Bulk ferroelectric thermal switch

Abstract

Methods and thermal switches (10) for modulating heat flow. The methods include applying a thermal gradient across a self-standing bulk relaxor ferroelectric material (12) (28) lacking a substrate and having a thickness of at least five micrometers, and applying an electric field across the ferroelectric material (12) (28) to modulate heat flow. The switches (10) include electrodes (14) and thermal couplers (18) operatively coupled to respective faces (22) (24) of the ferroelectric material (12) (28). The electrodes (14) are configured to apply an electric field to the bulk relaxor ferroelectric material (12) (28). The thermal couplers (18) are configured to define a thermal path through the ferroelectric material (12) (28) that includes at least a portion of the bulk relaxor ferroelectric material (12) (28) across which the electric field is applied. The electric field alters the thermal conductivity of the ferroelectric material (12) (28) between the thermal couplers (18), and thus the thermal conductivity of the thermal switch (10).

Description

BULK FERROELECTRIC THERMAL SWITCHCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U. S. Application No. 63 / 713,140 filed on October 29, 2024 and U. S. Application No. 63 / 753,166 filed on February 3, 2025, each entitled “Bulk Ferroelectric Thermal Switch” and having its disclosure incorporated by reference herein in its entirety.GOVERNMENT RIGHTS

[0002] This invention was made with government support under CBET 213 3718 awarded by the National Science Foundation. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] The present invention relates generally to thermal switches and, more particularly, to thermoelectric switches based on bulk relaxor ferroelectric materials.BACKGROUND

[0004] U. S. Patent No. 12,266,494 to Heremans et al. discloses ferromagnetic materials may be used to control heat flow with an externally applied electric field. Electric fielddependent phonon spectrum and heat conduction in ferroelectrics, by Wooten et al., Science Advances, Vol. 9, 2023 discloses that thermal switches operating at room temperature can be made from bulk ferroelectric materials with a thickness larger than 1 pm. Wooten further discloses that in a lead zirconium titanate (PZT) actuator, thermal conductivity decreases by increasing the field and reaches a maximum value close to zero electric field at 306 K. This measurement resulted in a thermal conductivity with the following coefficient of electric field dependency:r4o= -9.5where k' is the derivative of thermal conductivity with respect to the electric field E, and is the thermal conductivity when the electric field E = 0.

[0005] Experimental articles include studies of the thermal conductivity of PZT in the presence of an electric field and investigate ferroelectric domain wall and electromagnetic coupling in lead magnesium niobate-lead titanate (PMN-xPT). For example, Wooten experimentally demonstrated the effects of external electric fields on the thermal conductivity of bulk ferroelectric PZT. At 306 K, Wooten reported a thermal conductivity of 2.04 W / mK at zero field and 2.02 W / mK at 100 V. Although these findings represent an advancement inthe development of thermal switches due to their disclosures of materials responsive to electric fields, the switching ratios observed for those materials may be insufficient to enable the fabrication of practical thermal switches.

[0006] Ferroelectric Domain Wall Engineering Enables Thermal Modulation in PMN-PT Single Crystals, by Negi et al., Advanced Materials, vol. 35, 2023 experimentally investigates ferroelectric domain walls in alternating and direct current poled and unpoled PMN-.vPT in the (001) direction for 0.30 < x < 0.33. Negi compares the domain wall density of poled and unpoled samples, concluding that at intermediate poling states (0 < d33 < c, max) the domain wall density is lower due to an increase in domain size. Negi measured the piezoelectric coefficient of an unpoled sample to be 15+15 pC / N and a poled sample (after applying an electric field of 10 kV / cm for 40 cycles) to be 2400+25 pC / N. Negi observed a thermal conductivity enhancement of 27% using AC poling. These findings indicate that an enhancement in the piezoelectric coefficient d33 leads to a minimum domain wall density at an intermediate state.

[0007] Giant Electromechanical Coupling ofRelaxor Ferroelectrics Controlled by Polar Nanoregion Vibrations, by Manley et al., Science Advances, Sept. 16, 2016, investigates the lattice dynamics and local structure of PMN-30PT using neutron scattering. Manley uses poled PMN-30PT (x = 30%) single crystals with a volume of approximately 20 cm3and unpoled PMN-30PT single crystals with a volume of approximately 10 cm3, both oriented in the same direction and subjected to neutrons having an energy of 25 meV. Manley concludes that in ferroelectric-relaxor single crystals, electromechanical coupling can be enhanced by engineering the domains. Manley discloses that polar nanoregions move collectively with acoustic phonons, forming two hybrid modes. One mode is softer as compared to the original phonons (responsible for macroscopic shear softening) and the other mode is stiffer as compared to the original phonons. Furthermore, when a polar nanoregion mode aligns with part of the local structure in an electric field, shear softening increases, suggesting a method to enhance the ultrahigh piezoelectric response through the manipulation of elastic shear softening. However, Manley does not disclose thermal conductivity measurements in the presence of either a dynamic electric field or a static polarization.

[0008] U. S. Pub. No. 2004 / 0232893 to Odagawa discloses thermal switches that operate based on changes in thermal conductivity induced by phase transitions in the transfer element. Odagawa introduces the concept of thermal switches, which consist of a transition body and two electrodes, featuring a thermal switching element to regulate heat flow.Odagawa proposes controlling heat flow through various forms of energy, such asmechanical energy, magnetic fields, and electric fields, and describes methods for manufacturing a heat transfer element. Odagawa suggests the transition body can be made from any material capable of undergoing a phase transition when energy is applied. In addition to introducing the disclosed configuration for thermal switches, Odagawa highlights the significance of geometry and the phase transition in the transfer element. Odagawa asserts that the thermal conductivity of the electrodes is altered by the phase change in the transfer element, which is induced by applied energy.

[0009] There are several drawbacks associated with using phase change materials in thermal switches such as disclosed by Odagawa. These disadvantages include(1) functionality is limited to a narrow temperature range where the phase change occurs; (2) mechanical fatigue due to material strain, resulting in a limited cycle life; and (3) slow response times that cause delays in operation of the thermal switch. These limitations diminish the practicality of using the geometry disclosed by Odagawa in real-world applications.

[0010] Thus, in view of the above problems and limitations in the art, there is a need for improved methods and devices for controlling heat flow with a voltage.SUMMARY

[0011] In one aspect of the disclosure, an improved method of modulating heat flow is provided. The method includes applying a thermal gradient across a bulk relaxor ferroelectric material and an electric field across the bulk relaxor ferroelectric material to modulate the heat flow. The bulk relaxor ferroelectric material is self-standing, has a thickness of at least five micrometers, and does not include a substrate.

[0012] In one embodiment of the disclosed method, the method may further include controlling a temperature of the bulk relaxor ferroelectric material is such that the bulk relaxor ferroelectric material is maintained in a monoclinic phase.

[0013] In another embodiment of the disclosed method, the method may further include controlling the electric field applied to the bulk relaxor ferroelectric material such that the bulk relaxor ferroelectric material is maintained in the monoclinic phase.

[0014] In another embodiment of the disclosed method, the method may further include controlling both the electric field applied to the bulk relaxor ferroelectric material and the temperature of the bulk relaxor ferroelectric material such that the bulk relaxor ferroelectric material is maintained in the monoclinic phase.

[0015] In another embodiment of the disclosed method, the electric field may be between 0 and 3 kV / cm.

[0016] In another embodiment of the disclosed method, the heat flow and the electric field may each have a direction, and the direction of the heat flow and the direction of the electric field may be independent of each other.

[0017] In an another embodiment of the disclosed method, the bulk relaxor ferroelectric material may include Pb[Mgi / 3Nb2 / 3]O3- PbTiO3.

[0018] In another embodiment of the disclosed method, x may be greater than or equal to 0.00 and less than or equal to 0.37.

[0019] In another embodiment of the disclosed method, x may be greater than or equal to 0.30 and less than or equal to 0.33.

[0020] In another embodiment of the disclosed method, the bulk relaxor ferroelectric material may include a doped PMN-PT material.

[0021] In another embodiment of the disclosed method, the bulk relaxor ferroelectric material may be selected from the group consisting of Pb(Mg, Nb)O3 (PMN), Pb(Mg, Nb)C>3-PbTiO3(PMN-PT), Pb(In, Nb)O3(PIN), Pb(In, Nb)O3-Pb(Mg, Nb)O3-PbTiO3 (PIN-PMN-PT), Pb(Zn, Nb)O3(PZN), Pb(Zn, Nb)O3-PbTiO3(PZN-PT), (Pb, La)(Zr, Ti)O3(PLZ), (Pb, La)(Zr, Ti)O3-PbTiO3(PLZT), Pb(Sc, Nb)O3(PSN), Pb(Mg, Ta)O3(PMT), Pb(Fe, Nb)O3(PFN), Pb(Fe, W)O3(PFW), BaTiO3-Bi(Zn(Nb, Ta))O3(BT-BZNT), BaTiO3-(Ba, Sr)TiO3(BT-BST), and bismuth layer structured ferroelectrics (BLSF).

[0022] In another embodiment of the disclosed method, the bulk relaxor ferroelectric material may be a single crystal of PMN-33PT oriented in the (001) direction.

[0023] In another embodiment of the disclosed method, the temperature of the bulk relaxor ferroelectric material may be maintained between 225 K and 325 K, and the bulk relaxor ferroelectric material may be in a monoclinic C phase.

[0024] In another embodiment of the disclosed method, the temperature of the bulk relaxor ferroelectric material may be below 325 K, and the bulk relaxor ferroelectric material may be in a monoclinic A phase.

[0025] In another embodiment of the disclosed method, the temperature of the bulk relaxor ferroelectric material may be above 325 K, and the bulk relaxor ferroelectric material may be in a tetragonal phase.

[0026] In another aspect of the invention, a thermal switch for modulating heat flow is provided. The thermal switch includes a bulk relaxor ferroelectric material having a plurality of faces and a thickness of at least five micrometers, a first electrode operatively coupled to afirst face of the plurality of faces, a second electrode operatively coupled to a second face of the plurality of faces different from the first face, a first thermal coupler operatively coupled to one of the first face or a third face of the plurality of faces different from the first face and the second face, and a second thermal coupler operatively coupled to one of the second face or a fourth face of the plurality of faces different from the first face, the second face, and the third face. The first and second electrodes are configured to apply an electric field to at least a portion of the bulk relaxor ferroelectric material in response to a voltage being applied across the first and second electrodes, one of the first and second faces or the third and fourth faces defines a thermal path through the bulk relaxor ferroelectric material that includes at least a part of the portion of the bulk relaxor ferroelectric material across which the electric field is applied, and applying the electric field to at least the portion of the bulk relaxor ferroelectric material selectively alters a thermal conductivity of the thermal switch.

[0027] In one embodiment of the thermal switch, the bulk relaxor ferroelectric material may include one of a plurality of layers of bulk relaxor ferroelectric material each having the first face and the second face, the first electrode may include a plurality of first conductive layers, the second electrode may include a plurality of second conductive layers, the first face of each layer of the plurality of layers of bulk relaxor ferroelectric material may be operatively coupled to a respective first conductive layer, and the second face of each layer of the plurality of layers of bulk relaxor ferroelectric material may be operatively coupled to a respective second conductive layer. In this embodiment, the plurality of layers of bulk relaxor ferroelectric material, the plurality of first conductive layers, and the plurality of second conductive layers may define a stack of alternating layers of the bulk relaxor ferroelectric material and the first and second conductive layers.

[0028] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material may include one of a plurality of layers of bulk relaxor ferroelectric material each having the first face, the second face, the third face, and the fourth face, the first electrode may include a plurality of first conductive layers, the second electrode may include a plurality of second conductive layers, the third face of each layer of the plurality of layers of bulk relaxor ferroelectric material may be operatively coupled to a respective first conductive layer, the fourth face of each layer of the plurality of layers of bulk relaxor ferroelectric material may be operatively coupled to a respective second conductive layer, and the plurality of layers of bulk relaxor ferroelectric material, the plurality of first conductive layers, and the plurality of second conductive layers may define a stack of alternating layers of the bulk relaxor ferroelectric material and the first and second conductive layers.

[0029] In another embodiment of the thermal switch, the first face of the bulk relaxor ferroelectric material may be parallel to the second face of the bulk relaxor ferroelectric material.

[0030] In another embodiment of the thermal switch, the third face of the bulk relaxor ferroelectric material may be parallel to the fourth face of the bulk relaxor ferroelectric material, and the third face of the bulk relaxor ferroelectric material may be perpendicular to the first face of the bulk relaxor ferroelectric material.

[0031] In another embodiment of the thermal switch, at least a portion of the first thermal coupler may be operatively coupled to the first face of the bulk relaxor ferroelectric material through at least a portion of the first electrode, and at least a portion of the second thermal coupler may be operatively coupled to the second face of the bulk relaxor ferroelectric material through at least a portion of the second electrode.

[0032] In another embodiment of the thermal switch, the heat flow and the electric field may each have a direction, and the direction of the heat flow and the direction of the electric field may be independent of each other.

[0033] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material may include Pb[Mgi / 3Nb2 / 3]O3-xPbTiO3.

[0034] In another embodiment of the thermal switch, x may be greater than or equal to 0.00 and less than or equal to 0.37.

[0035] In another embodiment of the thermal switch, x may be greater than or equal to 0.30 and less than or equal to 0.33.

[0036] In another embodiment of the thermal switch, the temperature of the bulk relaxor ferroelectric material may be such that the bulk relaxor ferroelectric material is in the monoclinic phase.

[0037] In another embodiment of the thermal switch, the electric field applied to the bulk relaxor ferroelectric material may be such that the bulk relaxor ferroelectric material is in the monoclinic phase.

[0038] In another embodiment of the thermal switch, both the temperature of the bulk relaxor ferroelectric material and the electric field applied to the bulk relaxor ferroelectric material may be such that the bulk relaxor ferroelectric material is maintained in the monoclinic phase.

[0039] In another embodiment of the thermal switch, the electric field may be between 0 and 3 kV / cm.

[0040] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material may include includes a doped PMN-PT material.

[0041] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material maybe selected from the group consisting of Pb(Mg, Nb)O3, (Pb, La)(Zr, Ti)O3, Pb(Sc, Nb)O3, BaTiO3-Bi(Zn(Nb, Ta))O3, BaTiO3-(Ba, Sr)TiO3, and PIN-PMN-PT.

[0042] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material may be a single crystal of PMN-33PT oriented in the (001) direction.

[0043] In another embodiment of the thermal switch, the temperature of the bulk relaxor ferroelectric material may be between 225 K and 325 K, and the bulk relaxor ferroelectric material may be in in the monoclinic C phase.

[0044] In another embodiment of the thermal switch, the temperature of the bulk relaxor ferroelectric material may be below 325 K, and the bulk relaxor ferroelectric material may be in the monoclinic A phase.

[0045] In another embodiment of the thermal switch, the temperature of the bulk relaxor ferroelectric material may be above 325 K, and the bulk relaxor ferroelectric material may be in the tetragonal phase.

[0046] In another embodiment of the thermal switch, the bulk relaxor ferroelectric material is poled, and the thermal path is parallel to the poling.

[0047] The above summary presents a simplified overview of some embodiments of the invention to provide a basic understanding of certain aspects of the invention discussed herein. The summary is not intended to provide an extensive overview of the invention, nor is it intended to identify any key or critical elements, or delineate the scope of the invention. The sole purpose of the summary is merely to present some concepts in a simplified form as an introduction to the detailed description presented below.BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

[0049] FIG. 1 is a graphical view of phase transitions for relaxor ferroelectric material observed during thermal conductivity versus temperature measurements at zero electric field.

[0050] FIG. 2 is a graphical view of polarization versus electric field for the relaxor ferroelectric material of FIG. 1.

[0051] FIG. 3 is a graphical view of longitudinal resonant frequency versus electric field for a sample of the relaxor ferroelectric material of FIG. 1.

[0052] FIGS. 4-6 are graphical views of thermal conductivity versus electric field at room temperature for the relaxor ferroelectric material of FIG. 1 after different numbers of switching cycles.

[0053] FIGS. 7 and 8 are graphical views of piezoelectric strain versus electric field measurements on a fresh sample and a conditioned sample of the relaxor ferroelectric material of FIG. 1.

[0054] FIG. 9 is a schematic view of an electrically controlled thermal switch including a body comprising a layer of bulk relaxor ferroelectric material in which the thermal gradient and electrical gradient are aligned.

[0055] FIG. 10 is a schematic view of an alternative embodiment of the electrically controlled thermal switch of FIG. 9 in which the body comprises a plurality of bulk relaxor ferroelectric material layers alternating with electrically conductive layers.

[0056] FIG. 11 is a schematic view of an electrically controlled thermal switch including a body comprising a layer of bulk relaxor ferroelectric material in which the thermal gradient and electrical gradient are orthogonal.

[0057] FIG. 12 is a schematic view of an alternative embodiment of the electrically controlled thermal switch of FIG. 11 in which the body comprises a plurality of bulk relaxor ferroelectric material layers alternating with electrically conductive layers.

[0058] FIGS. 13 and 14 are graphical views of thermal conductivity measurements along different crystallographic orientations for both poled and unpoled samples of bulk relaxor ferroelectric materials.

[0059] FIGS. 15 and 16 are diagrammatic views showing relationships between the directions of thermal flux and poling for the graph of FIG. 14.

[0060] It should be understood that the appended drawings are not necessarily to scale, and may present a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, may be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments may have been enlarged or distorted relative to others to facilitate visualization and a clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.DETAILED DESCRIPTION

[0061] Ferroelectric materials have a spontaneous electric polarization which can be reoriented by an external electric field. The direction of the electric polarization typically varies in different regions of the ferroelectric material. Each region with a uniform polarization may be referred to as a “domain”, and the boundary between adjacent domains may be referred to as a “domain wall”. A domain wall can thus be considered as an interface that separates two regions having different polarization orientations in a ferroelectric material.

[0062] As used herein, the term heat flux generally refers to a flow of thermal energy per unit area per unit time, e.g., in watts per square meter (W / m2). Heat flux is a vector quantity having a direction and magnitude. Heat flow generally refers to a total rate of thermal energy transfer, e.g., in watts (W). Heat flow can be determined by integrating heat flux over the total area through which heat is flowing. Heat flow and heat flux are thus related, and the terms are sometimes used interchangeably.

[0063] Relaxor ferroelectrics are ferroelectric materials that exhibit high electrostriction. Relaxor ferroelectric materials have high dielectric constants, typically orders-of-magnitude higher than those of non-relaxor (or “ordinary”) ferroelectric materials. Both ordinary ferroelectrics and relaxor ferroelectrics show permanent dipole moments in domains.However, the size of these domains differs significantly. Ordinary ferroelectric domains are generally on the scale of microns, while relaxor ferroelectric materials have domains on the scale of nanometers. The domains of relaxor ferroelectric materials also typically take less energy to align than those of ordinary ferroelectric materials. As described in more detail below, embodiments of the present invention use bulk relaxor ferroelectric materials to implement thermal switches in ways that are neither disclosed nor appreciated by the prior art.

[0064] Previous attempts to use ferroelectric materials having an electric field dependent thermal conductivity to fabricate thermal switches have been made. (See e.g., Background references Hermans, Wooten, and Negri.) However, there at least three reasons why the previous results are not applicable the present disclosure. First, it has been determined that modulation of sound velocity in certain materials with an applied electric field (See e.g., FIG. 3) is consistent with the amount of thermal conductivity modulation. Sound velocity is a property at thermodynamic equilibrium and is thus not affected by scattering of phonons on domain walls. Second, the phonon mean free path in these materials is on the order ofnanometers, and the average distance between domain walls as disclosed by Negi is one micrometer. Thus, the domain wall scattering as described by Negi cannot be the mechanism behind the field dependent thermal conductivity disclosed below. Third, embodiments of the present invention also utilize relaxor ferroelectrics within temperature ranges that avoid phase changes, adjustments to the crystal orientation, or adjustments to the polarization direction relative to the principal axes of the crystal.

[0065] Relaxor ferroelectric materials include materials derived from the solid solution of (x) PbTiC>3-(l-x) I’bZrOj. Relaxor properties in PZT may be developed by incorporating foreign elements in A / B sites with isovalent and aliovalent dopants (donor and acceptor dopants). These dopants include: Isovalent types (Ba2+, Sr2+for Pb2+site & Sn4+for Zr4+and Ti4+sites), donor types (La3+, Nd3+, Sb3+for Pb2+sites & Nb5+, Ta5+, Sb5+, W6+for Zr4+and Ti4+sites), and acceptor types (K+, Na+for Pb2+& Fe3+, Al3+, Sc3+, In3+, Cr3+for Zr4+and Ti4+sites). Other lead-based relaxor ferroelectrics are Pb(Mg1 / 3Nb2 / 3)O3(PMN) or Pb(Sc1 / 2Ta1 / 2)O3(PST), Pb1-x(Lax)(ZryTiy)1-x / 4O3(PLZT), Pb(Zn1 / 3Nb2 / 3)O3(PZN), Pb(Mg1 / 3Ta2 / 3)O3(PMT), Pb(Sc1 / 2Nb1 / 2)O3(PSN), Pb(In1 / 2Nb1 / 2)O3(PIN), Pb(Fe1 / 2Nb1 / 2)O3(PFN), Pb(Fe1 / 2W1 / 2)O3(PFW) and the solid solutions:(1-x)Pb(Mg1 / 3Nb2 / 3)O3-xPbTiO3(PMN-PT) and (1-x)Pb(Zn1 / 3Nb2 / 3)O3-xPbTiO3(PZN-PT). It has been shown that a (1-x) [Pb(Mg1 / 3Nb2 / 3)O3] (x)[PbTiO3] solid solution exhibits normal ferroelectric properties near MPB (x = 0.30 to 0.35). Also, the structural fluctuation from rhombohedral to tetragonal through intermediate phases (i.e. monoclinic / orthorhombic / triclinic) has been observed with respect to x vary from 0.13 to 0.30. The relaxor properties in the above solid solution have been developed with substitution of optimum mole fraction (4%) of Sr2+in place of Pb2+.

[0066] Lead-free relaxor ferroelectrics include: (a) barium titanate (BaTiCb / BTO) based-, (b) potassium sodium niobate (Ko.5Nao.5Nb03 / KNN) based-, (c) bismuth sodium titanate (Bio.5Nao.5Ti03 / BNT) based- and (d) bismuth layer structured ferroelectrics (BLSFs). At room temperature, BTO exhibits stable electrical properties (dielectric and ferroelectric), good electrochemical coupling (fej ~ 0.50), a high-quality factor, and low dielectric loss, but may be limited by low Tc(120 °C-135 °C) and d33(-190 pC / N). Also, BTO follows the subsequent structural phase transitions from cubic (>120 °C-135 °C)-tetragonal (120 °C to 20 °C)-orthorhombic (20 °C to -80 °C)-rhombohedral (<-80 °C). In general,BaTiO3exhibits normal ferroelectric properties and follows the Curie-Weiss law at the ferroelectric to the paraelectric phase transition. However, relaxor ferroelectric behavior has been observed in a BaTiO3-BaSnO3 solid solution BTO based compound. Relaxor behaviorin BTO has been developed by designing the A and B-sites with incorporation of both heterovalent and isovalent ionic substitutions. Modified BTO ceramics with diffuse phase transition are available. The available BTO based relaxor ferroelectric systems include BaTiOa-CaTiOa, BaTiO3-BaZrO3-CaTiO3 [dss ~ 620pC / N for Ba0.85Ca0.15Ti0.90Zr0.10O3], BaTi03-BiFe03-Bi(Mgo.5Tio.5)03, BaTio.sSno.2O3,Ba(Tio.94Sno.o3Zro.o3)03 BaTiO3-La(Mg0.5Ti0.5)O3, BiTiO3-(x), Bi(Mg2 / 3Nbi / 3)O3 and so on.

[0067] The KNN system is another lead-free alternative having a high Tc(-410 °C), high Pr(~33pcm“2), and large Kp(-0.454). Basically, KNN is the solid solution of two perovskite compounds, i.e., KNbO? (orthorhombic: ferroelectric) and NaNbOs (orthorhombic: antiferroelectric). In general, the KNN forms the morphotropic phase boundary similar to that of PZT. KNN exhibits moderate dielectric, ferroelectric and piezoelectric properties as compared to PZT. Similar to BTO, the relaxor behavior of KNN has been developed by introducing other elements through interrupting the long range polar ordering and forms the PNRs. Some of the KNN based relaxor ferroelectrics with physical properties are (Ko.48Nao.535)o.942Lio.o58Nb03 \cl 3 ~ 314, K33 ~ 41, Tc- 490 °C], (Ko.44Nao.52Lio.o4)(Nao.86Tao.ioSbo.o4)03 \d33 ~ 416pC / N], 0.96(Ko,5Nao.5)o.95Lio,o5Nbi-xSbx03-0.04BaZrO3, 0.5wt%Mn-KNN (Tc- 416 °C, d33- 350 pCN’1), (Nao.44Ko.5i5Lio.o45)Nbo.9i5 Sbo.o45Tao.o503(*3 - 390pC / N, Tc- 320 °C K33 - 0.49), 0.96(Ko.4Nao.6)(Nbo.96Sbo.o4)03-0,04Bio.5Ko.5Zro.9Sno.i03 (<fe - 460pC / N, Tc- 250 °C, K33 - 0.47), (Nao.5Ko.5)o.975Lio.o25Nbo.76 Sb0.06Ta0.i8O3 (*3 - 352, Tc- 200 °C, K33 - 0.47).

[0068] BNT is another lead- free material that competes with PZT for actuator applications. It exhibits relaxor ferroelectric properties with relatively large remanent polarizations (Pr- 38 pC / cm2), large coercivity (Ec- 73 kV / cm), and high Curie temperature (-320 °C). It follows the character of an ergodic relaxor with room temperature rhombohedral crystal symmetry. BNT materials include BNT-ATiOs (A = Ca2+, Sr2-1-, Ba2+, and Pb2+), BNT-KNbCh, BNT-Bio.sLiosTiCh, BNT-Bio.5Ko.5Ti03(BNT-BKT), BNT-Ko.5Nao.5Nb03(BNT-KNN), BNT-BKT-KNN, BNT-BT-KNN, BNT-BKT-BiFeO3, BNT-BKT-BaTiO3-SrTiO3and so on.

[0069] BLSFs are considered as lead-free relaxor ferroelectrics. The general formula of BLSFs is (Bi20a)2+(Am-iBm03m + 1)2” with A-site occupy by mono-, di- or trivalent ions, B-site occupies by tetra-, penta- or hexavalent ions with appropriate size. The possible A-site elements could be K+, Na+, Ca+, Sr+, Pb2+, Ba2+, La3+, Bi3+, Ce3+, etc., and B-site could be Ti4+, Nb5+, Ta5+, W6+, Mo6+, etc. The number “m” (=1, 2, 3, 4, and 5) is the number of BOe octahedra in the (Am-iBmChm + 1)2” perovskite blocks. BLSFs exhibit high Tc, low tanb.low Er, and decent aging resistance. The above mentioned various classes of relaxor ferroelectrics may exhibit different unique relaxor behavior depending upon the formation of PNRs due to the compositional fluctuation in the crystallographic sites.

[0070] The term “switching ratio” refers to a figure of merit that indicates the ability of a thermal material or switch to selectively conduct heat. The switching ratio is the ratio of the thermal conductivity in the high conductivity state (or “on state”) to the thermal conductivity in the low conductivity state (or “off’ state). Switching ratios indicate how effectively a device can control heat flow, with a high switching ratio indicating heat flow can be blocked or passed more effectively than a low switching ratio. Embodiments of the present invention include thermal switches having relaxor ferroelectric materials with switching ratios substantially higher than those known in the art. In particular, thermal switches made from bulk relaxor ferroelectric PMN-xPT (x > 0.3) are shown to provide higher switching ratios than those made from PZT. The below disclosure also directly relates changes in thermal conductivity to changes in the piezoelectric coefficients d33 and d3i as the electric field is applied to the sample.

[0071] Samples of PMN-33PT (0.30 < x < 0.33) with a (001) poling direction and a thickness of 0.5 mm were obtained from MSE Supplies of Tucson Arizona. To facilitate application of an electric field to the PMN-33PT samples, a 10 nm layer of Ti was evaporated onto the primary surfaces of the sample, and a 90 nm layer of Au evaporated onto the layer of Ti. Each layer was deposited on the sample using a standard electron beam (E-beam) evaporator, which may be obtained from CHA Industries, Inc. of Elgin, Illinois.

[0072] The thermal conductivity of a PMN-33PT single crystal with a lattice constant of 4.026 A at room temperature was determined. PMN-33PT undergoes several crystallographic phase transitions depending on the electric field and temperature. Thermal conductivity measurements were conducted using the static heater-and-sink method. Without an applied electric field, the sample remains tetragonal (T) above 320 K and transitions to monoclinic (M) below this temperature. The monoclinic phase includes monoclinic A (MA) and monoclinic C (Mc) phases, where the subscripts denote different polarization vector directions. Thermal conductivity measurements, with heat flux applied through the (001) direction, clearly indicate the temperatures at which these phase transitions occur.

[0073] FIG. 1 depicts a graph illustrating temperature dependent phase transitions observed in thermal conductivity measurements at zero electric field. In the graph, the tern “1st zero” refers to the thermal conductivity value at zero electric field as the electric field isdecreasing, and the term “2nd zero” refers to the value at zero electric field as the electric field is increasing. MA, Me, and T refer to the monoclinic A phase, monoclinic C phase, and tetragonal phase (respectfully) that PMN-33PT passes through as the temperature changes. The functionality of a thermal switch may rely on selecting a temperature range where the relaxor ferroelectric material maintains a consistent crystallographic phase.

[0074] PMN-33PT may undergo phase transitions in an electric field, as can be demonstrated by strain vs. electric field curves. An example of this may be found in Electric-field-, temperature-, and stress-induced phase transitions in relaxor ferroelectric single crystals, by M. Davis et al., Phys Rev B Condensed Matter Mater Physis, vol. 73, no. 1, 2006. Davis discloses that just above room temperature, PMN-33PT transitions from the MAphase to the Mcphase with increasing electric field. Davis and Negi further disclose that these phase transitions can be influenced by synthesis methods, composition, and poling conditions.

[0075] Polarization along the (001) direction of the PMN-33PT sample was measured as a function of the electric field at room temperature. FIG. 2 depicts a graph illustrating the resulting polarization of the PMN-33PT sample, where arrows indicate the direction of the swept electric field. As can be seen, the coercive field is about 3 x 105V m-1and the saturation field is about 6 X 105V m-1with a polarization of about 0.06 coulombs per square meter (Cm-2). This provides information about the maximum field required to achieve saturation, avoiding the need to measure thermal conductivity concerning the electric field in the minor loops. Negi also relates the poling conditions to d33piezoelectric coefficient, disclosing that the coefficient can be affected by the poling electric field strength and AC / DC poling conditions.

[0076] To measure the sound velocity, a fresh sample of PMN-PT material, previously unexposed to an electric field, was cleaved into a parallelepiped shape with dimensions of 3 mm x 1 mm. Coiled copper wires (Omega, diameter of 25 pm) were attached to the electrodes to enable the application of an electric field along the (001) direction. A Data Precision 8200 voltage calibrator, which may be obtained from Stanford Research Systems of Sunnyvale California, was used to apply voltages to the sample. The longitudinal frequency associated with the longitudinal acoustic phonon was calculated based on the Young’s modulus and density provided by the manufacturer, and the sample’s dimensions. The sample was then placed in a Resonant Ultrasound Spectroscopy (RUS) instrument, which may be obtained from Alamo Creek Engineering, of Santa Fe, New Mexico. The sample wasexposed to ultrasound frequencies, which were amplified if they matched a natural resonant frequency. The frequency / of the resonance of the longitudinal compressive mode of the whole sample along the (100) direction was measured in the RUS instrument. The peak corresponding to the longitudinal frequency (around 320 kHz) was tracked as it shifted in response to the applied voltage.

[0077] FIG. 3 depicts a graph illustrating measurement results obtained from the RUS instrument of the frequency of the longitudinal compressive mode on the parallelepiped sample of PMN-33PT at 290 K. The relative change of this longitudinal resonant frequency is tracked as a function of electric field. The relative change of frequency equals the relative change of the ratio v7vo of the longitudinal acoustic phonon along the (100) direction of the crystal. The gray bar indicates where the slope was taken to obtain the value of v7vo, wherein v' is the derivative of sound velocity with respect to the electric field E, and vo is the sound velocity when the electric field E - 0. The graph includes a plot showing the relative changes in the frequency ( (E)- / o) / o versus electric field at room temperature as the field is varied from maximum positive field to zero and then to maximum negative field and vice versa. The frequency f0corresponds to the frequency at zero field. Resonant frequency was used instead of sound velocity because they are both proportional to the square roots of the elastic constant cn, so thatwhere v is the sound velocity of longitudinal acoustic phonons along the (100) direction. As a result, the relative field dependence of the phonon’s sound velocity v7vo was calculated as 2.63X10’8mV-1.

[0078] Thermal conductivity was measured under dynamically applied DC voltage by monitoring the temperature with two type T thermocouples attached to separate samples, referred to herein as sample one (“fresh sample”) and sample two (“cycled sample”). The thermocouples were made of copper and constantan wire and were secured to sample one using what is commonly referred to as GE varnish and to sample two using StyCast epoxy (available from Henkel North American Consumer Goods of Rocky Hill, Connecticut) to avoid short-circuiting. A 1200 resistive heater was placed on each sample to provide heat. Both samples were positioned on an alumina base (which acted as a heat sink) using the aforementioned GE varnish for sample one and StyCast epoxy for sample two. Each sample was then placed in a vacuum environment inside a nitrogen-cooled cryostat obtained from Lake Shore Cryotronics of Westerville, Ohio.

[0079] For each experiment, temperature was allowed to stabilize for an hour and the electric field was maintained for five minutes before measuring the thermal conductivity to prevent any pyroelectric artifacts. The measurement of thermal conductivity using the heater and sink method carries possible errors around 10%, including radiative heat losses and geometrical uncertainties. Geometrical uncertainties, which may be primarily due to the distance between the two thermocouples, were calculated to be between 1.4% and 1.8%. Radiative heat losses were assessed by measuring the thermal conductivity of an iron bar, which is known to be 0.10 mW / K at 200 K, 0.44 mW / K at 300 K, and 0.85 mW / K at 360 K. For the PMN-33PT samples, radiative losses were found to be less than 7% at 200 K, less than 29% at 300 K, and less than 55% at 400 K. These losses were subtracted from the measured conductance, though the difference in emissivity between the iron bar and PMN-33PT may introduce an additional error of about 30%. Consequently, the uncertainty in radiative losses for the PMN-33PT samples may be approximately 2% at 200 K, 10% at 300 K, and up to 18% at 360 K. As discussed below, fresh and cycled samples show different behavior for the thermal conductivity with respect to the electric field.

[0080] FIGS. 4-6 depict graphs of room temperature thermal conductivity measurements for a fresh sample on its first cycle (FIG. 4) and for a cycled sample at its tenth (FIG. 5) and twentieth cycle (FIG. 6). As can be seen, the data at the tenth run is essentially the same as the data for the twentieth run. Thus, the data indicates that while a transition occurs during the first 10 cycles, the sample is stable after about 10 cycles of sweeping the electric field. The arrows indicate the direction of sweep while the gray bar represents where the slope was taken to calculate the K'IKO values displayed below each plot. For the fresh sample of FIG. 4, K' / K0— 3.94 X 10"8m V-1. However, the cycled sample of FIG. 6, which was swept by the electric field for about 20 cycles at room temperature, provides the final value of K' / KQ= -3.86 x lO-7m V-1. This ultimate result seems to be stable and reproducible after about 10 cycles. The major point of the experiment conducted on these two samples is that the coefficient of electric field dependency K' / KO changes by cycling and the coefficient is enhanced by an order of magnitude. The coefficient of electric field dependency of a PMN-33PT sample conditioned by electric field cycling is two orders of magnitude higher than the coefficient observed on the PZT sample (K' / K0= — 7 X 10-9m V-1) disclosed by Wooten. Thus, devices made from PMN-33PT are expected to have significantly higher switching ratios than devices made with currently known

[0081] The d31coefficient was measured using both a conditioned sample that was previously utilized in a thermal conductivity experiment and a fresh (or “unconditioned”) sample. The fresh sample dimensions were 10 mm x 10 mm x 0.5 mm. The fresh sample was affixed to a microscope glass slab using GE varnish. Due to the thickness of the fresh sample, a piece of Kapton tape was placed on the top surface of the fresh sample to provide a broader area for a laser beam, thus reducing measurement error. Displacement was measured using an LK-G32 laser sensor, which may be obtained from the Keyence Corporation of Elmwood Park, New Jersey. An AC voltage was applied via copper wires attached to the electrodes using a Techron 5050 Linear Amplifier in tandem with an Agilent 33120A Waveform Generator. The Techron 5050 linear amplifier may be obtained from AE Techron of Elkhart, Indianna. The Agilent 33120A waveform generator may be obtained from Agilent Technologies, Inc. of Santa Clara, California. Changes in displacement as the AC voltage was applied were monitored with a Rigol MSO5074 oscilloscope, which may be obtained from Rigol Technologies of Tigard, Oregon.

[0082] The same experiment was conducted on the conditioned sample, with the primary difference being the sample mount. Instead of Kapton tape, a strain gauge was placed on top of the conditioned sample, providing a broad area for the laser beam. This gauge was attached using StyCast epoxy. The aim of these experiments was to verify that cycling a sample through multiple electric field sweeps alters the d31piezoelectric coefficient. FIGS. 7 and 8 depict graphs of the piezoelectric strain measurements on a fresh PMN-33PT sample (FIG. 7) and a conditioned PMN-33PT sample (FIG. 8). The slopes taken in the decreasing field sweep, indicated with arrows, were used to calculate the piezoelectric constant du values. The graphs of FIGS. 7 and 8 show the changes in strain relative to the electric field. The electric field was applied along the (001) direction, and the sample length change was measured along the (100) direction. The d i piezoelectric coefficient was calculated by deriving the strain tensor components with respect to the electric field, as the change in sample length corresponds to changes in the strain tensor.

[0083] FIG. 9 depicts an exemplary solid-state electrically controlled thermal switch 10 in accordance with an embodiment of the present invention. The thermal switch 10 includes a body 12 comprising a layer of bulk relaxor ferroelectric material, a plurality of electrodes 14 (e.g., upper and lower electrodes 14), and a plurality of thermal couplers 18 (e.g. upper and lower thermal couplers 18). The body 12 includes an upper face 22 that is operatively coupled to one electrode 14 (e.g., the upper electrode 14), and a lower face 24 generally opposite the upper face 22 that is operatively coupled to another electrode 14 (e.g., the lowerelectrode 14). The body 12 may have a typical thickness (i.e., distance between lower and upper faces 22, 24) of between 2 and 4 mm, however embodiments are not limited to this range of thicknesses. In practice, the thickness of the body 12 may be determined by the operational specifications of the thermal switch 10, and may range, for example, from 5 urn to 10 mm or more, depending on the application.

[0084] The body 12 may be coupled to the electrodes 14 by direct contact or through a thermally conductive layer (not shown). By way of example, each electrode 14 may be formed by depositing an electrically conductive material such as a metal (e.g., silver, gold, copper, aluminum, etc.) an epitaxial conductive oxide (e.g., SrRuCh, (La, Sr)MnO3, etc.) or other suitable conductive material directly on the respective face 22, 24 of the bulk relaxor ferroelectric material, or to an adhesion layer (e.g., titanium) deposited on the respective face 22, 24 of the bulk relaxor ferroelectric material. The electrodes 14 may thereby be configured to apply an electric field to (e.g., across) at least a portion of the bulk relaxor ferroelectric material (e.g., the portion between the electrodes 14) in response to a voltage being applied to the electrodes 14.

[0085] Each thermal coupler 18 may be operatively (e.g., thermally) coupled to a respective face 22, 24 through a respective one of the electrodes 14. To this end, each thermal coupler 18 may be coupled to a respective electrode 14 by a respective dielectric layer 26 to electrically isolate the thermal couplers 18 from voltages applied to the electrodes 14. Suitable dielectric layers 26 may include thin films of silicon dioxide, aluminum oxide (sapphire), silicon or aluminum nitride, diamond, etc. In an alternative embodiment, one or both of the dielectric layers 26 may be omitted. In either case, the thermal couplers 18 may thereby be operatively coupled to the bulk relaxor ferroelectric material through the dielectric layer 26 (when present) and the electrodes 14 such that the thermal resistance between each thermal coupler 18 and the body 12 of thermal switch 10 is less than the thermal resistance through the body 12 of thermal switch 10. In any case, the faces to which the thermal couplers 18 are operatively coupled may define a thermal path through the bulk relaxor ferroelectric material forming the body 12 of thermal switch 10 such that heat can flow from one thermal coupler 18 to the other thermal coupler 18 along the thermal path.

[0086] FIG. 10 depicts another exemplary solid-state electrically controlled thermal switch 10 in accordance with another embodiment of the present invention. The thermal switch 10 includes a body 12 comprising a plurality of bulk relaxor ferroelectric material layers 28 alternating with electrically conductive layers 30. The electrically conductivelayers 30 are alternately coupled to one of a plurality of electrode terminals 32, thereby defining the electrodes 14 of thermal switch 10.

[0087] FIG. 11 depicts yet another exemplary solid-state electrically controlled thermal switch 10 in accordance with another embodiment of the present invention. The thermal switch 10 includes a body 12, a plurality of thermal couplers 18 (e.g., upper and lower thermal couplers 18), and a plurality of electrodes 14 (e.g., left and right electrodes 14). The body 12 includes a left face 34 that is operatively coupled to one electrode 14, and a right face 36 generally opposite the left face 34 that is operatively coupled to another electrode 14. Each thermal coupler 18 may be operatively (e.g., thermally) coupled to a respective face 22, 24 of body 12. To this end, each thermal coupler 18 may be coupled to the body 12 directly (shown) or through a respective dielectric layer 26 (not shown).

[0088] FIG. 12 depicts yet another exemplary solid-state electrically controlled thermal switch 10 in accordance with another embodiment of the present invention. The thermal switch 10 includes a body 12 comprising a plurality of bulk relaxor ferroelectric material layers 28 alternating with electrically conductive layers 30. The electrically conductive layers 30 are alternately coupled to one of a plurality of electrode terminals 32, thereby defining the electrodes 14 of thermal switch 10.

[0089] In contrast to the generally parallel orientation between directions of the electric field and heat flow in the thermal switches 10 depicted by FIGS. 9 and 10, in the thermal switches 10 depicted by FIGS. 11 and 12, the direction of the heat flow and electric fields are at intersecting angles (e.g., orthogonal) to each other. Thus, it should be understood that embodiments of the invention are not limited any particular angular relationship between the electric field and the heat flow in the thermal switch. That is, the direction of the heat flow and the direction of the electric field may be selected independently of each other when designing thermal switches 10. Advantageously, this directional independence between heat flow and controlling electric field may provide an extra degree of freedom that enables optimization of thermal switches 10 for specific applications.

[0090] The smallest dimension of each relaxor ferroelectric layer (typically referred to herein as the thicknesses) may be greater than 1 pm, and preferably greater than 5 pm. The total thickness of the bulk relaxor ferroelectric material through which the heat flux must travel in each the thermal switch 10 may be selected according to an on and / or off thermal resistance design criteria. As described above, this thickness may depend on both the thermal conductivity of the bulk relaxor ferroelectric material and the design specifications of the thermal switch 10. In any case, dividing the total thickness of the bulk relaxor ferroelectricmaterial into multiple relaxor ferroelectric layers 28 (as in the bodies 12 of thermal switches 10 depicted by FIGS. 10 and 12) may enable relatively high coercive electric fields Ec to be generated by relatively low control voltages as compared to having a body 12 comprising a single relaxor ferroelectric layer (as in the thermal switches 10 depicted by FIGS. 9 and 11).

[0091] In operation, one thermal coupler 18 of the thermal switch 10 may be thermally coupled to a thermal load 38, and the other thermal coupler 18 may be thermally coupled to a heat sink 40. The thermal load 38 may be either a cooling load (i.e., heat is to be removed from the thermal load 38 such as in a cryogenic application) or a heating load (i.e., heat is to be transferred into the thermal load, such as in a heat leveling application). The heat sink 40 may be a thermal reservoir which is configured to either receive heat from the thermal switch 10 or supply heat to the thermal switch 10. A control voltage 42 may be applied to the electrodes 14 to selectively cycle the thermal impedance of the body 12 so as to thermally couple the thermal load 38 to the heat sink 40 in a controlled manner.

[0092] Ferroelectric materials with spontaneous and reversible polarization offer new opportunities for manipulating thermal transport via an external electric field. The present disclosure demonstrates that electric fields can significantly affect the thermal conductivity of ferroelectric materials by altering the dispersion relation of a subset of phonons known as “ferrous”, as well as by altering the phonon-phonon scattering. Such control may enable innovative applications, including thermal transistors, dynamic thermal management, solid-state energy conversion, and thermal logic devices.

[0093] A foundational study on Pb(Zr, Ti)O3 (PZT) revealed a 2% reduction in thermal conductivity at 2 MV / m due to field-induced changes in lattice dynamics, including thermal diffusivity and sound velocity. A predictive theory based on piezoelectric coefficients and Griineisen parameters quantitatively describes this behavior without adjustable parameters. These findings are extended to the relaxor ferroelectric 0.66Pb[Mgi / 3Nb2 / 3]O3-0.33PbTiG3 (PMN-33PT), where larger effects, up to 10%, are observed at lower fields (0.4 MV / m). Phase transitions and hysteretic behavior in PMN-33PT confirm that electric-field-driven changes in phonon spectra and scattering rates are responsible for these effects.

[0094] Complementary work on PMN-30PT reveals significant softening in the lower part of the transverse acoustic (TA) phonon branch and a reduction in phonon lifetime, likely due to an increased phase space for phonon scattering. Thermal conductivity measurements along the (001), (100), and (011) crystallographic orientations for both poled and unpoled samples indicate changes in thermal conductivity by up to a factor of two. These effects may be attributed to electric-field-induced changes in sound velocity and the phonon-phononscattering, rather than domain wall scattering. While dynamic field measurements are pending, the observed differences between poled and unpoled samples indicate that PMN-30PT is a suitable material for electrically actuated thermal switches with practical switching ratios.

[0095] FIG. 13 depicts a graph of thermal conductivity measurements along the (001), (100), and (011) crystallographic orientations for both poled and unpoled samples. The measurements reveal changes in thermal conductivity by up to a factor of two. These variations are attributed to electric-field-induced changes in sound velocity and phonon scattering, rather than domain wall scattering. Data collection for the unpoled (110) sample is still ongoing. However, the similar thermal conductivity values observed for the unpoled samples along the (010) and (100) directions indicate that PMN-30PT remains cubic within this temperature range, without undergoing any phase transitions.

[0096] FIG. 14 depicts a graph of thermal conductivity measurements along the (100), and (010) crystallographic orientations for additional samples of PMN-30PT poled along the (100) direction. The top plot shows thermal conductivity measurements for samples in which the heat flux is aligned with the (100) direction and is thus parallel to polarization. The bottom plot shows thermal conductivity measurements for samples in which the heat flux is aligned with the (010) direction and is thus perpendicular to polarization. FIG. 15 depicts the orientations of the polarization and heat flux for parallel measurements, and FIG. 16 depicts the orientation of the polarization and heat flux used for perpendicular measurements. As can be seen from the graph of FIG. 14, the thermal conductivity K of PMN-30PT samples polarized in the (100) direction is approximately three times higher along the poling direction than it is perpendicular to the poling direction. This ratio of thermal conductivity K is maintained over a wide range of temperatures. The thermal conductivity K along the perpendicular direction closely matches that of the unpoled PMN-xPT near the morphotropic phase boundary.

[0097] The data depicted by FIG. 14 was obtained from a sample of a Bulk PMN-PT single crystal grown using a modified Bridgman growth method. The crystal was aligned along the major crystallographic orientations

[0001] c, [01 l]c, and [11 l]cusing Laue X-ray and X-ray diffractometry techniques. Sample discs having a thickness of 0.5 cm and a diameter of 5 cm were sliced from the bulk crystal using an inner diameter saw. The sample discs were polarized by first cleaning and coating portions of the sample discs with an approximately 50 nm thick layer of chromium followed by an approximately 200 nm thick layer of gold. Each layer was deposited using a sputtering process. The coated portions ofthe discs were then poled by submerging them in an oil bath having a high dielectric strength and exposing the samples to a 4 kV / cm DC electric field applied at room temperature.Thermal conductivity was measured along directions parallel and perpendicular to the poling axis using a static heater-sink method on two distinct samples.

[0098] The research disclosed herein may be used to establish a foundation for polarization caloritronics — a field exploring thermal fluctuations in ferroelectric materials — and paves the way for advanced thermal management systems, energy-efficient cooling, and electrocaloric heat engines. The influence of the poling direction on thermal conductivity can have a large effect on the performance of the thermal switch. Measurements on poled samples indicate that higher thermal conductivity can be achieved when the sample is poled along the (001) or (100) directions, with thermal conductivity measured either parallel to the poling direction or along the (110) or (010) direction. Previously, measurements were primarily conducted perpendicular to the poling direction. This finding provides new insights into the relationship between crystallographic orientations, electric fields, and thermal conductivity, without the occurrence of any phase transitions, and may be applied advantageously to produce thermal switches having improved performance.

[0099] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include both the singular and plural forms, and the terms “and” and “or” are each intended to include both alternative and conjunctive combinations, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” or “comprising,” when used in this specification, specify the presence of stated features, integers, actions, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, or groups thereof.Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, “comprised of’, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

[0100] While all the invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described.Accordingly, departures may be made from such details without departing from the spirit or scope of the Applicant’s general inventive concept.

Claims

CLAIMSWhat is claimed is:

1. A method of modulating a heat flow, comprising:applying a thermal gradient across a bulk relaxor ferroelectric material that is selfstanding, does not include a substrate, and has a thickness of at least five micrometers; and applying an electric field across the bulk relaxor ferroelectric material to modulate the heat flow through the bulk relaxor ferroelectric material.

2. The method of claim 1, further comprising:controlling a temperature of the bulk relaxor ferroelectric material such that the bulk relaxor ferroelectric material is maintained in a monoclinic phase.

3. The method of claim 1, further comprising:controlling the electric field applied to the bulk relaxor ferroelectric material such that the bulk relaxor ferroelectric material is maintained in a monoclinic phase.

4. The method of claim 1, further comprising:controlling both a temperature of the bulk relaxor ferroelectric material and the electric field applied to the bulk relaxor ferroelectric material such that the bulk relaxor ferroelectric material is maintained in a monoclinic phase.

5. The method of claim 1, wherein the electric field is between 0 and 3 kV / cm.

6. The method of claim 1, wherein the heat flow and the electric field each have a direction, and the direction of the heat flow and the direction of the electric field are independent of each other.

7. The method of any of claims 1-6, wherein the bulk relaxor ferroelectric material includes Pb[Mg1 / 3Nb2 / 3]O3-xPbTiO3.

8. The method of claim 7, wherein x is greater than or equal to 0.00 and less than or equal to 0.37.

9. The method of claim 7, wherein x is greater than or equal to 0.30 and less than or equal to 0.33.

10. The method of any of claims 1 -6, wherein the bulk relaxor ferroelectric material includes a doped PMN-PT material.

11. The method of any of claims 1 -6, wherein the bulk relaxor ferroelectric material is selected from the group consisting of Pb(Mg, Nb)Oa (PMN), Pb(Mg, Nb)O3-PbTiC>3 (PMN-PT), Pb(In, Nb)O3(PIN), Pb(In, Nb)O3-Pb(Mg, Nb)O3-PbTiO3(PIN-PMN-PT), Pb(Zn, Nb)O3(PZN), Pb(Zn, Nb)O3-PbTiO3(PZN-PT), (Pb, La)(Zr, Ti)O3(PLZ), (Pb, La)(Zr, Ti)O3-PbTiO3(PLZT), Pb(Sc, Nb)O3(PSN), Pb(Mg, Ta)O3(PMT), Pb(Fe, Nb)O3(PFN), Pb(Fe, W)O3(PFW), BaTiO3-Bi(Zn(Nb, Ta))O3 (BT-BZNT), BaTiO3-(Ba, Sr)TiO3(BT-BST), and a bismuth layer structured ferroelectric (BLSF).

12. The method of any of claims 1-6, wherein the bulk relaxor ferroelectric material is a single crystal of PMN-33PT oriented in the (001) direction.

13. The method of claim 12, wherein a temperature of the bulk relaxor ferroelectric material is maintained between 225 K and 325 K, and the bulk relaxor ferroelectric material is in a monoclinic C phase.

14. The method of claim 12, wherein a temperature of the bulk relaxor ferroelectric material is below 325 K, and the bulk relaxor ferroelectric material is in a monoclinic A phase.

15. The method of any of claims 1 or 4-6, wherein the bulk relaxor ferroelectric material is a single crystal of PMN-33PT oriented in the (001) direction, a temperature of the bulk relaxor ferroelectric material is above 325 K, and the bulk relaxor ferroelectric material is in a tetragonal phase.

16. The method of any of claims 1-6, wherein the bulk relaxor ferroelectric material is poled, and the thermal gradient is applied across the bulk relaxor ferroelectric material in a direction parallel to the poling.

17. A thermal switch for modulating heat flow, comprising:a bulk relaxor ferroelectric material having a plurality of faces and a thickness of at least five micrometers;a first electrode operatively coupled to a first face of the plurality of faces;a second electrode operatively coupled to a second face of the plurality of faces different from the first face;a first thermal coupler operatively coupled to one of the first face or a third face of the plurality of faces different from the first face and the second face; anda second thermal coupler operatively coupled to one of the second face or a fourth face of the plurality of faces different from the first face, the second face, and the third face, wherein:the first and second electrodes are configured to apply an electric field to at least a portion of the bulk relaxor ferroelectric material in response to a voltage being applied across the first and second electrodes,one of the first and second faces or the third and fourth faces defines a thermal path through the bulk relaxor ferroelectric material that includes at least a part of the portion of the bulk relaxor ferroelectric material across which the electric field is applied, and applying the electric field to at least the portion of the bulk relaxor ferroelectric material selectively alters a thermal conductivity of the thermal switch.

18. The thermal switch of claim 17, wherein:the bulk relaxor ferroelectric material comprises one of a plurality of layers of bulk relaxor ferroelectric material each having the first face and the second face,the first electrode includes a plurality of first conductive layers,the second electrode includes a plurality of second conductive layers,the first face of each layer of the plurality of layers of bulk relaxor ferroelectric material is operatively coupled to a respective first conductive layer,the second face of each layer of the plurality of layers of bulk relaxor ferroelectric material is operatively coupled to a respective second conductive layer, andthe plurality of layers of bulk relaxor ferroelectric material, the plurality of first conductive layers, and the plurality of second conductive layers define a stack of alternating layers of the bulk relaxor ferroelectric material and the first and second conductive layers.

19. The thermal switch of claim 17, wherein:the bulk relaxor ferroelectric material comprises one of a plurality of layers of bulk relaxor ferroelectric material each having the first face, the second face, the third face, and the fourth face,the first electrode includes a plurality of first conductive layers,the second electrode includes a plurality of second conductive layers,the third face of each layer of the plurality of layers of bulk relaxor ferroelectric material is operatively coupled to a respective first conductive layer,the fourth face of each layer of the plurality of layers of bulk relaxor ferroelectric material is operatively coupled to a respective second conductive layer, andthe plurality of layers of bulk relaxor ferroelectric material, the plurality of first conductive layers, and the plurality of second conductive layers define a stack of alternating layers of the bulk relaxor ferroelectric material and the first and second conductive layers.

20. The thermal switch of claim 17, wherein the first face of the bulk relaxor ferroelectric material is parallel to the second face of the bulk relaxor ferroelectric material.

21. The thermal switch of claim 17, wherein the third face of the bulk relaxor ferroelectric material is parallel to the fourth face of the bulk relaxor ferroelectric material, and the third face of the bulk relaxor ferroelectric material is perpendicular to the first face of the bulk relaxor ferroelectric material.

22. The thermal switch of claim 17, wherein at least a portion of the first thermal coupler is operatively coupled to the first face of the bulk relaxor ferroelectric material through at least a portion of the first electrode, and at least a portion of the second thermal coupler is operatively coupled to the second face of the bulk relaxor ferroelectric material through at least a portion of the second electrode.

23. The thermal switch of claim 17, wherein the heat flow and the electric field each have a direction, and the direction of the heat flow and the direction of the electric field are independent of each other.

24. The thermal switch of any of claims 17-23, wherein the bulk relaxor ferroelectric material includes Pb[Mgi / 3Nb2 / 3]O3-xPbTiO3.

25. The thermal switch of claim 24, wherein x is greater than or equal to 0.00 and less than or equal to 0.37.

26. The thermal switch of claim 24, wherein x is greater than or equal to 0.30 and less than or equal to 0.33.

27. The thermal switch of any of claims 17-23, wherein a temperature of the bulk relaxor ferroelectric material is such that the bulk relaxor ferroelectric material is in a monoclinic phase.

28. The thermal switch of any of claims 17-23, wherein the electric field applied to the bulk relaxor ferroelectric material is such that the bulk relaxor ferroelectric material is in a monoclinic phase.

29. The thermal switch of any of claims 17-23, wherein both a temperature of the bulk relaxor ferroelectric material and the electric field applied to the bulk relaxor ferroelectric material are such that the bulk relaxor ferroelectric material is maintained in a monoclinic phase.

30. The thermal switch of any of claims 17-23, wherein the electric field is between 0 and 3 kV / cm.

31. The thermal switch of any of claims 17-23, wherein the bulk relaxor ferroelectric material includes a doped PMN-PT material.

32. The thermal switch of any of claims 17-23, wherein the bulk relaxor ferroelectric material is selected from the group consisting of Pb(Mg, Nb)O3, (Pb, La)(Zr, Ti)O3, Pb(Sc, Nb)O3, BaTiO3-Bi(Zn(Nb, Ta))O3, BaTiO3-(Ba, Sr)TiO3, and PIN-PMN-PT.

33. The thermal switch of any of claims 17-23, wherein the bulk relaxor ferroelectric material is a single crystal of PMN-33PT oriented in the (001) direction.

34. The thermal switch of claim 33, wherein a temperature of the bulk relaxor ferroelectric material is between 225 K and 325 K, and the bulk relaxor ferroelectric material is in a monoclinic C phase.

35. The thermal switch of claim 33, wherein a temperature of the bulk relaxor ferroelectric material is below 325 K, and the bulk relaxor ferroelectric material is in a monoclinic A phase.

36. The thermal switch of claim 33, wherein a temperature of the bulk relaxor ferroelectric material is above 325 K, and the bulk relaxor ferroelectric material is in a tetragonal phase.

37. The thermal switch of any of claims 17-23, wherein the bulk relaxor ferroelectric material is poled, and the thermal path is parallel to the poling.

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