EP4643385A1 - Thermal transfer article - Google Patents

Abstract

A thermal transfer article having a first major surface, a second major surface opposite the first major surface, and a thickness (x) therebetween. The article comprises a matrix material extending to the first major surface and to the second major surface and a corrugated layer at least partially embedded within the matrix material. The corrugated layer exhibits anisotropic thermal conductivity. The corrugated layer is formed of alternating peaks and valleys and has a thickness (t) and a pitch (p). The article thickness (x), corrugated layer thickness (t), and corrugated layer pitch (p) are designed to achieve high effective thermal conductivity of the article from the first major surface to the second major surface.

Description

THERMAL TRANSFER ARTICLE Background [0001] Thermal management of electrical devices (e.g., semiconductor devices, batteries, or motors) typically requires the effective transport of heat from a source (e.g., integrated circuit chip, windings, or electrochemical cell) to a sink (e.g., cold plate, heat exchanger, or finned heatsink). Thermal interface materials (TIMs), such as greases, adhesives and/or pads, are often placed between the source and sink to facilitate heat transport. More recently, TIMs include composite materials having polymeric binder materials filled with inorganics (e.g., ceramic particles or carbon fibers) that boost the thermal conductivity relative to that of the binder materials. In the design of such composites, it is often necessary to balance effective thermal conductivity against other desirable features, such as mechanical compliance, processability, and/or electrical insulation resistance. Summary [0002] Thermal transfer articles (alternatively called heat transfer articles) comprising a corrugated material and a polymeric matrix material have been offered as an alternative to composite TIMs with binders having inorganic particles or fibers dispersed therein. The motivation behind the corrugated construction is the ability to leverage the high in-plane thermal conductivity of the corrugated material across the thickness of the component while leveraging the mechanical properties of the matrix, such as conformability and compressibility. Examples of such thermal transfer articles can be found, for example, in WO 2018/164671 and U.S. Patent No.4,839,227. [0003] However, these prior disclosures fail to recognize the impact that specific levels of directional thermal conductivity within the corrugated material, along with the matrix material thermal conductivity and article geometric design, have on the effective thermal conductivity of the thermal transfer article, and can lead one to unfavorable design selections. In contrast, the present disclosure uncovers complex and unexpected dependencies of performance on component design (including practical material property tradeoffs), unlocking high thermal performance. While heat transfer performance of such articles comprising a corrugated layer and matrix material can be envisioned to increase monotonically by selecting a corrugated layer of ever-increasing thermal conductivity and ever- increasing thickness, doing so is not practicable due to inherent limitations of materials used in the construction of thermal transfer articles. The present disclosure provides a thermal transport article that effectively balances the in-plane thermal conductivity and through-plane thermal conductivity of the corrugated layer within realistic material constraints to enhance the effective thermal conductivity of the article while providing desirable mechanical and/or electrical properties of the article (e.g., low electrical conductivity or low dielectric loss). [0004] In one embodiment, the present disclosure provides a thermal transfer article having a first major surface, a second major surface opposite the first major surface, and a thickness (x) therebetween, the article comprising: a matrix material extending to the first major surface and to the second major surface; and a corrugated layer at least partially embedded within the matrix material, the corrugated layer forming alternating peaks and valleys and having a thickness (t) and a pitch (p), wherein x is from 0.2 mm to 5 mm, wherein t is from 0.005x to 0.2x, wherein p is: 2t to x + 30t when 0.005x < t < 0.0358x, -0.167 + 6.67t to x + 30t when 0.0358x < t < 0.05x, and -0.167 + 6.67t to 2.5x when 0.05x < t < 0.2x, and wherein the corrugated layer exhibits anisotropic thermal conductivity. [0005] In another embodiment, the present disclosure provides an assembly comprising the thermal transfer article. [0006] As used herein: [0007] The term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Such terms will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of.” Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present. By “consisting essentially of” is meant including any elements listed after the phrase, and limited to other elements that do not interfere with or contribute to the activity or action specified in the disclosure for the listed elements. Thus, the phrase “consisting essentially of” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present depending upon whether or not they materially affect the activity or action of the listed elements. [0008] The terms “a,” “an,” and “the” are used interchangeably with “at least one” to mean one or more of the components being described. [0009] The term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements. [0010] The term “some embodiments” means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of such phrases in various places throughout this specification are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiments. [0011] The terms “preferred” and “preferably” refer to embodiments of the disclosure that may afford certain benefits, under certain circumstances; however, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the disclosure. [0012] The recitations of numerical ranges by endpoints include all numbers subsumed within that range as well as the endpoints (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). The phrase “up to” a number (e.g., up to 50) includes the number (e.g., 50). [0013] The words “top” and “bottom” are relative terms that are not meant to apply a particular orientation in space. [0014] The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The description that follows more particularly exemplifies illustrative embodiments. Brief Description of Drawings [0015] Fig.1A is a schematic perspective view of one embodiment of a thermal transfer article in the present disclosure; [0016] Fig 1B is a schematic cross-sectional view of the thermal transfer article in FIG.1A; [0017] Fig.2 is a schematic cross-section view of an assembly comprising the thermal transfer article in Fig.1; [0018] Fig.3A is a schematic cross-sectional view of an exemplary model thermal transfer article in the present disclosure; [0019] Fig.3B is a schematic cross-sectional view of an exemplary model assembly comprising a two-dimensional model element from the model thermal transfer article in FIG.3A; [0020] Fig.3C is a schematic cross-sectional view of the two-dimensional model element in Fig. 3B; [0021] Fig.4 is a schematic cross-sectional view of another two-dimensional model element; [0022] Fig.5 is a plot of the effective thermal conductivity, k_eff, of the design model thermal transfer articles 25-28 versus the ratio of the through-plane thermal conductivity, k_z, to the in-plane thermal conductivity, k_xy, for the model corrugated layer; [0023] Fig.6 is a plot of the effective thermal conductivity, k_eff, of the design model thermal transfer articles 165-168 versus the ratio of the through-plane thermal conductivity, k_z, to the in-plane thermal conductivity, k_xy, for the model corrugated layer; and [0024] Fig.7 shows the relationship between thermal transfer article design parameters and the maximum modeled effective thermal conductivity, k_eff., and percentage factors by which modeled k_eff was improved by appropriately tailoring k_xy and k_z of the corrugated layer (vs. using a corrugated layer with k_xy = k_z). [0025] With reference to the figures, like reference numbers offset by multiples of 100 (e.g., 114, 314 and 414) indicate like elements. Unless otherwise indicated, all figures and drawings in this document are not to scale and are chosen for the purpose of illustrating different embodiments of the invention. In particular, the dimensions of the various components are depicted in illustrative terms only, and no relationship between the dimensions of the various components should be inferred from the drawings, unless so indicated. Detailed Description [0026] In the following description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. [0027] The thermal transfer article 100 of the present disclosure generally comprises a first major surface 110, a second major surface 112 opposite the first major surface 110, and a thickness (x) therebetween, as illustrated in Figs.1A and 1B. A matrix material 120 extends to the first major surface 110 and to the second major surface 112 of the article 100. A corrugated layer 114 is at least partially embedded in the matrix material 120. The corrugated layer 114 forms alternating peaks 116 and valleys 118 and has a thickness (t) and a pitch (p). The dimensions of the thermal transfer article 100 are such that: x is from 0.2 mm to 5 mm; t is from 0.005x to 0.2x; and p is 2t to x + 30t when 0.005x < t < 0.0358x, -0.167 + 6.67t to x + 30t when 0.0358x < t < 0.05x, and -0.167 + 6.67t to 2.5x when 0.05x < t < 0.2x. The corrugated layer exhibits anisotropic thermal conductivity. Each of the components will be discussed in greater detail below. [0028] Corrugated Layer [0029] The corrugated layers of the present disclosure have a pleated profile where the corrugated layers weave directionally from one side of the article to the other. The pleats can be sharp (e.g., triangular profile), curved (e.g., sinusoidal), or combinations thereof. In some embodiments, the pleats may extend across the entire length and/or width of the corrugated layer. In other embodiments, the pleats may extend across only a portion of the length and/or width of the corrugated layer (e.g., corrugated regions interspersed with flat, non-corrugated regions). In preferred embodiments, as illustrated in Fig.1A, the pleats extend across the entire corrugated layer. [0030] As illustrated in Fig.1B, the corrugated layer 114 has alternating peaks 116 and valleys 118. The peaks 116 extend towards the first major surface 110 of the article 100 and the valleys 118 extend towards the second major surface 112 of the article 100, thus providing a pathway for thermal conductance in the corrugated layer material and through the thickness of thermal transfer article. [0031] The corrugated layers of the present disclosure exhibit anisotropic thermal conductivity. Anisotropic thermal conductivity means that the in-plane thermal conductivity of the corrugated layer is different than the through-plane conductivity. With reference to Fig.1B, the thermal conductivity of the corrugated layer is divided into two components: a through-plane thermal conductivity (k_z); and an in- plane thermal conductivity (k_xy). Anisotropic thermal conductivity occurs when the value of k_z is different from the value of k_xy. By contrast, isotropic thermal conductivity occurs when the values for k_z and k_xy are equal. [0032] Thermal transfer articles comprising corrugated layers have traditionally focused on maximizing the in-plane thermal conductivity of the corrugated layer. However, it has been found in the present disclosure that the through-plane thermal conductivity is no less important. Optimal heat transfer follows from an appropriate balance between the in-plane and through-plane thermal conductivities of the corrugated layer. As provided in greater detail in the Example section herein, it was found that effective thermal conductivity of an article could be improved if the ratio of through-plane and in-plane thermal conductivities were constructed to fall within a certain range. [0033] The corrugated layers disclosed herein exhibit a through-plane thermal conductivity (k_z) and an in-plane thermal conductivity (k_xy), wherein the ratio of k_z to k_xy is from 0.02 to 0.75, from 0.03 to 0.50, or from 0.04 to 0.40. [0034] In some embodiments, the value of k_xy is at least 20, 40, 60, 80, or 100 W/mK. [0035] Materials that are well-suited to form the corrugated layer include composites comprising crystallites that impart high thermal conductivity to the layer. Whether through processing effects or because the crystallites are anisotropic in their own thermal conductivity or both, the magnitude of anisotropy, as reflected in the different in in-plane and through plane thermal conductivities, can be varied. [0036] In some embodiments, the corrugated layer is a composite film comprising hexagonal boron nitride (h-BN), a transition metal dichalcogenide, carbon fibers, metal fibers, graphite, or combinations thereof. The h-BN may include boron nitride nanosheets (BNNS). Exemplary transition metal dichalcogenides include MoS₂, WS₂, MoSe₂, WSe₂, MoTe₂. Exemplary metal fibers include nanowires. Suitable forms of graphite include pyrolytic graphite and graphite foils. In some preferred embodiments, the corrugated layer is a composite film comprising h-BN. [0037] The corrugated layer may further comprise a binder. In some embodiments, the binder is a thermoplastic polymer. Exemplary thermoplastic polymers include polyurethane, polyester (e.g., polyethylene terephthalate, polybutylene terephthalate, and polylactic acid), polyamide (e.g., nylon 6, nylon 6,6, nylon 12 and polypeptide), polyether (e.g., polyethylene oxide and polypropylene oxide), polycarbonate (e.g., bisphenol-A-polycarbonate), polyimide, polysulphone, polyethersulphone, polyphenylene oxide, polyacrylate (e.g., thermoplastic polymers formed from the addition polymerization of monomer(s) containing an acrylate functional group), polymethacrylate (e.g., thermoplastic polymers formed from the addition polymerization of monomer(s) containing a methacrylate functional group), polyolefin (e.g., polyethylene and polypropylene), styrene and styrene- based, random and block copolymer, chlorinated polymer (e.g., polyvinyl chloride), fluorinated polymer (e.g., polyvinylidene fluoride; copolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride; copolymers of ethylene, tetrafluoroethylene; hexafluoropropylene; and polytetrafluoroethylene), and copolymers of ethylene and chlorotrifluoroethylene. In some embodiments, thermoplastic polymers include homopolymers or copolymers (e.g., block copolymers or random copolymers). In some embodiments, thermoplastic polymers include a mixture of at least two thermoplastic polymer types (e.g., a mixture of polyethylene and polypropylene or a mixture of polyethylene and polyacrylate). In some embodiments, the polymer may be at least one of polyethylene (e.g., ultra-high molecular weight polyethylene), polypropylene (e.g., ultra-high molecular weight polypropylene), polylactic acid, poly(ethylene-co-chlorotrifluoroethylene) and polyvinylidene fluoride. In some embodiments, the thermoplastic polymer is a single thermoplastic polymer (i.e., it is not a mixture of at least two thermoplastic polymer types). In some embodiments, the thermoplastic polymers consist essentially of, or consist of, polyethylene (e.g., ultra-high molecular weight polyethylene). [0038] In some embodiments, the binder is a crosslinked polymer, for example a thermoset. Examples of suitable crosslinked polymers include appropriately formulated and processed poly(meth)acrylates, epoxies, polyurethanes, polydicyclopentadienes, polyureas, melamine, benzoxazines, polyimides, silicones, and vinyl esters. [0039] In some preferred embodiments, the corrugated layer is electrically insulating. In electrical assemblies such as electronic packages (e.g., circuit boards with surface mounted integrated circuit carriers in thermal contact with metal heat sinks) or battery packs (e.g., electric vehicle battery packs containing multiple electrochemical cells each housed in a metal housing or can), effective heat removal must be achieved while preserving electrical isolation. For example, in the case of a battery pack, it would defeat the independent charging and discharging of each electrochemical cell if the metal housings of the cells (each usually being an energized cell terminal in its own right) were electrically connected to each other through the TIM or thermal transport article. In the case of electronic packages, for example those supporting transistors supplying switched power to the motor of an electric vehicle, heat needs to be dissipated from electrically conducting components of the power semiconductor device without short- circuiting those components with each other or to other components of the vehicle. TIMs or thermal transfer articles that transport heat effectively from those electrically conducting electronic components to cooling plates, which are typically metallic (electrically conductive), without short-circuiting are required. Thus, in order to be used in these important commercial applications, the thermal transfer articles, and their respective material components, are preferably electrically insulating. Accordingly, in some preferred embodiments, the corrugated layer is preferably comprised of h-BN particles, preferably in the form of platelet or flake particles with their thin dimension (crystallographic c-axis) disproportionately aligned (i.e., not randomly oriented) parallel to the thickness direction of the corrugated layer material (i.e., parallel to the direction of the k_z component of thermal conductivity). [0040] Corrugated layers can be made from the corrugated material using a number of well-known techniques, including those disclosed in U.S. Patent Nos. US 3,789,299; US 3,682,736; and US 8,075,832. [0041] Matrix Material [0042] The corrugated layer is at least partially embedded within matrix material. In a preferred embodiment, as illustrated in Figs.1A and 1B, the peaks 116 of the corrugated layer 114 are flush with the upper surface of the matrix material 120 that extends to the first major surface 110 (i.e., the peaks and matrix material together form the first major surface 110 of the article 100), and the valleys 118 of the corrugated layer 114 are flush with the bottom surface of the matrix material 120 that extends to the second major surface 112 (i.e., the valleys and matrix material together forming the second major surface 112 of the article 100). This configuration provides for optimal effective thermal transfer through the article, while simultaneously taking advantage of the conformable matrix material to provide for good heat transfer interfaces (high interface thermal conductance) between the thermal transfer article and each of a heat source and a heat sink. [0043] In other embodiments, the matrix material can overcoat the peaks and/or valleys of the corrugated material. In this instance, the matrix overcoat would form the first and/or second major surface of the thermal transfer article. Since the corrugated layer is a conduit for thermal conduction, enabling higher transport performance than the matrix material would offer on its own, the greater the thickness of the overcoat, the lower the effective thermal conductivity of the thermal transfer article. Increasing the thermal conductivity of the matrix material can help offset some of the loss of thermal transfer article performance (e.g., as expressed by its effective thermal conductivity k_eff) attributed to the overcoat. However, such increase in conductivity is typically associated with an increase in conductive particles and/or fibers dispersed within the matrix material, which can detrimentally impact the mechanical properties of the matrix material, especially compliance and conformability as are necessary for making good thermal contact with a heat source or heat sink. Therefore, any offset is practically limited. [0044] In some embodiments, the matrix material forms an overcoat on the peaks of the corrugated layer, the valleys of the corrugated layer, or a combination thereof. In some embodiments, the overcoat extends up to 100, 50, 40, 30, 20, 10 micrometers above the peaks of the corrugated layer. In the same or alternative embodiments, the overcoat extends up to 100, 50, 40, 30, 20, 10 micrometers below the valleys of the corrugated layer. [0045] In some less preferred embodiments, the peaks and/or valleys protrude from the top and/or bottom surfaces of the matrix material. In this instance, the peaks and/or valleys would define the portions of the first and/or second surface of the thermal transfer material that are contactable by a flat heat transfer surface of a heat source or a flat heat transfer surface of a heat sink. This would allow direct contact between the corrugated layer and a heat source and heat sink, but may reduce the interface thermal conductance between the thermal transfer article and heat source and/or heat sink, thus reducing overall thermal conductance between heat source and heat sink. [0046] The thermal conductivity of the matrix material is preferred to be high, within the constraints of tradeoffs in processability and mechanical properties (e.g., compliance and conformability). The matrix material typically has a thermal conductivity (k_matrix) from 0.1 to 10 W/mK, 0.2 to 9 W/mK, 0.5 to 7.5 W/mK, or 1.0 to 5 W/mK. [0047] Suitable matrix materials for the thermal transfer articles provided herein include silicones, poly(meth)acrylates, polyurethanes, polyolefins (including cyclo-olefins), acrylonitrile-butadiene rubbers, hydrogenated acrylonitrile-butadiene rubbers, ethylene propylene diene rubbers, fluorinated elastomers, chloroprene rubbers, styrene-butadiene rubbers, epoxies, polyurethanes, polydicyclopentadienes, polyureas, melamine, benzoxazines, polyimides, and vinyl esters, natural rubbers, or combinations thereof. [0048] Suitable matrix materials also include composites comprising a polymer binder and thermally conductive particles dispersed therein. Any of the aforementioned polymers may be used as the binder. Thermally conductive particles should be included in high enough quantities (e.g., volume percent) that they substantially boost the thermal conductivity of the composite binder into the ranges of k_matrix disclosed above, but not so high as to render the composite mechanically noncompliant and to process due to high viscosity before solidification (e.g., curing). In some embodiments, the thermally conductive particles include metals, semiconductors, and ceramics. Suitable thermally conductive particles include aluminum, copper, silver, graphite, diamond, SiC, Si₃N₄, A1N, BeO, MgO, Al₂O₃, aluminum trihydroxide, aluminum oxyhydroxide, hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), ZnO, natural aluminosilicate, synthetic aluminosilicate, or combinations thereof. Preferably, the thermally conductive particles are not electrically conductive (i.e., are electrically insulating). Suitable electrically insulating thermally conductive particles include diamond, Si₃N₄, A1N, BeO, MgO, Al₂O₃, aluminum trihydroxide, aluminum oxyhydroxide, hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), natural aluminosilicate, synthetic aluminosilicate, or combinations thereof. [0049] Exemplary sizes of the thermally conductive particles range from 100s of nanometers to 100s of micrometers in size. Exemplary shapes of the thermally conductive particles include irregular, platy, acicular, spherical shapes, and as well as agglomerated forms. Agglomerates can range in size, for example, from a few micrometers up to, and including, a few millimeters. The particles can be mixed to have multimodal size distributions which may, for example, allow for optimal packing density. [0050] In some embodiments, the thermally conductive particles have an average particle size (average length of longest dimension) in a range from 100 nm to 2 mm (in some embodiments, in a range from 150 nm to 200 micrometers, in some embodiments from 200 nm to 100 micrometers, or in some embodiments from 250 nm to 50 micrometers). [0051] In some embodiments, the thermally conductive particles have bimodal or trimodal particle size distribution. Multimodal size distributions of particles can allow for higher packing density, improved particle-to-particle contact and thereby improved thermal conductivity. In some embodiments, the volume percent (vol-%) loading of thermally conductive particles in the polymer binder ranges from 10 to 80 vol-%, in other embodiments from 15 to 60 vol-%, and in yet other embodiments from 20 to 50 vol-%. [0052] In some embodiments, the matrix material comprises an electrically insulating polymer binder and a plurality of electrically insulating particles (e.g., ceramic particles comprising hexagonal boron nitride, aluminum trihydroxide, silica carbide, and metal oxides (e.g., aluminum oxide and iron oxide)). [0053] In other embodiments, the matrix material comprises a polymer binder (either conductive or insulating) and a plurality of electrically conductive particles (e.g., metal particles comprising aluminum, copper, nickel, and gold). [0054] Article [0055] The thermal transfer articles of the present disclosure can be made by at least partially embedding the corrugated layer in the matrix material. [0056] The thermal transfer articles of the present disclosure with a corrugated layer having appropriately selected anisotropic thermal conductivity (i.e., k_xy does not equal k_z) typically have a through-plane effective thermal conductivity (k_eff) that is at least 10%, 20%, 30% or 40% greater than would be obtained for a similar article that differs only in that the corrugated layer has isotropic thermal conductivity (i.e., k_z = k_xy) equal to the average of k_xy and k_z for the anisotropic cases. [0057] The thermal transfer articles typically have a through-plane effective thermal conductivity (k_eff) of at least 5, 10, 15, or 20 W/mK. [0058] The through-plane effective thermal conductivity (k_eff) of an article of the present disclosure can be measured according to ASTM D5470-17 “The Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials”. The Test Method measures the thermal impedance of a material and calculates the thermal conductivity therefrom. The method is described as useful for measuring the thermal impedance of viscous liquids, viscoelastic solids, and elastic solids. The method is based on an apparatus comprising, in series, a heat source, a hot meter bar, the specimen being tested, a cold meter bar, and a heat sink. The heat source is based on, for example, wire wound cartridge heaters. Each meter bar is constructed from a high thermal conductivity material (e.g., > 50 W/mK) integrated with a series of temperature sensors (e.g., thermocouples) that can measure the temperature gradient along the meter bar. The temperature on each side of the specimen during the measurement is determined using temperature readings for sensors within each meter bar. The heat sink is reported to be commonly a metal block cooled by temperature-controlled circulating fluid. Once a steady state temperature gradient is established within the apparatus, a thermal impedance can be determined using the heat flow through the specimen, the temperature gradient across the sample, and its cross-sectional area, as fully described in the test method. An apparent thermal conductivity of the sample (through its thickness in the heat flow direction) can then be calculated by dividing the thickness of the sample by the calculated thermal impedance. Alternatively, as described in the standard, the apparent conductivity can be obtained from a plot of thermal impedance for single and multiple layered specimens against the respective specimen thickness. The so plotted curve is a straight line whose slope is the reciprocal of the apparent thermal conductivity. [0059] In preferred embodiments, the thermal transfer article is electrically insulating (e.g., with an effective electrical resistivity of at least 10⁴ ohm-meter, preferably at least 10⁷ ohm-meter, more preferably at least 10¹⁰ ohm-meter). [0060] The thermal transfer articles of the present disclosure are typically conformable and flexible, thus providing for a good thermal interface (i.e., high interface thermal conductance) with heat sources and/or heat sinks, thus reducing unwanted conductance loss at the article/heat sink and article/heat source interfaces. For good conformability of its surface to the non-planarities of a heat source or heat sink, the thermal transfer article preferably has a Shore A hardness of less than 80, more preferably less than 70, even more preferably less than 55, yet more preferably less than 40. [0061] Assembly [0062] The thermal transfer articles of the present disclosure can be assembled with other components to provide thermal transfer from one part of the assembly to another. Generally, the assembly comprises the thermal transfer article interposed between a first heat transfer surface (e.g., heat transfer surface of an electronic component or electrochemical cell) and a second heat transfer surface (e.g., heat transfer surface of a heat sink or cooling plate). [0063] One embodiment of such an assembly 205 is illustrated in Fig.2. The assembly 205 comprises the thermal transfer article 100 in Fig.1 in combination with a heat source 226 and heat sink 228. Exemplary heat sources may include integrated circuit chips or chip packages; motor windings, cores or housings; or electrochemical cells. Exemplary heat sinks may include cooling plates, heat exchangers, or finned heat sink components. The orientation of the assembly 205 is illustrative only. In other embodiments, the position of the heat source and heat sink may be reversed. The heat source 226 has a first heat transfer surface 230 adjacent the first major surface 110 of the article 100. The heat sink 228 has a second heat transfer surface 232 adjacent the second major surface 112 of the article 100. It is within the scope of the assemblies of this disclosure that the first heat transfer surface 230 or the second heat transfer surface 232 is defined by a coating or film applied to heat source or heat sink. The thermal transfer article 100 can be in direct contact with first heat transfer surface 230 and second heat transfer surface 232. Alternatively, thermally conductive material (e.g., grease, paste, or adhesive) can be placed between the thermal transfer article 100 and one or both of the heat transfer surfaces 230, 232. In a preferred embodiments, the thermal transfer article is in direct contact with each of the first and second heat transfer surfaces. [0064] The thermal transfer articles of the present disclosure find use in, for example, passenger and freight vehicles (e.g., battery packs, power electronics, and electric machines), consumer electronics (e.g., personal computers, gaming consoles, and mobile telephones and smartphones), telecommunications infrastructure (e.g., wireless base stations and routers), and durable equipment (e.g., heating, ventilation, and air conditioning (HVAC), appliances, and lift equipment). Examples [0065] Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. [0066] The following abbreviations are use in this section: m = meter; K = Kelvin; W = watt. [0067] Modeling Methodology [0068] Various designs of the thermal transfer article (100) in Figs.1A, 1B, and 2 were computationally modelled using the simulation package COMSOL MULTIPHYSICS, from COMSOL, Inc. in Burlington, Massachusetts (herein also referred to as “COMSOL”). The effective thermal conductivity across the thickness of the article (i.e., the z direction) was derived from the simulation and is expressed herein as an effective thermal conductivity value, k_eff, in units of W/mK. The k_eff of the thermal transfer article is the value of thermal conductivity for a uniformly thermally conductive homogenous material with equivalent thermal transport performance to the modeled thermal transfer article with its corrugated layer. The higher the value of k_eff, the higher the thermal conductivity of the thermal transfer article. [0069] A cross-section of a model thermal transfer article 300 is illustrated in Fig.3A. The thermal transfer article comprises a corrugated layer 314 embedded in a matrix material 320. The thermal transfer article is bordered on the top and bottom by the first and second major surfaces of the article 310, 312, respectively, and has a thickness (x’) therebetween. For purposes of the simulation, the design was simplified to a two-dimensional representative element 322 extending from one peak 316 in the corrugated layer to the adjacent valley 318 (i.e., ½ period of the corrugated layer). Based on reflection symmetry and translational symmetry, modeling the representative element 322 is sufficient for estimating the properties across the entire cross section of Fig.3A, as well as across the three- dimensional thermal transfer article 300. [0070] Representative elements 322 having the design geometries set forth in Table 1 below were generated within the simulation package and placed between two modeled aluminum blocks 330, 332 of thickness 1mm in the thickness direction z’ of the thermal transfer article to simulate a heat source and a heat sink applied to the major surfaces 310, 312 of the thermal transfer article 300, forming an overall model assembly 305, as illustrated in Fig.3B. The overall model assembly was meshed automatically into area elements using the “user controlled mesh,” “predefined,” and “extremely fine,” and “calibrated for general physics” in the simulation package. In order to calculate k_eff for the article design, a 100K temperature gradient was first imposed on the overall model assembly 305 in the z’ direction as a fixed temperature of 100K at the modeled top surface 338 and a fixed temperature of 200K at the modeled bottom surface 340. A thermal transport simulation was carried out for each design, driven by this temperature gradient. An average heat flux through the modeled top surface 338 of the assembly 305 was determined within COMSOL by solving Fourier’s Law across the geometry of the overall assembly (i.e., finite element analysis). An overall thermal resistance of the assembly 305 along the z’ direction was calculated based on the temperature difference and the heat flux. An overall area specific thermal resistance (m²K/W) of the assembly 305 along the z’ direction was calculated based on the temperature difference and the heat flux (i.e., equal to the temperature (K) difference divided by the heat flux (W/m²)). The theoretical area specific thermal resistance (0.0000084 m²K/W) of each of the modeled aluminum blocks 330, 332 was subtracted from the overall area specific thermal resistance to yield an effective area specific thermal resistance for the representative element 322 of the modeled thermal transfer article 300. Finally, based on the overall dimensions of the representative element 322 of the thermal transfer article 300, its effective area specific thermal resistance was converted to a value of k_eff (W/mK) in the z’ direction (i.e., equal to the overall thermal transfer article thickness (x’) divided by the thermal transfer article effective area specific resistance (m²K/W)). [0071] For the various designs of modeled thermal transfer article, the corrugated layer 314 and the matrix material 320 were assigned various values of thermal conductivity. The matrix material 320 was assumed in all cases to have a thermal conductivity that is isotropic, describable by a single thermal conductivity value (k_matrix). The corrugated layer 314 was assumed to have independently assignable values of i) thermal conductivity, k_xy, in the direction parallel to its major surfaces 342, 344 and ii) thermal conductivity, k_z, in the direction perpendicular to its major surfaces 342, 344. Corrugated layer materials with different values of k_xy and k_z are described herein as having anisotropic thermal conductivity (also referred to herein as directional thermal conductivity). Corrugated layer materials with equal values of k_xy and k_z are described herein as having isotropic thermal conductivity. k_xy and k_z are described herein as the in-plane and through-plane components of thermal conductivity of the corrugated layer material, respectively. [0072] The assignment of in-plane and through-plane component values of thermal conductivity (k_xy and k_z, respectively) to area elements within the corrugated layer 314 included orientating the components so that k_xy was parallel to its major surfaces 342, 344 and k_z was perpendicular to its major surfaces 342, 344 throughout the entire area of the corrugated layer 314. The latter was achieved using the “curvilinear coordinate system” feature of the simulation package and its embedded “diffusion method” option. That is, the in-plane thermal conductivity (k_xy) of the corrugated layer 314 followed the contour of the corrugation layer, and the through-plane thermal conductivity (k_z) followed the normal to the corrugation layer. [0073] A uniform thickness of the corrugated layer 314 in the thermal transfer article models was achieved as follows. A sinusoidal curve serves as the centerline 346 of the corrugated layer 314, as illustrated in Fig.3C. The centerline 346 has amplitude A. The modeled surfaces 342, 344 of the corrugated layer 314 were defined by their distance d from the centerline 346, also referred to herein as the corrugated layer half-thickness. The centerline 346 and corrugated layer 314 have a pitch p’. Parametric equations (EQUATIONS 1 and 2) were used to calculate coordinates (x_c, y_c) of the centerline 346 of the corrugated layer 314. Parametric equations (EQUATIONS 3 and 4) were used to calculate coordinates (x_u, y_u) of the upper modeled surface 342 of the corrugated layer 314. Parametric equations (EQUATIONS 5 and 6) were used to calculate coordinates (x_l, y_l) of the lower modeled surface 344 of the corrugated layer 314. For the parametric equations, the parameter ^^ runs from 0 to 1 for period of the sinusoidal curve. The inequality expressed as EQUATION 7 describes the design space that will avoid any kinks in the inside surface of bends in the corrugated layer, although some designs reported herein do include such a kink at the inside surface of bends. ^^_^ ൌ ^^ ^^ (EQUATION 1) ^^_^ ൌ ^^ sin^2 ^^ ^^^ (EQUATION 2) ^^ ^{ଶగ ௗ ^ ୡ୭^^ଶగ ^^} ^^ ^^ ^ ^{ଶగ ௗ ^ ୡ୭^^ଶగ ^^} 4 ^^^ଶ ^^ ^^ ^ ^^^ଶ (EQUATION 7) [0074] The corrugated layer 314 has a “peak-to-valley” height dimension in the z’ direction equal to two times (A + d). In some examples described herein, the peaks 316 and valleys 318 extend to the first major surface 310 and second major surface 312 of the article, respectively. In other words, (2A + 2d) is equal to the thickness x’ of the article. In other examples, as illustrated in Fig.4, the matrix material 420 extends beyond the peaks and/or valleys to form an overcoat. In other words, (2A’ + 2d’) < x’’. An article can have an overcoat thickness above the peaks for the corrugated layer 448, and overcoat thickness below the valleys of the corrugated layer 450, or a combination of both. The first and second matrix overcoat thicknesses 448, 450 can be the same or different. [0075] Designs of Thermal Transfer Articles and Materials [0076] Four series of example thermal transfer article designs were explored by thermal simulation of the modeled thermal transfer articles as described above, focusing on calculation of the effective thermal conductivity, k_eff, through the thickness of the articles. [0077] Design Models 1-168 [0078] Thermal simulation and calculation of effective thermal conductivity, k_eff, were carried out for a first series of modeled thermal transfer articles. The first series of thermal transfer article designs span a range of values of each of matrix thermal conductivity (k_matrix), corrugated layer thickness (t’, t’’), and corrugated layer pitch (p’, p’’). Specific values of these design parameters are given in Table 1. For each of the modeled thermal transfer articles in Table 1, the overall component thickness was 1 millimeter. [0079] For Design Models 17-40, the corrugated peaks and valley extend to the first and second major surface of the article, respectively, as illustrated in Fig.3C. In the remaining Design Models, the matrix material overcoats both the peaks and valleys of the corrugated layer, as illustrated in Fig.4. This configuration mitigates meshing and solving challenges within the simulation package. For Design Models 1-16, 41-60, 69-88 and 93-112, the matrix material overcoats each of the peaks and valleys of the corrugated material by 0.1 micrometer. For Design Models 61-68, 89-92 and 113-168, the matrix material overcoats each of the peaks and valleys of the corrugated material by 0.5 micrometers. [0080] For each modeled thermal transfer article design parameter combination in Models 1-168, a series of combinations of corrugated layer material in-plane thermal conductivity (k_xy) and corrugated layer through-plane thermal conductivity (k_z) were modeled. Table 2 includes the 91 specific combinations that were used, along with a unique value of k_z/k_xy that was calculated for each combination. k_z/k_xy is an expression of the thermal conductivity anisotropy ratio of the modeled corrugated layer material. The series of combinations of k_xy and k_z in Table 2 is a model of the tradeoff that accompanies the use of anisotropic thermally conductive particle based corrugated layer materials such as hexagonal boron nitride (h-BN) filled layer materials or graphite filled layer materials. The term tradeoff as used here, together with the values of k_z and k_xy in Table 2, capture the understanding that as one loads a material layer (e.g., corrugated material layer) with anisotropically thermally conductive filler particles such as h-BN or graphite having controllable preferred orientation, increasing the in-plane thermal conductivity is accompanied by reducing the through-plane thermal conductivity, and vice versa. Table 1. Corrugated Layer Thickness Corrugated Layer Pitch Design Model k_matrix (t’, t’’) (p’, p’’) (DM) (W/mK) (millimeters) (millimeters) 1 0.1 0.02 0.25 2 1 0.02 0.25 3 5 0.02 0.25 4 10 0.02 0.25 5 0.1 0.04 0.25 6 1 0.04 0.25 7 5 0.04 0.25 8 10 0.04 0.25 9 0.1 0.08 0.25 10 1 0.08 0.25 11 5 0.08 0.25 12 10 0.08 0.25 13 0.1 0.12 0.25 14 1 0.12 0.25 15 5 0.12 0.25 16 10 0.12 0.25 17 0.1 0.02 0.5 18 1 0.02 0.5 19 5 0.02 0.5 20 10 0.02 0.5 21 0.1 0.04 0.5 22 1 0.04 0.5 23 5 0.04 0.5 24 10 0.04 0.5 25 0.1 0.08 0.5 26 1 0.08 0.5 27 5 0.08 0.5 28 10 0.08 0.5 29 0.1 0.12 0.5 30 1 0.12 0.5 31 5 0.12 0.5 32 10 0.12 0.5 33 0.1 0.16 0.5 34 1 0.16 0.5 35 5 0.16 0.5 36 10 0.16 0.5 37 0.1 0.24 0.5 38 1 0.24 0.5 39 5 0.24 0.5 40 10 0.24 0.5 0.1 0.02 1 1 0.02 1 5 0.02 1 10 0.02 1 0.1 0.04 1 1 0.04 1 5 0.04 1 10 0.04 1 0.1 0.08 1 1 0.08 1 5 0.08 1 10 0.08 1 0.1 0.12 1 1 0.12 1 5 0.12 1 10 0.12 1 0.1 0.16 1 1 0.16 1 5 0.16 1 10 0.16 1 0.1 0.2 1 1 0.2 1 5 0.2 1 10 0.2 1 0.1 0.24 1 1 0.24 1 5 0.24 1 10 0.24 1 0.1 0.02 1.5 1 0.02 1.5 5 0.02 1.5 10 0.02 1.5 0.1 0.04 1.5 1 0.04 1.5 5 0.04 1.5 10 0.04 1.5 0.1 0.08 1.5 1 0.08 1.5 5 0.08 1.5 10 0.08 1.5 0.1 0.12 1.5 1 0.12 1.5 5 0.12 1.5 10 0.12 1.5 0.1 0.16 1.5 1 0.16 1.5 5 0.16 1.5 10 0.16 1.5 0.1 0.24 1.5 1 0.24 1.5 5 0.24 1.5 10 0.24 1.5 0.1 0.02 2 1 0.02 2 5 0.02 2 10 0.02 2 0.1 0.04 2 1 0.04 2 5 0.04 2 10 0.04 2 0.1 0.08 2 1 0.08 2 5 0.08 2 10 0.08 2 0.1 0.12 2 1 0.12 2 5 0.12 2 10 0.12 2 0.1 0.16 2 1 0.16 2 5 0.16 2 10 0.16 2 0.1 0.2 2 1 0.2 2 5 0.2 2 10 0.2 2 0.1 0.24 2 1 0.24 2 5 0.24 2 10 0.24 2 0.1 0.02 2.5 1 0.02 2.5 5 0.02 2.5 10 0.02 2.5 0.1 0.04 2.5 1 0.04 2.5 5 0.04 2.5 10 0.04 2.5 0.1 0.08 2.5 1 0.08 2.5 5 0.08 2.5 10 0.08 2.5 0.1 0.12 2.5 1 0.12 2.5 5 0.12 2.5 10 0.12 2.5 0.1 0.16 2.5 1 0.16 2.5 5 0.16 2.5 10 0.16 2.5 0.1 0.24 2.5 1 0.24 2.5 5 0.24 2.5 10 0.24 2.5 0.1 0.02 3.5 1 0.02 3.5 5 0.02 3.5 10 0.02 3.5 149 0.1 0.04 3.5 150 1 0.04 3.5 151 5 0.04 3.5 152 10 0.04 3.5 153 0.1 0.08 3.5 154 1 0.08 3.5 155 5 0.08 3.5 156 10 0.08 3.5 157 0.1 0.12 3.5 158 1 0.12 3.5 159 5 0.12 3.5 160 10 0.12 3.5 161 0.1 0.16 3.5 162 1 0.16 3.5 163 5 0.16 3.5 164 10 0.16 3.5 165 0.1 0.24 3.5 166 1 0.24 3.5 167 5 0.24 3.5 168 10 0.24 3.5 Table 2. k_xy k_z k_z/k_xy (W/mK) (W/mK) 1 100 100.000 2.1 98.9 47.095 3.2 97.8 30.563 4.3 96.7 22.488 5.4 95.6 17.704 6.5 94.5 14.538 7.6 93.4 12.289 8.7 92.3 10.609 9.8 91.2 9.306 10.9 90.1 8.266 12 89 7.417 13.1 87.9 6.710 14.2 86.8 6.113 15.3 85.7 5.601 16.4 84.6 5.159 17.5 83.5 4.771 18.6 82.4 4.430 19.7 81.3 4.127 20.8 80.2 3.856 21.9 79.1 3.612 23 78 3.391 24.1 76.9 3.191 25.2 75.8 3.008 26.3 74.7 2.840 27.4 73.6 2.686 28.5 72.5 2.544 29.6 71.4 2.412 30.7 70.3 2.290 31.8 69.2 2.176 8.1 2.070 67 1.9715.9 1.8774.8 1.7903.7 1.7082.6 1.6301.5 1.5570.4 1.4889.3 1.4228.2 1.3607.1 1.301 56 1.2444.9 1.1913.8 1.1402.7 1.0911.6 1.0450.5 1.0009.4 0.9578.3 0.9177.2 0.8776.1 0.840 45 0.8043.9 0.7692.8 0.7351.7 0.7030.6 0.6729.5 0.6428.4 0.6137.3 0.5866.2 0.5595.1 0.533 34 0.5072.9 0.4831.8 0.4600.7 0.4379.6 0.4158.5 0.3937.4 0.3726.3 0.3525.2 0.3324.1 0.313 23 0.2951.9 0.2770.8 0.2599.7 0.2428.6 0.2267.5 0.2106.4 0.1945.3 0.1794.2 0.1643.1 0.149 12 0.1350.9 0.121 91.2 9.8 0.107 92.3 8.7 0.094 93.4 7.6 0.081 94.5 6.5 0.069 95.6 5.4 0.056 96.7 4.3 0.044 97.8 3.2 0.033 98.9 2.1 0.021 100 1 0.010 [0081] Design Model 169 [0082] Thermal simulation and calculation of effective thermal conductivity (k_eff) were carried out for a second series of modeled thermal transfer articles. To verify that the design rules discovered for 1 millimeter total thickness of thermal transfer articles (results below for Design Models 1-168) can be extended to different size scales of the components, a model was constructed by starting with Design Model 27 and then doubling the dimensions of each of the corrugated layer thickness t’ (from 0.08 millimeter to 0.16 millimeter), the corrugated layer pitch p’ (from 0.5 millimeter to 1 millimeter), and the overall thermal transfer article thickness x’ (from 1 millimeter to 2 millimeters). The modeled thermal transfer article was modeled to be placed between 2 millimeter thick blocks of aluminum. In a simulation, a 100K thermal difference was applied to the modeled assembly, and k_eff for the thermal transfer article was calculated in the same manner as for Design Model 27. [0083] Design Model 170-187 [0084] Thermal simulation and calculation of effective thermal conductivity (k_eff) were carried out for a third series of modeled thermal transfer articles. The third series of thermal transfer article designs span a range of values of each of the matrix thermal conductivity (k_matrix), a first matrix overcoat thickness 448, and a second matrix overcoat thickness 450. Specific values of these design parameters are given in Table 3. The modeled corrugated layer thickness t’’ and pitch p’’ were held constant at 0.08 millimeters and 1 millimeter, respectively. The modeled corrugated layer peak-to-valley height (2A + 2d) was held constant at 1 millimeter. For each example modeled thermal transfer article design parameter combination in Design Models 170-187, a series of combinations of corrugated layer material in-plane thermal conductivity (k_xy) and corrugated layer through-plane thermal conductivity (k_z) were modeled. Table 2 includes the 91 specific combinations that were used. Table 3. First Matrix Second Overcoat Matrix Overall Thickness Overcoat Thermal (% of Second Thickness transfer First Matrix Overall Matrix (% of Design article Overcoat Component Overcoat Overall Model k_matrix Thickness Thickness Thickness) Thickness Component (DM) (W/mK) (millimeters) (millimeters) (millimeters) Thickness) 170 1 1.0004 0.0002 0.02% 0.0002 0.02% 171 1 1.004 0.002 0.2% 0.002 0.2% 172 1 1.04 0.02 1.9% 0.02 1.9% 173 1 1.1 0.05 4.5% 0.05 4.5% 174 1 1.2 0.1 8.3% 0.1 8.3% 175 1 1.4 0.2 14.3% 0.2 14.3% 176 5 1.0004 0.0002 0.02% 0.0002 0.02% 177 5 1.004 0.002 0.2% 0.002 0.2% 178 5 1.04 0.02 1.9% 0.02 1.9% 179 5 1.1 0.05 4.5% 0.05 4.5% 180 5 1.2 0.1 8.3% 0.1 8.3% 181 5 1.4 0.2 14.3% 0.2 14.3% 182 10 1.0004 0.0002 0.02% 0.0002 0.02% 183 10 1.004 0.002 0.2% 0.002 0.2% 184 10 1.04 0.02 1.9% 0.02 1.9% 185 10 1.1 0.05 4.5% 0.05 4.5% 186 10 1.2 0.1 8.3% 0.1 8.3% 187 10 1.4 0.2 14.3% 0.2 14.3% [0085] Design Models 188-192 [0086] A series of films were obtained from the Joinset Company (Gyeonggi-do, Korea) under the tradename Low Dielectric Thermal Spreader Sheet. The product code and thickness of each film is listed in Table 4. The in-plane and through-plane thermal conductivity values of the films were determined by measuring thermal diffusivity with laser flash analysis (“HYPERFLASH LFA 467” from Netzsch Instruments North America LLC, Boston, MA) and measuring specific heat capacity using a Differential Scanning Calorimeter (“DSC2A-00883/RCS” from TA Instruments, New Castle, DE). Table 4. Design Product Code Thickness Model (micrometers) (DM) 188 LDTS-P1000030-50 30 189 LDTS-P1000045- 2₀₀ ⁴⁵ 190 LDTS-P1000100-50 100 191 LDTS-P450045-200 45 192 LDTS-P450100-50 100 [0087] Results [0088] Design Models 1-168 [0089] Thermal simulation of the modeled thermal transfer articles enabled calculation of the effective thermal conductivity (k_eff) for each design in the first series (Design Models 1-168). Figs.5 and 6 illustrate results for Design Models 25-28 and Design Models 165-168, respectively. Inspection of such results across the entire first series led to the discovery of specific ranges of k_z:k_xy ratio (k_z/k_xy) for anisotropically thermally conductive corrugated layers that dramatically improved k_eff for specific thermal transfer article design parameters. [0090] As shown in Fig.5, for one exemplary combination of corrugated layer thickness (0.08 millimeter) and corrugated layer pitch (0.5 millimeter), careful selection of k_z/k_xy within the range of 0.02 to 0.75 yielded k_eff values that far exceeded those constructed of isotropically thermally conductive corrugated layers (k_z = k_xy; k_z/k_xy = 1) or either maximally anisotropically thermally conductive cases (k_z/k_xy = 0.01 or 100). In fact, at k_matrix values of 0.1, 1, 5, and 10 W/mK for Design Models 25, 26, 27, and 28 respectively, selection of k_z/k_xy of 0.393, 0.332, 0.277, and 0.259 respectively yielded improvements of 12% (11.48 W/mK vs.10.23 W/mK), 17% (15.03 W/mK vs.12.88 W/mK), 19% (20.64 W/mK vs.17.31 W/mK), and 18% (25.34 W/mK vs.21.44 W/mK) respectively vs. the isotropically thermally conductive corrugated layer case (k_z = k_xy). That is, a noticeable improvement was made by careful selection of k_z/k_xy in combination with other thermal transfer article design parameters. For the overall thermal transfer article thickness, corrugated layer thickness, and corrugated layer pitch of Design Models 25-28, careful selection yielded k_eff for some designs that was simultaneously i) improved by over 10% compared to the otherwise identical case except with isotropically thermally conductive corrugated layer (k_z = k_xy = 50.5 W/mK), and ii) greater than 5 W/mK, an important and challenging performance level for technological applications. [0091] As shown in Fig.6, for another exemplary combination of corrugated layer thickness (0.24 millimeter) and corrugated layer pitch (3.5 millimeter), at k_matrix values of 0.1, 1, 5, and 10 W/mK for Design Models 165, 166, 167, and 168 respectively, selection of k_z/k_xy of 0.313, 0.277, 0.352, and 0.507 respectively yielded improvements of 9% (1.67 W/mK vs.1.54 W/mK), 15% (4.68 W/mK vs.4.06 W/mK), 7% (9.80 W/mK vs.9.15 W/mK), and 2% (15.05 W/mK vs.14.71 W/mK) respectively vs. the isotropically thermally conductive corrugated layer case (k_z = k_xy). For the overall thermal transfer article thickness, corrugated layer thickness, and corrugated layer pitch of Design Models 165-168, careful selection yielded k_eff for no designs that both i) improved by over 10% compared to the otherwise identical case except with isotropically thermally conductive corrugated layer (k_z = k_xy), and ii) exhibited greater than 5 W/mK. [0092] The analysis described above for the results data plotted in Figs.5 and 6 was also carried out for all of the other results data for the first series Design Models 1-168. This enabled a categorization of certain thermal transfer article design regimes according to the i) degree of improvement in k_eff by selecting k_z/k_xy between 0.02 and 0.75 (over the isotropically thermally conductive corrugated layer case (k_z = k_xy = 50.5 W/mK)), and ii) whether or not the technologically important k_eff absolute magnitude of 5 W/mK had been achieved. Fig.7 reports such categorizations of designs according to their selections of corrugated layer thickness and corrugated layer pitch for 1 millimeter total thermal transfer article thickness (design groups). Design groups in Fig.7 are categorized according to whether, for any of the k_matrix values studied (0.1, 1.5, and 10 W/mK), there is at least one design in the group where k_eff is i) both ≥ 10% to < 20% improved over the isotropic case and > 5 W/mK; ii) both ≥ 20% to < 30% improved over the isotropic case and > 5 W/mK; iii) both ≥ 30% to < 40% improved over the isotropic case and > 5 W/mK; iv) both ≥ 40% improved over the isotropic case and > 5 W/mK; or v) either < 10% improved over the isotropic case or < 5 W/mK. This enabled the graphical selection of design spaces 570, 572, 574, 576 that were discovered to be preferred. [0093] It has been discovered herein that preferably high levels of through-plane effective thermal conductivity can be attained for thermal transfer articles comprising a corrugated layer at least partially embedded in a matrix material, particularly for designs that do not include an electrically conductive corrugated layer or matrix, when appropriate selections of corrugated layer in-plane and through-plane thermal conductivity (k_xy and k_z, respectively), matrix thermal conductivity (k_matrix), corrugated layer design (thickness, pitch, and peak-to-valley height), and overall thermal transfer article thickness are selected. Preferred article designs have been identified on the basis of i) reaching a through-plane effective thermal conductivity of the thermal transfer article of at least 5 W/mK, and ii) delivering an improvement (e.g., at least 10%, 20%, 30% or 40%) in through-plane effective thermal conductivity of the thermal transfer article over a base case design by judicious selection within a tradeoff between corrugated layer k_xy vs. k_z values (wherein the base case design is where k_xy and k_z are equal, the simple isotropic case). [0094] Selection of appropriate k_matrix was also discovered to be important for realizing both of the bases listed above for identification of the preferred design spaces. That is, it was discovered that for some combinations of corrugated layer thickness, corrugated layer pitch, and overall thermal transfer article thickness, followed by judicious selection within a tradeoff between corrugated layer k_xy vs. k_z values, achievement of both of the bases above depended also on appropriate selection of k_matrix (within a range of 0.1 to 10 W/mK). For other combinations of corrugated layer thickness, corrugated layer pitch, and overall thermal transfer article thickness, followed by judicious selection within a tradeoff between corrugated layer k_xy vs. k_z values, both of the bases were found across the entire range of k_matrix from 0.1 to 10 W/mK. [0095] Design Model 169 [0096] Thermal simulation of the modeled thermal transfer articles enabled calculation of the effective thermal conductivity (k_eff) for each design in the second series (Design Model 169). The same values of k_eff (for each of the different combinations of the values of k_z and k_xy of the corrugated layer material in Table 2) were found for this “2x” scaled component as for the “1x” scaled component in the Design Model 27. This result confirmed that design regimes, with their various performance levels, can be expressed in terms of a single absolute dimension together with the other dimensions expressed as proportions or multiples of that one absolute dimension. [0097] Design Models 170-187 [0098] Thermal simulation of the modeled thermal transfer articles enabled calculation of the effective thermal conductivity (k_eff) for each design in the third series (Design Models 170-187). For each example, the highest value of k_eff found within the corrugated layer material anisotropic thermal conductivity series (k_z/k_xy) was determined (maximum k_eff). Further, that maximum value of k_eff within the corrugated layer anisotropic thermal conductivity series was compared with the value of k_eff for the isotropic corrugated layer material thermal conductivity (k_z = k_xy = 50.5 W/mK). The latter yielded a value of improvement in k_eff over the isotropic case that was enabled by selecting a preferred combination of k_xy and k_z (anisotropy in corrugated layer material thermal conductivity). The maximum k_eff value and its degree of improvement over the isotropic case are given Table 5. The results demonstrate that for a number of designs, both ≥ 10% improvement over the isotropic case and > 5 W/mK for k_eff were maintained for matrix overcoat thickness of up to 0.1 millimeter or 8.3% of the thermal transfer article total thickness. Table 5. Design Model Maximum k_eff Improvement in k_eff over the (DM) (W/mK) Isotropic Case 170 8.0 19.5% 171 6.7 17.7% 172 4.1 10.6% 173 3.2 7.2% 174 2.5 5.1% 175 2.0 3.4% 176 13.3 21% 177 13.0 21% 178 11.7 17% 179 10.5 14% 180 9.4 11% 181 8.2 8% 182 17.9 16% 183 17.9 16% 184 17.1 14% 185 16.2 12% 186 15.3 10% 187 14.1 8% [0099] Design Models 188-192 [00100] Table 6 below gives measurement results for the commercially available anisotropically thermally conductive films, along with calculated values of kz/kxy, demonstrating the availability of suitable materials to fabricate the modeled thermal transfer articles. Table 6. Design Model (DM) k_xy (W/mK) k_z (W/mK) k_z/k_xy 188 108 2 0.019 189 77 2.9 0.038 190 91 4 0.044 191 61 2.2 0.036 192 42 2.5 0.060 [00101] Hypothetical Examples- Series 1 (H1-H8) [00102] Materials [00103] Corrugated Layer Material 1: A thermally conductive film commercially available from the Joinset Company (Gyeonggi-do, Korea) under the tradename LDTS-P1000100-50, a Low Dielectric Thermal Spreader Sheet (LDTS) having thickness of 100 micrometers and an advertised in-plane thermal conductivity (k_xy) of 100 W/mK, reported by Joinset Company to be a hybrid sheet made of boron nitride powder and polymer. The in-plane (k_xy) and through-plane (k_z) thermal conductivity values of the films were determined by measuring thermal diffusivity with laser flash analysis (“HYPERFLASH LFA 467” from Netzsch Instruments North America LLC, Boston, MA) and measuring specific heat capacity using a Differential Scanning Calorimeter (“DSC2A-00883/RCS” from TA Instruments, New Castle, DE). Measurement of the in-plane thermal conductivity (k_xy) and through-plane thermal conductivity (k_z) of the Corrugated Layer Material 1 yielded values of 91 W/mK and 4 W/mK, respectively. [00104] Corrugated Layer Material 2: A thermally conductive film commercially available from the Joinset Company (Gyeonggi-do, Korea) under the tradename LDTS-P1000030-50, a Low Dielectric Thermal Spreader Sheet (LDTS) having thickness of 30 micrometers and an advertised in-plane thermal conductivity (k_xy) of 100 W/mK, reported by Joinset Company to be a hybrid sheet made of boron nitride powder and polymer. The in-plane (k_xy) and through-plane (k_z) thermal conductivity values of the films were determined by measuring thermal diffusivity with laser flash analysis (“HYPERFLASH LFA 467” from Netzsch Instruments North America LLC, Boston, MA) and measuring specific heat capacity using a Differential Scanning Calorimeter (“DSC2A-00883/RCS” from TA Instruments, New Castle, DE). Measurement of the in-plane thermal conductivity (k_xy) and through-plane thermal conductivity (k_z) of the Corrugated Layer Material 2 yielded values of 108 W/mK and 2 W/mK, respectively. [00105] Matrix Material 1: A thermally conductive silicone material commercially available from Henkel Corporation (Düsseldorf, Germany) under the tradename Bergquist Gap Filler TGF 4500CVO, a two-part dispensable liquid silicone composition that cures to become an elastic solid after mixing, with a reported thermal conductivity of 4.5 W/mK. [00106] Matrix Material 2: A thermally conductive polyurethane material commercially available from Parker Lord Corporation (Cary, North Carolina) under the tradename CoolTherm® UR-2002, a two-part dispensable liquid urethane composition that cures to become an elastic solid after mixing, with a reported thermal conductivity of 2.0 W/mK. [00107] Release Liner: A polyester film with a non-silicone release coating applied to its surface commercially available from 3M Company (St. Paul, Minnesota) under the tradename 3M^TM Non- Silicone Secondar Release Liner 4935, with a thickness of 3 mils (approximately 75 micrometers). [00108] Methods [00109] Method 1: A thermal transfer article can be formed according to methods described in JP6886543 B1, paragraphs [0044] through [0047]. To implement this method, a Matrix Material can first be molded into a sheet by mixing its Part A and Part B and compressing to the desired thickness between release liners using a common laboratory press. Once cured, the molded sheet of Matrix Material can be cut into elastic members with shape that, once combined with a Corrugated Layer Material, yields the desired cross-section geometry of the intended final corrugated layer (e.g., trapezoidal, sinusoidal). For the hypothetical examples described herein as derivable from this Method 1, the elastic members can be cut from sheet of Matrix Material with thickness of 1 millimeter and area dimensions of 5 centimeters by 5 centimeters. The elastic members can be formed by cutting the sheet parallel to a selected 5 centimeter long edge into strips using alternating cut angles of positive and negative 45 degrees to the major surfaces of the sheet at a pitch of 1.2 millimeters in the direction along the sheet surface and perpendicular to the selected 5 centimeter long edge. Such cutting will yield prism shaped elastic members with approximately trapezoidal cross-section. As described in JP6886543 B1, the thermal transfer article comprising an embedded corrugated layer and matrix material filling the grooves of the corrugated layer can be formed by pressing the Corrugated Layer Material between interlocking elastic members that were formed by cutting the molded sheet of Matrix Material. [00110] Method 2: A thermal transfer article can be formed by first converting a Corrugated Layer Material into a corrugated layer with a top side and a bottom side, each having grooves, for example according to methods described in US3789299, US3682736, or US8075832. Then the grooves of the corrugated layer can be filled according to methods in US20190281726 A1, paragraph [0088], which describes pouring a filled reactive silicone system over a corrugated sheet and hand casting over a set gap. In hypothetical examples according to this Method 2, the uncured Matrix Material can be poured over the top side of a corrugated layer formed of a Corrugated Layer Material, hand cast into the grooves of the corrugated layer top side using a squeegee or doctor blade, and then cured in place to form a top- side-filled article. Subsequently, such a top-side-filled article can be inverted, and the same process of US20190281726 of pouring a reactive system (in this case, the Matrix Material) into the exposed empty grooves of the bottom side and thus filling them can be carried out, followed again by casting into the grooves of the corrugated layer using a squeegee or doctor blade and then curing. For the hypothetical examples described herein as derivable from this Method 2, the corrugated layer can be formed with a sinusoidal shape having a pitch (also referred to herein as wavelength) of 1 millimeter and a peak to peak height (also referred to herein as two times the amplitude, plus the corrugated layer material thickness) of 1 millimeter. The profiled tools for forming this corrugated layer, as described for example in US3789299, can be formed by stereolithography using known methods. [00111] Examples H1-H8 [00112] The materials and methods used to produce each of hypothetical examples H1-H8 are provided in Table 7 below. Table 7. Hypothetical Examples H1-H8 Materials and Method of Making Hypothetical Example Corrugated Layer Material Matrix Material Method H1 Corrugated Layer Material 1 Matrix Material 1 Method 1 H2 Corrugated Layer Material 1 Matrix Material 1 Method 2 H3 Corrugated Layer Material 1 Matrix Material 2 Method 1 H4 Corrugated Layer Material 1 Matrix Material 2 Method 2 H5 Corrugated Layer Material 2 Matrix Material 1 Method 1 H6 Corrugated Layer Material 2 Matrix Material 1 Method 2 H7 Corrugated Layer Material 2 Matrix Material 2 Method 1 H8 Corrugated Layer Material 2 Matrix Material 2 Method 2 [00113] Hypothetical Examples- Series 2 (H9-H28) [00114] Another series of hypothetical example thermal transfer article designs were analyzed by thermal simulation, focusing on calculation of the effective thermal conductivity, k_eff, through their thickness (i.e., parallel to their thickness direction), as described above. Each of this second series of hypothetical thermal transfer article designs incorporates a corrugated layer material with measured thermal transport properties of a commercially available anisotropically thermally conductive film selected from Model Designs 188-192 (as reported in Table 6). Each thermal transfer article design is further defined by the thermal conductivity of the matrix and the pitch of the corrugated layer. Specific values of these design parameters are given in Table 8. The modeled component overall thickness was held constant at 1 millimeter in this series of hypothetical examples. The modeled corrugated layer peak- to-peak height (2A + 2d) was held constant at 1 millimeter (i.e., matrix overcoat thickness of 0 millimeter). The calculated effective thermal conductivity of each thermal transfer article of hypothetical examples H9-H28, k_eff, through their thickness and parallel to their thickness direction is given in Table 8. Table 8. Hypothetical Examples H9-H28 Materials, Designs, and Results Example Corrugated Corrugated Corrugated Corrugat k_matrix Corrugated Calculated Layer layer layer ed layer (W/mK) Layer Pitch effective Material material material material (mm) thermal k_xy k_z thickness conductivit (W/mK) (W/mK) (mm) y of the heat transfer component k_eff (W/mK) H9 DM 188 108 2 0.030 1 0.5 7.5 H10 DM 188 108 2 0.030 1 1.5 3.4 H11 DM 188 108 2 0.030 10 0.5 16.9 H12 DM 188 108 2 0.030 10 1.5 11.6 H13 DM 189 77 2.9 0.045 1 0.5 7.7 H14 DM 189 77 2.9 0.045 1 1.5 3.6 H15 DM 189 77 2.9 0.045 10 0.5 16.8 H16 DM 189 77 2.9 0.045 10 1.5 11.6 H17 DM 190 91 4 0.100 1 0.5 10.1 H18 DM 190 91 4 0.100 1 1.5 5.3 H19 DM 190 91 4 0.100 10 0.5 19.1 H20 DM 190 91 4 0.100 10 1.5 13.2 H21 DM 191 61 2.2 0.045 1 0.5 6.4 H22 DM 191 61 2.2 0.045 1 1.5 3.1 H23 DM 191 61 2.2 0.045 10 0.5 14.9 H24 DM 191 61 2.2 0.045 10 1.5 10.7 H25 DM 192 42 2.5 0.100 1 0.5 6.2 H26 DM 192 42 2.5 0.100 1 1.5 3.4 H27 DM 192 42 2.5 0.100 10 0.5 13.0 H28 DM 192 42 2.5 0.100 10 1.5 9.9 [00115] Thus, the present disclosure provides, among other things, thermal transfer articles and assemblies comprising the same. Various features and advantages of the present disclosure are set forth in the following claims.

Claims

What is claimed is: What is claimed is: 1. A thermal transfer article having a first major surface, a second major surface opposite the first major surface, and a thickness (x) therebetween, the article comprising: a matrix material extending to the first major surface and to the second major surface; and a corrugated layer at least partially embedded within the matrix material, the corrugated layer forming alternating peaks and valleys and having a thickness (t) and a pitch (p), wherein x is from 0.2 mm to 5 mm, wherein t is from 0.005x to 0.2x, wherein p is: 2t to x + 30t when 0.005x < t < 0.0358x, -0.167 + 6.67t to x + 30t when 0.0358x < t < 0.05x, and -0.167 + 6.67t to 2.5x when 0.05x < t < 0.2x, and wherein the corrugated layer exhibits anisotropic thermal conductivity.

2. The article of claim 1, wherein the corrugated layer exhibits a through-plane thermal conductivity (k_z) and an in-plane thermal conductivity (k_xy), and wherein the ratio of k_z to k_xy is from 0.02 to 0.75.

3. The article of claim 1 or claim 2, wherein the value of k_xy is at least 20 W/mK.

4. The article of any one of claims 1 to 3, wherein the matrix material has a thermal conductivity (k_matrix) from 0.1 to 10 W/mK.

5. The article of any one of claims 1 to 4, wherein the article has a through-plane thermal conductivity (k_eff) of at least 5 W/mK.

6. The article of claim 5, wherein the article through-plane effective thermal conductivity (k_eff) is at least 10% greater than would be obtained for a similar article that differs only in that the corrugated layer has isotropic thermal conductivity equal to the average of k_xy and k_z.

7. The article of any one of claims 1 to 6, wherein the corrugated layer has a profile that is pleated.

8. The article of any one of claims 1 to 7, wherein the corrugated layer is a composite film comprising hexagonal boron nitride (h-BN), a transition metal dichalcogenide, carbon fibers, metal fibers, graphite, or combinations thereof.

9. The article of any one of claims 1 to 8, wherein the corrugated layer is a composite film comprising h-BN.

10. The article of claim 8 or claim 9, wherein the corrugated layer further comprises a binder.

11. The article of any one of claims 1 to 10, wherein the matrix material comprises silicones, poly(meth)acrylates, polyurethanes, polyolefins (including cyclo-olefins), acrylonitrile-butadiene rubbers, hydrogenated acrylonitrile-butadiene rubbers, ethylene propylene diene rubbers, fluorinated elastomers, chloroprene rubbers, styrene-butadiene rubbers, epoxies, polyurethanes, polydicyclopentadienes, polyureas, melamine, benzoxazines, polyimides, and vinyl esters, natural rubber, or combinations thereof.

12. The article of any one of claims 1 to 11, wherein the matrix material comprises an electrically insulating polymer binder and a plurality of electrically insulating particles.

13. The article of any one of claims 1 to 11, wherein the matrix material comprises a polymer binder and a plurality of electrically conductive particles.

14. The article of any one of claims 1 to 12, wherein the article is electrically insulating.

15. The article of any one of claims 1 to 14, wherein the peaks of the corrugated layer are flush with the matrix material extending to the first major surface, the valleys of the corrugated layer are flush with matrix material extending to the second major surface, or both.

16. The article of any one of claims 1 to 14, wherein the matrix material forms an overcoat on the peaks of the corrugated layer, the valleys of the corrugated layer, or a combination thereof.

17. The article of claim 16, wherein the overcoat extends up to 100 micrometers above the peaks of the corrugated layer, below the valleys of the corrugated layers, or a combination thereof.

18. An assembly comprising the article of any one of claims 1 to 17.

19. The assembly of claim 18, further comprising a thermally conductive material on at least one of the first and second major surfaces.

20. The assembly of claim 18 or claim 19, wherein the article is interposed between a first heat transfer surface of a heat source and a second heat transfer surface of a heat sink.