger than the width of the van der Waals channels, then the electrolyte will not be able to penetrate the channels and no double layer will be formed within the channels. Accordingly, a material having van der Waals channel widths which are less than the solvated ionic radius must be trained to accommodate the larger ions, and thereby trained to accommodate the electrolyte itself. The van der Waals channel width of bismuth chalcogenides is too narrow for electrolytes of interest to penetrate them, and thus a training" of bismuth chalcogenide is required. A detailed training sequence is described below.
Electrode materials, like activated carbon, which are not intercalation compounds, do not require such a training process, because they do not contain van der Waals regions. As explained above, activated carbon provides an enhanced surface area with the surfaces of its internal pores, which accommodate electrolyte to form a double layer of charge.
The charge density capacity and the internal resistance of electrode materials, and particularly intercalation materials, are dramatically impacted by the purity and defect density of the chosen material. This is manifested in the degree of ability to manipulate van der Waals channels to increase electrode surface area Impurities and crystal lattice defects distort the geometry, as well as the field characteristics, of the van der Waals channels, rendering them less accessible to intercalating species, degrading the channel surface structure and thus degrading the electrical and mechanical properties of the channels.
Accordingly, it is ideally preferred that the material chosen for the capacitor electrodes be prepared using unique processes, developed by the inventors herein, yielding a highly pure and as defecbfree as possible material. To that end, the following single crystal growth process is preferred for bismuth chalcogenide materials. Alternative processes, providing less than ideally pure and defectefree material, may nonetheless be acceptable for particular cooling device applications. Those skilled in the art will recognize critical material parameters and corresponding performance results.
In the preferred intercalation compound preparation process, stoichiometric quantities of highly purified (99.9999% pure) bismuth and tellurium (or other selected chalcogenide) are first charged into a quartz ampoule. If necessary, the materials are zone refined before use. Off-stoichiometry results in an n- or pdoped material with the resultant degradation of the lattice structure and the associated performance. The ampoule is evacuated to 10-7 minlig and barkfilled to a pressure of 104 mmHg with a small amount of inert gas, such as argon, or a reducing gas, such as hydrogen (3-10 cycles), and then sealed.Hydrogen is particularly preferred because it reacts with oxygen during processing to prevent oxidation and decrease the segregation of chalcogenide by reducing its vapor pressure.
A highly homogeneous polycrystalline material is prepared in a first processing step. The sealed ampoule is placed in a furnace at room temperature and heated to a temperature 5-10 C above its melting point. The ramp rate, temperature and reaction time are dependent upon the final compound. The reaction conditions are listed in Table I for the preparation of polycrystailine Bi2S3, Bi2Se9, and Bi2Te3. The temperature of the furnace over the entire length of the ampoule is controlled to within +0.5 C. Careful and accurate control of the temperature is important because of the high volatility of chalcogenides.
Temperature variations along the ampoule length causes segregation of chalcogenide which leads to off-stoichiometry. To optimize the temperature control along the length of the ampoule, a long furnace can be used. Additional heating coils can be used at furnace ends to reduce the temperature gradient at the furnace exits.
Table I. Processing conditions for polycrystaline material.
processing conditions Bi9Teq Bi9Seq Bi9S3 heating rate to Tliq ( C/h) 30 20 15 exposure time (h) 10 15 20 at Tiq + 10 C cooling rate ( C/h) 50 40 35
During the last hour of reaction time, the ampoule is agitated or vibrated to insure complete mixing of the ampoule components. The ampoule vibration is in the range of 25-100 Hz and is accomplished by fixing one end of the ampoule to an oscillation source. Any conventional vibration means is contemplated by the present invention. After reaction is complete, the ampoule is cooled at a slow controlled rate.
Once a homogeneous polycrystaline material is obtained, it can be further processed into a highly defect-free bismuth chalcogenide single crystal. Any known method of growing single crystals can be used, such as Bridgeman techniques, Czolchralski process and zone refinement (recrystallization). In particular zone refinement has proved to be highly effective in obtaining high purity single crystals.
Zone refinement is preferably carried out in a quartz boat containing a seed crystal of the desired lattice structure, e.g., the hexagonal lattice structure.
It is recommended that clean rooms levels of Class 100 be maintained The seed crystal is oriented in the boat such that crystal layers are horizontal. The entire apparatus should be shock-mounted to insulate against environmental vibrations. The boule of polycrystalline material is positioned in surface contact with the seed crystal.
The furnace comprises two parts, an outer furnace for maintaining an elevated temperature along the entire boule length and a narrow zone movable in a direction for heating a small portion of the polycrystalline material. The outer flirnace is maintained at 350C below the melting point and the zone, which is 2-3 Cm in length, is held at 100C above the melting point of the polycrystalline material. Unlike for the preparation of the polycrystalline material, in the first processing step described above, the boule can be rapidly heated to the operating temperature. The zone is initially positioned at the seed crystal/boule interface and this region is heated to the melting point of the material. The zone is moved slowly down the length of the boule.Zone travel rate varies with composition, and exemplary rates are shown, along with other processing parameters, in Table IL Zone travel rate is an important processing parameter. If it is too great, crystallization is incomplete and defects are formed If it is too slow, layer distortions result. The lower portion of the heattreated boule in contact with the quartz boat is preferably removed before use.
Table II. Processing conditions for hexagonal single crystal growth.
The above process can be modified slightly to produce crystals of rhombohedral structure, in which case a rhombohedral seed crystal is used in the zone refinement process. In addition, to obtain rhombohedral crystals, the finance temperature is held at 300C below the melting point and the zone is maintained at the melting point of the polycrystalline material.
Preferably, a monocrystalline intercalation compound, and most preferably, bismuth chalcogenide, is grown using the process described above to produce monocrystalline electrode structures. For example, as one embodiment of the invention, monocrystalline bismuth chalcogenide electrodes are produced having a rectangular geometry with sides of 4 millimeters-long and 5 millimeters-long, and having a thickness of between 0.5-1 millimeters. It is preferable to metalize one of the faces of the monocrystalline material which is perpendicular to the plane of the van der Waals channels within the crystal.
This metalization may consist of, for example, a nickel paste, which is spread on the crystal to form a 10-20 micron-thick metal layer. The metalization provides both a good electrical contact to the crystalline piece and enhances the rigidity of the crystalline piece.
Alternatively, the monocrystalline bismuth chalcogenide material may be ground into a powder for forming the electrodes; such a powdered material is more easily manipulated than the single crystal material. The crystal grinding process may be carried out using, for example, a ball milling device, or other grinding device, to produce single crystal particles each having a diameter of preferably approximately 70 microns. Other particle diameters may be more preferable in specific instances. The crystal particles are then mixed with an appropriate compound to bind them together. While the binder acts, in effect, to "glue" the particles together, it must not completely electrically insulate the particles from each other. The binder material is selected according to the electrolyte.When an aprotic electrolyte solvent is used, the binder preferably consists of a 3% aqueous solution of carboxymethylcellulose, in which the particles are mixed; for other electrolytes, alternative binding agents, e.g., a 5% polyethylene dispersion in normal hexane, may be used. The resulting powderbinder mixture is placed into an electrode mold and then dried at room temperature. The electrode geometry, as determined by the mold, may be, for example, disc-shaped, as is conventional for capacitors, with an electrode thickness of between 0.3-1 millimeters. Alternative electrode geometries are also feasible.
The grinding process described above produces some amount of crystal damage, and corresponding crystal defects. However, because of the weakness of the van der Waals attractive force between the crystal layers of intercalation compounds, these compounds cleave readily along the axis of the channels without much danger of lattice damage or distortion.
The inventors herein have also developed a preferable process for producing electrodes of material combinations of bismuth chalcogenides, activated carbon, and heat treated graphite. In this scheme, a bismuth chalcogenide, such as Bi2Te3 may be present in the range of from 0 to 100%.
The activated carbon material (ACM) may also be present in the range of 0 to 100% and the heat treated graphite (HTG) may be present in the range of from O to 40% of the ultimate composition. An electrode with higher performance but with higher cost will include a higher proportion of the bismuth chalcogenide whereas an electrode with lower performance and lower cost will include a higher proportion of the ACM. The particular proportions chosen will thus be based on a cost-performance analysis. The composition is held together with a binder such as polytetrafluoroethylene (teflon).
If the electrode is to include a bismuth chalcogenide such as Bi2Te3, the process described above is employed to produce monocrystalline powder particles. The starting material for the ACM is a cellulose fiber. The cellulose fiber is heat treated at 800-950 C for one to two hours in an oxygen free environment. Thereafter, it is activated in a steam atmosphere at 800-9000C for two to three hours. The material is then crushed into particles having diameters in the range of 4-8 mm and a length of 0.5 mm. This material will have a specific surface area of 1300-2500 m2/g. The heat treatment of the cellulose fiber material will result in an 85-90% carbon content.
Production of the heat treated graphite (HUG) begins with crystalline flake graphite having an ash content of not more than 2%. This material is ground and sorted to have a maximum diameter of 200 pm. The particulate material is then heat-treated in an electric furnace at a temperature in the range of 2200-2500 C in a vacuum atmosphere. The preferred vacuum is 104 mmHg. The material is maintained in the furnace for a time sufficient to reduce residual ash content to less than 0.1%. After this heat treatment, the powdered material is sifted in a vibration sifting device, resulting in particles having a diameter in the range of 80-200 pm.
The selected binder material, for example, polytetrafluoroethylene, is first processed in a mixer to obtain a bulk volume of 3-5 cm2/g. The binder is added to the powder material in a quantity not to exceed 20 wt% of the electrode weight.
Once the starting materials are prepared, the following steps are carried out to fabricate an ACM electrode. First, the monocrystal bismuth chalcogenide powder, if any is to be included, the heat treated graphite, and the teflon are put into a mixer and mixed for 2-3 minutes at a speed of rotation not less than 4,000 rpm. Thereafter the selected quantity of ACM is added and the mixing process is continued for another 2-3 minutes at the same speed to obtain a material having a bulk volume of 4.3-4.7 cm3/g. A suitable material begins with 97% HTG and 3% teflon; ACM is then added to provide 80% of the total weight.
This electrode material is then compressed onto a metallic substrate in a hydraulic press with a compression pressure of approximately 270 MPa. A suitable electrode size is 10 cm by 10 cm by 0.1 cm. A suitable substrate material is perforated nickel foil having holes whose diameter is in the range of 0.7-1.0 mm. The distance between the holes is in the range of 0.6-0.8 mm.
Other perforation schemes are also acceptable. A preferred thickness of the metallic substrate is 0.1 mm.
After the electrode composition is pressed onto the metallic substrate, the resulting electrode is annealed for 1-2 hours at a temperature of 100-140 C in a chamber evacuated to 10.1 mmRg.
After annealing, the electrode is impregnated with a suitable electrolyte such as a 28-30% water solution of potassium hydroxide. The electrode is immersed in the electrolyte at room temperature under pressure for approximately 5-10 minutes.
Using a suitable electrode production process, including either of the processes described above or another suitable process, the capacitive cooling device of the invention is assembled into the following scheme. Referring to Fig. 1, in the preferred device configuration 60, two identical electrodes, most preferably being bismuth chalcogenide electrodes 20, are separated by an electrolyte 30. The electrolyte 30 suitably consists of an aqueous solution of, e.g., alkali, or 1.0 M of LiClO4 in propylene carbonate, using bismuth chalcogenide electrodes. In this case, a separator, consisting of 2 layers of non woven polypropylene, each layer 100 pm-thick, and saturated with the electrolyte, provides mechanical support of the electrolyte.Alternatively, for various electrode materials, the electrolyte may comprises a 1.2 M solution of organic cation of perchlorate in a mixture of propylene carbonate in dimethoxyethane, an aqueous solution of potassium hydroxide, an aqueous solution of single valence metal sulphates, or other aqueous solution. Using the LiClO4 propylene carbonate electrolyte discussed above, a polypropylene separator is suitably impregnated with the electrolyte solution and is positioned between the electrodes 20. Because the separator material adds to the overall internal resistance of the device, the separator thickness should be minimized while at the same time taking dependent device parameters into consideration.
The electrodes, held apart by the separator, are inserted into a supporting frame (not shown) and sealed in a pressing fond, As explained above, depending on the cooling device electrode material and electrolyte, the material may need to be "trained"; in particular, some intercalation compounds must be trained to allow the electrolyte to penetrate the van der Waals channels and form a double layer of charge with the channels' 8surfaces. Bismuth chalcogenides, while being preferred for their electrical properties, are one type of intercalation material which requires this training process. Accordingly, "training" is aprocess, described below, whereby electrolyte (and ions) are driven within the van der Waals channels to facilitate flow of electrolyte into and out of the channels.While the process will be described for this material, it must be recognized that the same procedure may be applied to other materials.
Referring to Fig. 2A, there is shown a capacitive cooling device 60 having two bismuth chalcogenide electrodes 20a, 20b at the start of the training process. The dimensions of the electrodes' van der Waals channels 70a, 70b are greatly exaggerated for clarity, and it must be recalled that each electrode is comprised of on the order of 106-107 such channels. Between the two electrodes is positioned a LiClO4-based electrolyte 30. During the training process, the power supply 40 is set to provide a voltage which is greater than the faraday potential for cation intercalation, and thus the voltage depends directly on the particular combination of device electrode material and electrolyte employed.
Given a particularly chosen electrode-electrolyte combination, those skilled in the art will recognize that the corresponding faraday potential may be determined in a standard table of material systems and faraday voltages.
At the start of the electrode training, when a voltage above the faraday voltage is applied to the device, the electrode 20b connected to the positive terminal of the power supply accumulates a positive surface charge. The surfaces of the van der Waals channels 70b of the electrode likewise accumulate this positive surface charge. Correspondingly, both the macroscopic surface and the surfaces of the van der Waals channels 70a of the electrode 20a connected to the negative terminal of the power supply accumulate a negative surface charge.
In response to this surface charge configuration, free Li+ ions 72 readily intercalate the negatively charged electrode 20a, because of the favorable charge and energy configuration, and because their ionic radius is relatively smaller than the width of the van der Waals channels. In addition, solvated Li+ complexes 74 move toward the negatively charged electrode surface and solvated Cl04 complexes 76 move toward the positively charged electrode surface.The positively charged electrode's van der Waals channels 70b, being 3-4 A-wide (as occurring before the training process) are too small for the ClO4- complexes to penetrate within them; the solvated Li' complexes, however, do to a small degree penetrate the 3-4 A-wide channels 70a of the negatively charged electrode 20a, effectively being transported along with the free Li+ ions to the electrode surface and within the electrode channels. As a result, the solvated Li+ complexes slightly widen the channels that they partially enter in the negatively charged electrode.
In order to cause the solvated Li+ complexes to penetrate the opposite electrode 20b, the polarity of the power supply is reversed. Then, the accumulated surface charge distribution reverses; the previously positively charged electrode now accumulates negative surface charge, and attracts the free Lif ions 72 and solvated complexes 74. The free Lif ions 72 readily intercalate the channels and the solvated complexes 74 again partially enter the corresponding van der Waals channels, and thereby slightly widen the channels.
Referring to Fig 2B, repetition of this process of voltage polarity switching progressively widens the van der Waals channels of each of the electrodes 20a, 20b. Throughout the process, the voltage may be increased, depending on the initially applied voltage, to thereby increase the attraction of the ions and electrolyte to the van der Waals channels.At an intermediate point in the training process, as depicted in the figure, the solvated Li+ complexes 74, as well as the free Lil ions, will be able to completely penetrate the widened channels 70b of the electrode 70b, which is shown to be currently negatively charged The solvated ClO4 complexes, being of a larger size than the solvated Li complexes, will not yet be able to completely penetrate the channels of the currently positively charged electrode 70a, however.
At the end of the training process period, referring to Fig. 2C, both the solvated ClQ complexes 76 and the solvated Li+ complexes 74 are able to completely penetrate the van der Waals channels 70a, 70b, of both electrodes, 20a, 20b. As shown in Fig. 2'), at this time, electrically neutral electrolyte (including both ClQ complexes 76 and Li+ complexes 74) is thereby able to completely penetrate the van der Waals channels and create an electric double layer of charge 80, 82 and 84, 86 at the electrode-electrolyte interface throughout the van der Waals channels of each electrode, in a manner similar to that which occurs at the macroscopic surface of the electrodes. This penetration of electrolyte throughout the crystal channels forms the basis for achieving dramatic charge density increases that result in the cooling effect achieved by the invention.
The extent of training required to achieve penetration of the electrolyte within the electrodes' van der Waals channels is critically dependent on the particular combination of electrode material and electrolyte employed. The width of the electrode van der Waals channels before undergoing any training process and the radius of the solvated complexes in the electrolyte determine the training required; the larger the complex radius and the smaller the van der Waals channels' width, the longer the training time requirement. For the electrode material Bi2Te3 and an LiCl04-based electrolyte, the training preferably consists of about 20 training cycles of approximately 30 minutes each, where the polarity of the power supply is reversed with each cycle. For specific capacitive device requirements, this training may be adjusted, however.With less training, a lower degree of electrolyte penetration within the channels would be achieved, and a correspondingly lower double layer capacitance would result. Thus, for achieving the maximum possible capacitance of a given electrode, the training should be maximized. Those skilled in the art will recognize that a preferable training procedure may be empirically determined for a given electrode-electrolyte combination and charge density accommodation goal.
Alternative training processes are within the intended scope of the invention. For example, the voltage polarity may be maintained constant in the above process, or a charge-discharge process may be employed to widen the van der Waals channels. In such a process, a voltage above the faraday potential is applied between the electrodes, in the manner discussed above, for a period of time, and then the capacitor is discharged across an appropriate load. If the voltage polarity is maintained constant during this process, or if the voltage polarity is not switched during the training process first described, one of the electrodes may not achieve widened channels, depending on the electrode material and electrolyte composition.For example, using Bi2Te3 electrodes and a LiClO4-based electrolyte in a training procedure in which the voltage polarity is constant, the electrode having the negative polarity will be intercalated with free and solvated Li+ complex (and thereby accommodate electrolyte), but the electrode of positive polarity will not have the benefit of free Li+ ions and complexes beginning to open its lattice channels, and thus the solvated ClOd complexes will not widen those channels to accommodate electrolyte; as a result, the electrode of positive polarity will not provide the extended van der Waals surfaces.
Of particular importance is the fact that the training process does not deform or distort the crystal planes of the layered crystalline electrode material to any significant extent. The extent of crystal plane deformation is related to the starting purity and defect density of the electrode material, as well as other properties resulting from the growth process; fewer initial defects in the crystal result in fewer crystal plane deformation sites caused by the training. With little or no crystalline plane distortion at the end of training, the electrodes' van der Waals channel surfaces are uniform and stable, and can correspondingly charge and discharge the double layer in a short time period. In addition, the internal resistance of the material is maintained very low. These conditions provide for the adiabatic discharge mechanism relied on by the cooling device of the invention.Also of importance is the fact that the after the training process widens the van der Waals channels to accommodate the electrolyte, the channels do not later shrink to their original width.
It should be emphasized that alternative materials, like compounds including activated carbon, are also suitable electrode materials and do not require a training process
Referring now to Fig. 3, the cooling device of the invention is operated in the following configuration. The capacitive device structure 60 is thermally isolated from the ambient via a calorimeter 35, or any suitable insulating container. The device is connected in a parallel charging-discharging circuit via, for example a switch 62. This circuit is external to the calorimeter 35. The charging circuit comprises a power supply 40 and an electrical control component 64, e.g., a current limiter, for limiting the speed with which the power supply charges the double layer of the capacitive device.The discharging circuit comprises an appropriate load 65, e.g., a wire, resistor, or other load element. The load element is suitably selected based on the charge density accumulation of the capacitive device to achieve the highest possible discharge speed A thermocouple device 66 may be positioned in proximity to the cooling device 60 to measure the cooling effect. Such a thermocouple device might typically include a meter 68 for indicating the cooling effect measurement.
In operation, the capacitive structure 60 is first connected to the power supply via an appropriate control of the switch 62, at which time the capacitive structure accommodates a charged double layer. The current limiter 64 produces a "trickle charge" effect and thereby controls the speed of the charging operation such that it proceeds slowly enough for isothermal conditions to be maintained at the device. A resistor or transistor-based device may suitably be used as a current limiter. Once the capacitive structure has accumulated a characteristic charge density, the switch 62 is controlled to discharge the capacitive structure across the load 65. Due to the very high charge density and very low internal resistance of the capacitive structure, and using a preferably low load resistance, the structure discharges at a speed approaching adiabatic conditions.As a result, during the discharge operation, the temperature of the capacitive structure decreases. Repetition of the charge-discharge process results in accumulation of this cooling effect.
Based on thermodynamic principles, the cooling mechanism results from the adiabatic entropy increase produced when the capacitive structure is discharged. As required by the second law of thermodynamics, an adiabatic increase in entropy results in a temperature decrease. The isothermal capacitive charging process results in an ordering, and corresponding entropy decrease, which is, in effect, released during the discharge process. Based on this phenomena, it is recognized that electrode materials exhibiting a high degree of order are more effective in the cooling device than materials of lesser degrees of order. "Degree of order is here meant to describe the impact of crystal purity, defect density, uniformity, and other related structural and electrical properties. For example, a monocrystalline electrode structure is understood to provide a larger entropy change and corresponding temperature decrease than a polyerystalline electrode structure.
A BiaTe3, LiCl04-based cooling device of 20 mm in size has been demonstrated to achieve a cooling effect of 0.5-0.7 C per charge-discharge cycle (each cycle consists of one charging and discharging process). This significant cooling effect is cumulative as cycles are continued, and has been demonstrated to proceed until the electrolyte froze. Preferably, each charge and discharge phase of each cycle is given 1 second to complete.The superior charge accommodation and low internal resistance of this device are understood to enhance the cooling mechanism--the device charge capacity is between 30-100 farads per cubic centimeter and the device internal resistance approaches approximately 0.02 Q-cm2. This extremely low internal resistance provides the ability to achieve high power in the capacitor discharge. Theoretically, a monocrystalline capacitor structure of pure and defect-free bismuth chalcogenide would exhibit 1000 farads per cubic centimeter and would provide an even higher discharge rate.
It must be understood that the capacitive cooling device of the invention may employ other materials and electrolytes and still achieve a cooling effect; the device performance is, however, effected by the particularly chosen combination of materials and electrolytes. For example, a cooling device consisting of activated carbon electrodes and, employing, e.g., sulphuric acid as an electrolyte, while still achieving the cooling effect, is somewhat less efficient than a device consisting of bismuth chalcogenide electrodes.
Considering that the Bi2T#, liiClO4-based cooling device achieves a degree of cooling which is great enough to freeze the electrolyte, alternative, low freezing-point electrolyte systems may be employed for appropriate electrode materials. For example, in theory, the use of propylene carbonate, which has a freezing point of -60 C, as an electrolyte would provide the ability to achieve a greater degree of cooling.
The inventors herein have recognized that the cooling capability provided by the capacitive cooling device of the invention may be employed in a myriad of applications where a solid state cooling device is preferable over a conventional condensation-tgpe cooling device. Such applications are characterized by space, weight, power, or material limitations (as in the case of replacements for freon- based systems) and include cooling of, e.g., electronic components, medical equipment, optical fiber systems, food refrigeration devices, chemical analysis devices, and an expansive variety of other systems.
Other embodiments of cooling device materials, training schemes, modes of operation, and application are intended as included within the spirit and scope of the invention.
RECHARGEABLE BATTERY
WO0235617
Inventor: TOVSTJUK KORNEI DENISOVICH // MICOV MIRO
HEAVY-POWER CAPACITOR WHICH HAS DOUBLE ELECTRIC LAYER
RU2098879
Inventor: TOVSTYUK KORNEJ DENISOVICH // CHERNILEVSKIJ IGOR KONSTANTINO
ELECTROLYTIC DOUBLE LAYER CAPACITOR
WO9309552
Inventor: GRIGORTCHAK IVAN // TOVSTJUK KORNEI
ENERGY STORAGE DEVICE
WO9309571
Inventor: KOZMIK IVAN // TOVSTJUK KORNEI
Electrolytic double layer capacitor
US5351164
Inventor: GRIGORTCHAK IVAN // TOVSTJUK KORNEI D
Energy storage device
US5368957
Inventor: KOZMIK IVAN // TOVSTJUK KORNEI D
CAPACITIVE THERMOELECTRIC DEVICE
CN1076809
Inventor: GRIGORTCHAK IVAN // TOVSTJUK KORNEI