Patent Publication Number: US-2015075182-A1

Title: Methods to improve the performance of electrocaloric  ceramic dielectric cooling device

Description:
FIELD OF THE INVENTION 
     The present disclosure is directed to electrocaloric ceramics and cooling devices, heat pumps, other devices employing the same, and methods of making 
     BACKGROUND OF THE INVENTION 
     The electrocaloric effect has the potential to provide high efficiency and environmentally friendly cooling technology, especially if the effect is large. The electrocaloric effect (ECE) is a result of direct coupling between the thermal properties, such as entropy, and the electric properties, such as electric field and polarization, in a dielectric material. In the ECE material, a change in the applied electric field induces a corresponding change in polarization, which in turn causes a change in the dipolar entropy as measured by the isothermal entropy change ΔS in the dielectrics. If the electric field varies in an adiabatic condition, the dielectric material will experience an adiabatic temperature change ΔT. To date, ECE materials having achieved only a small ECE (ΔT&lt;2 K), near room temperature, is commercially impractical for use in cooling devices. 
     Recently, it was demonstrated that by operating near ferroelectric phase transitions, a giant EC response can be realized (Bret Neese, Baojin Chu, Sheng-Guo Lu, Yong Wang, E. Furman, and Q. M. Zhang, Large Electrocaloric Effect in Ferroelectric Polymers Near Room Temperature. Science, 321, 821-823, 2008; S. G. Lu, B. Ro{hacek over (z)}i{hacek over (c)}, Q. M. Zhang, Z. Kutnjak, Xinyu Li, E. Furman, Lee J. Gorny, Minren Lin,B. Mali{hacek over (c)}, M. Kosec, R. Blinc, and R. Pirc. Organic and Inorganic Relaxor Ferroelectrics with Giant Electrocaloric Effect. Appl. Phys. Lett. 97, 162904, 2010). It has also been shown that a giant ECE can be obtained by designing a dielectric material near an invariant critical point (ICP), which allows the coexistence of a large number of coexistence phases and at which the energy barrier for the switching between different phases is lowered markedly (see Z. K. Liu, Xinyu Li, and Q. M. Zhang. Maximizing the number of coexisting phases near invariant critical points for giant electrocaloric and electromechanical responses in ferroelectrics. Appl. Phys. Lett.101, 082904 (2012)). 
     SUMMARY OF THE INVENTION 
     The advantages of the present disclosure include a cooling device comprised of ceramic ECE materials with a large number of co-existing phases. The ceramic EC modules are comprised of a plurality of layers formed by two or more EC ceramic single layers. The EC ceramic layers can be same or different materials. The thickness of the EC layers may be the same or different. 
     These and other advantages are satisfied, in part, by a cooling device comprising at least one high EC ceramic. The ceramic materials include but are not limited to modified BaTiO 3 , such as Ba(Ti 1-x Zr x )O 3 , Ba(Ti 1-x Sn x )O 3 , Ba(Ti 1-x Hf x )O 3 , (Ba 1-x Sr x )(Ti 1-x Zr x )O 3 , (Ba 1-x Sr x )TiO 3 , (Ba 1-x Sr x )(Ti 1-y Sn y )O 3 . The ceramic material can be modified by varying the end member R in the formula BaTiO 3 —R. The ceramic materials can also involve sintering to expand the composition variables. Furthermore, the ceramic materials can be La-modified Pb(ZrTi)O 3  ((PbLa x )(ZrTi)O 3 ), and bismuth based solid solution with PbTiO 3  such as(1-x)BiRO 3 -xPbTiO 3 , where R is selected from Fe, Mn, Cu, Sc, In, Ga, Yb, Mg 1/2 Ti 1/2 , Zn 1/2 Ti 1/2 , Co 1/2 Ti 1/2 , Mg 1/2 Zr 1/2 , Zn 1/2 Zr 1/2 , Mg 1/2 , Sn 1/2 , Mg 2/3 Nb 1/3 , Zn 2/3 Nb 1/3 , Mg 2/3 Ta 1/3 , Zn 2/3 Nb 1/3 , Co 2/3 Nb 1/3 , Co 2/3 Ta 1/3 , Mg 3/4 W 1/4 , Co 3/4 W 1/4  where 0.05≦x≦50.95. Of particular interest for electrocaloric cooling are compositions near the morphotropic boundary where a large number of phases may coexist and large electrocaloric cooling was predicted theoretically and was observed experimentally. For example the morphotropic boundary of xPbTiO3-(1-x)Bi (Mg 3/4 W 1/4) O 3 ) is located approximately at x=0.48. ICP also may lie near MPB although not limited to MPB as pinch off type ICP are also known (see C. Stringer et at Journal of Applied Physics, 97, 024101 (2005)).These are a few examples of the materials. Many other modifications are possible. 
     Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent similar elements throughout. Understanding that these drawings describe only several embodiments of the disclosure and are, therefore, not to be considered limiting of its scope and wherein: 
         FIG. 1 . Schematic illustration of one embodiment of a E-T-σ 1 -σ 2 -x PT -x PM multi-dimensional phase diagram with an ICP, using PM-PN-PT as an example with all the phases reported presented (the labeling of the phases refers to the zero-electric field phases). Here, x is the composition, σ a is stress, E is electric field and T is temperature. For simplicity, x, σ, and E are drawn on the same axis.  FIG. 2 . Phase diagram of one embodiment of Ba(Ti 1-x Zr x )O 3 , showing an ICP at composition x near 0.2. 
         FIG. 3 . Schematic phase diagram of one embodiment of AZrO 3 modified (K z Na y Li 1-z-y )NbO 3  ceramics (x(AZrO 3 )-(1-x)(K z Na y Li 1-z-y )NbO 3 . Here, 0&lt;z&lt;1; 0&lt;y&lt;1; 0&lt;x&lt;1; A=Ca, Sr, Ba or the combination thereof, and Zr can be substituted by the combination of (ZrHf), (ZrSn), (ZrTi), but must include Zr. 
         FIG. 4 . Schematic diagram of one embodiment of a ceramic multilayer EC module. In the example, the multilayer has ABABAB . . . sequence, where A (410) can be one ceramic composition and B (420) can be another ceramic composition, both of them having a large ECE; or A and B have the same composition but different thickness. Although six ceramic layers are shown, the number of ceramic layers in a multilayered EC module can be any number equal or greater than 2. 
     
    
    
     DETAILED DESCRIPTION OFTHE INVENTION 
     One embodiment relates to ceramic EC materials having large numbers of coexisting phases to achieve a large (ΔT≧2 K) electrocaloric effect (ECE). The advantages in some embodiments are achieved by cooling devices comprised of ceramic ECE materials with a large number of co-existing phases. The ceramic EC modules are comprised of multilayers formed by two or more EC ceramic single layers. The EC ceramic layers can be same or different. The thickness of each EC layer may be the same or different. 
     These and other advantages are satisfied, in part, by a cooling device comprising at least one high EC ceramic. In some embodiments, the EC ceramic materials include but are not limited to modified BaTiO 3 , e.g., Ba(Ti 1-x Zr x )O 3 , Ba(Ti 1-x Sn x )O 3 , Ba(Ti 1-x Hf x )O 3 , (Ba 1-x ,Sr x )(Ti 1-x Zr x )O 3 , (Ba 1-x Sr x )TiO 3 , (Ba 1-x Sr x )(Ti 1-y Sn y )O 3 . In some embodiments, the ceramic materials can also involve sintering aid to expand the composition variables. In some embodiments, the ceramic materials can be La-modified Pb(ZrTi)O 3  ((PbLa x )(ZrTi)O 3 ) and bismuth based solid solution with PbTiO 3  such as (1-x)BiTiO 3 -xPbTiO 3 . These are a few examples of the materials. Other modifications are possible. 
     Taking the ceramic (PM y -PN 1-y ) 1-x -PT x (P=Pb, M=Mg, N=Nb, and T=Ti) as an example, near the morphotropic phase boundary (x˜0.3, y˜13), multiple phases (p max =6) could coexist near ICP as illustrated in  FIG. 1 .  FIG. 1  shows that multiple phases can co-exist and energy barriers for the phase transformations near ICP are low, thus leading to large EC responses. The disordered phase (paraelectric phase) is a random mixture of a five ferroelectric phases with local polarization directions randomly distributed along symmetry allowed directions. For example, the five ferroelectric phases can be rhombohedral (Rh), two monoclinic (M), orthorhombic (O), tetragonal (T), and cubic. Using the entropy for a ferroelectric phase, along with the concept of phase mixture where a disordered paraelectric phase is considered as a random mixture of various dipole orientations, the entropy of a dipolar system can be written as 
     
       
         
           
             
               
                 
                   
                     
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     where k is the Boltzmann constant, c i  is the volume fraction of the i th  phase, Ω i  is the number of polar-states in the i th  phase, and v i  is the average volume associated with each dipolar unit in the i th  phase (smallest unit of v i is given by the volume per molecular unit). With Rh phase (Ω=8), O phase (Ω=12), T phase (Ω=6), and two monoclinic phases MC and MB (Ω=24 for both of them) in a PM-PN-PT thin film near an ICP, and, for simplicity, c i =⅕ and v i =v 0  is assumed for all the phases, Eq. 1 shows that the entropy becomes Sdip=4.15k/v 0 , which is much larger compared with a composition near pure PMN (pure Rh phase, Ω=8), which has a Sdip=2.08k/v 0 . If a high electric field can lead to a total saturation of polarization (Ω=1), then Sdip(Emax)=0 can be obtained, and thus a much higher entropy change ΔS in EC response near ICP can be achieved. For the example, here, ΔSdip(near ICP)/ΔSdip(Rh) is ˜2, which is an enhancement of 100%. These predictions indeed have been experimentally observed in the PMN-PT relaxor ceramics (Z. K. Liu, Xinyu Li, and Q. M. Zhang.  Appl. Phys. Lett.  101, 082904 (2012)). For example, for P(MN) 1-x -PT x  thin films at compositions near pure PMN (for example, x&lt;0.15), ΔS in the range of 1.7 Jmol −1 K −1  to 3.6 J mol −1 K −1  was induced under high electric fields (E=720 kV/m to 900 kV/m),while for P(MN) 1-x -PT x  thin films at composition near morphotropic phase boundary (x˜0.3), it was reported that a much higher ΔS from 3.7 Jmol −1 K −1  to 8.7 J mol − K −1  was induced under fields in the range from 600 kV/m to 750 kV/m. Here the average ratio ΔS dip (near ICP)ΔS dip (Rh)=2.3. 
     These results and considerations demonstrate the promise of working with multi-phase and multi-component material systems near ICPs to enhance the ECE. There are many material systems which can exhibit such an ICP in which multi-phases can coexist (more than two phases coexist). One class of dielectric ceramics which shows large ECE is the modified BaTiO 3  ceramics, which are lead-free and hence are desirable because they are environmentally friendly. 
     There are four different phases (rhombohedral (Rh), orthorhombic (O), tetragonal (T), and cubic) in BaTiO 3 , with phase transitions occurring at various temperatures as shown in  FIG. 2 . When Zr concentration is at 15%, the system BaZr x Ti 1-x O 3 exhibits a pinched phase transition, i.e., all the above three phase transition temperatures (T1, T2 and TC) correspond to pure BaTiO3 are merged or pinched into single diffuse phase transition as shown in  FIG. 2 . The large number of coexisting phases (4 different phases) at a single transition point near room temperature indicates that a very large ECE at room temperature can be obtained in this class of material. 
     The advantages of this embodiment is operating an EC material near its ICP, wherein a maximum number of available phases in the material can coexist and the energy barriers for switching between different phases become vanishingly small (compared with the thermal energy k B T, where k B  is the Boltzmann constant and T is the temperature in Kelvin). At room temperature (300K), the thermal energy is 25 meV, leading to very large ECE. One embodiment relates to modifying the composition to achieve large ECE in modified BaTiO 3 (BTO) which possess ICP at certain compositions. One example includes Ba(Ti 1-x Zr x )O 3  for x in the range from 0.1 to 0.25, preferably from 0.15 to 0.20. Other examples are environmentally friendly lead-free EC ceramics with ICPs. These materials include but are not limited to Ba(Ti 1-x Sn x )O 3  for 0.08≦x≦0.20, preferably 0.1≦x≦0.15, Ba(Ti 1-x Hf x )O 3  for 0.08≦x≦0.25, preferably 0.15≦x≦0.20, (Ba 1-x Sr x )(Ti 1-y Zr y )O 3  for composition with 0.08≦x≦0.2, and preferably 0.09≦x≦0.15, and with 0.1≦y≦0.25, (Ba 1-x Sr x )TiO 3  where 0.15≦x≦0.4, and (Ba 1-x Sr x )(Ti 1-y Sn y )O 3 where 0.1≦x≦0.3 and 0.05≦y≦0.3. 
     Another embodiment relates to flux systems often used in BTO based ceramic materials to improve the sintering such as lowering the sintering temperature while maintaining high dielectric responses and obtaining good densification. Addition of flux systems to BTO based ceramics also increase the number of possible composition variables which can increase the possible number of coexisting phases (see discussion of phase rule in the next paragraph and in Z. K. Liu, Xinyu Li, and Q. M. Zhang.  Appl. Phys. Lett.  101, 082904 (2012)). These flux systems include glass-forming flux such as B 2 O 3 , H 3 BO 3 , SiO 2 , and GeO 2 , and flux such as PbO, BaO, SrO, MnO 2 , Li 2 O 3 , LiBiO 2  and CaO which are very soluble in the lattice, and CdO, ZnO, Li 2 O,CuO, BaO, SrO, CaO, Na 2 O, K 2 O, and Bi 2 O 3  which are the A-site modifiers, and Bi 2 O 3 , Y 2 O 3 , Sb 2 O 5 , WO 3 , and Nb 2 O 5  which are B-site modifiers. These two types of modifiers (A-site and B-site) have limited solubility in the ceramic lattice (&lt;5 mol %). Other lead-free ceramics such as(Ba 0.3 Na 0.7 )(Ti 0.3 Nb 0.7 )O 3 , Na 0.5 Bi 0.5 TiO 3 , (1-y)(Na 0.5 Bi 0.5 )TiO 3 -y BaTiO 3  where 0.1≦y ≦0.9, xBaTiO 3 -(1-x)((K 1/2 Na 1/2 )NbO 3 ) where 0.1≦x≦0.9, and SrBiTa 2 O 9 can also exhibit multiphase co-existence region, leading to large ECE. 
     From thermodynamics consideration, the number of co-existing phases is constrained not only by the number of the available phases, but also by the variables such as the compositions, chemical potentials, temperature, stresses, and electrical field. For the material of two compositions, such as Ba(Zr x Ti 1-x )O 3 , composition x, temperature and electric field perpendicular to the ceramic layers are all variables. The number of maximum coexistence phases p max  is 3 (p max =m+v−f), where m is the independent composition variable, v is the all possibly available other variables including temperature, stresses, and electrical field, and f is the fixed variable which can&#39;t be changed for the material system. Taking Ba(Zr x Ti 1-x )O 3  ceramic as an example, m=1 (since the composition x can be varied),available variables for v include 6 for stresses, 3 for electrical field and one for temperature, thus, v=10, and fixed variable f=8 (6 for stresses (stress=0) since the material is in room pressure which is fixed, and 2 for electric field since ceramics are isotropic material. In order to induce a 4-phase coexistence point, one needs to introduce one more composition variable or other variables. 
     Another embodiment relates to using the flux to serve as additional variables to accommodate more phases to coexist. The flux can be added to the ceramics to increase the composition variables and improve the sintering. These include the glass-forming flux such as B 2 O 3 , SiO 2 , and GeO 2 , the flux such as PbO, BaO, SrO, and CaO which are very soluble in the lattice, and CdO, ZnO, Li 2 O, and CuO which are the A-site modifiers, and Bi 2 O 3 , Y 2 O 35  Sb 2 O 55  WO 3 , and Nb 2 O 5  which are B-site modifiers. 
     Another embodiment relates to mixing different ceramics to increase the number of variables. Examples include but not limited to (1-y)Ba(Zr x Ti 1-x )O 3 -y(Ba 1-z Ca z )TiO 3 where 0.15 23  x≦0.25, z in the range from 0.2≦z≦0.4, and 0.2≦y≦0.4, and xKNbO 3 — (1-x)(BaTiO 3 — (Bi 0.5 Na 0.5 )TiO 3 ) where 0≦x≦0.2. The large number of composition variables will increase the number of possible coexisting phases and lead to larger ECE. This approach can also be applied to other ceramics. 
     Another embodiment relates to polar-ceramic systems of (1-x)(K z Na y Li 1-z-y )NbO 3  (0&lt;z&lt;1; 0&lt;y&lt;1, 0&lt;x&lt;l)-xAZrO 3 , wherein A=Ca, Sr, Ba or the combination thereof, wherein Zr can be the combination of (ZrHf), (ZrSn) and(ZrTi)and must include Zr. These ceramics also possess similar ICP behavior (see  FIG. 3 ) and can exhibit a large ECE. 
     Another embodiment relates to increasing the number of co-existing phases near ICP of lead-based ceramics to enhance the ECE. These lead-based ceramics include, for example, La-modified Pb(Zr x Ti 1-x )O 3  where 0.3≦x≦0.7, to enhance the ECE. The forming of solid solution from different ceramics to increase the variables so that a large number of phases can coexist will also be used here. The ceramics that exhibit ICP and have promise to achieve ECE include solid solutions with PbTiO 3 . For these ceramics, there exist cubic, rhombohedral, orthorhombic and tetragonal phases at certain composition, in addition there exist four-phase coexistent composition regions near ICP. The solid solutions with PbTiO 3 can be expressed in (1-x)ABO 3 +xPbTiO 3 , where ABO 3  includes Pb(Mg 1/2 W 1/2 )O 3  with 0.4≦x≦0.65, Pb(Mg 1/3 Ta 2/3 )O 3  with 0.3≦x≦0.5, Pb(Ni 1/3 Nb 2/3 )O 3  with 0.3≦x≦0.5, Pb(Fe 1/2 Nb 1/2 )O 3  with 0.04≦x≦0.15, Pb(Mg 1/3 Nb 2/3 )O 3  with 0.25≦x≦0.45, Pb(Zn 1/3 Nb 2/3 )O 3  with 0.05≦x≦0.2, Pb(Mn 1/3 Nb 2/3 )O 3  with 0.15≦x≦0.35, Pb(Sc 1/2 Ta 1/2 )O 3  with 0.35≦x≦0.55, Pb(Co 1/3 Nb 2/3 )O 3  with 0.3≦x≦0.5, Pb(Sc 1/2 Nb 1/2 )O 3  with 0.35≦x≦0.52, Pb(Co 1/2 W 1/2 )O 3  with 0.35≦x≦0.55, Pb(In 1/i2 Nb 1/2 )O 3  with 0.30≦x≦0.45, Pb(Na 1/2  Bi 1/2 )O 3  with 0.05≦x≦0.25, Pb(Yb 1/2 Nb 1/2 )O 3  with 0.4≦x≦0.6, PbSnO 3  with 0.3≦x≦0.5, and PbHfO 3  with 0.4≦x≦0.6. 
     Another embodiment of this disclosure is bismuth based solid solution with PbTiO 3 . These materials have the potential to have a large number of co-existing phases near ICP, thus, to have a high ECE. These ceramics include but not limited to (1-x)BiRO 3 -xPbTiO 3 , where R is selected from Fe, Mn, Cu, Sc, In, Ga, Yb, Mg 1/2 Ti 1/2 , Co 1/2 Ti 1/2 , Mg 1/2 Zr 1/2 , Zn 1/2 Zr 1/2 , Mg 1/2 Sn 1/2 , Mg 2/3 Nb 1/3 , Zn 2/3 Nb 1/3 , Mg 2/3 Ta 1/3 , Zn 2/3 Nb 1/3 , Co 2/3 Nb 1/3 , Co 2/3 Ta 1/3 , Mg 3/4 W 1/4 , Co 3/4 W 1/4 , where 0.05≦x≦0.95. 
     Another embodiment of this invention is to use multilayers to introduce additional variables (stresses and electric fields) to increase the number of coexistence phases, as illustrated in  FIG. 3 . In the example illustrated, there are two types of EC ceramics A and B, A is, for example, BaTi 0.85 Zr 0.15 O 3 , and B is BaTi 0.5 Zr 0.2 O 3 . Ceramics A and B can be fabricated into multilayers with proper sintering aids. 
     Another embodiment of this invention is to fabricate the EC element in multilayer form, as illustrated in  FIG. 4 . Since the EC response of EC ceramics is related to the applied electric field on each EC layer. The relationship between the electric field (E), voltage (V) and thickness (d) of the ceramic is E=V/d. Thin EC layer in the multilayer can reduce the operation voltage while maintaining the required electric field. For example, for a multilayer EC element (or module) with each EC layer 5 μm thick, an applied voltage of 100 V will induce a 20 MV/m electric field in each EC ceramic layer. In addition, thin ceramic layer (less than 10 μm, for example) displays higher dielectric strength than that of a thick ceramic layer (100 μm, for example). In general, the thickness of each EC layer in a multilayer EC module can range 1 μm to 100 μm or thicker. The maximum number of layers can be 100, 200 or even 1000. 
     Only the preferred embodiment of the present invention and examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances, procedures and arrangements described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims.