Patent Application
20240383814
This invention generally relates to a ceramic material usable in a capacitor and, in particular, to an antiferroelectric ceramic material for a capacitor having improved energy efficiency and increased dielectric breakdown strength.
Capacitors are ubiquitous in modern electronics for both industrial and consumer applications. For instance, some smartphone designs incorporate more than 700 capacitors. Ceramic capacitors account for about 90% of the capacitor market by volume, with more than two trillion multilayer ceramic capacitors (MLCC) manufactured each year. High energy density capacitors are urgently needed in such areas as the storage of electricity generated from renewable sources and power electronics for the electric grid and in electric cars. As an example, of all the components used in power conditioning modules, capacitors take up the largest volume and contribute the greatest weight.
Ferroelectrics are extensively used in capacitors, particularly in the form of MLCCs. As one of the most heavily studied ferroelectrics, BaTiO3-based ceramic is the MLCC industry standard because of its high dielectric constant. However, despite possessing a comparatively high power density, ferroelectric materials have a comparatively low energy density compared to other energy storage devices, such as batteries and fuel cells. Thus, a capacitor able to provide a higher energy density while maintaining a high power density is desirable, particularly for such applications as electric vehicle charging and in windmills.
Embodiments of the disclosure relate to an improved ceramic material that is particularly suitable for use in a capacitor. In particular, the ceramic material is antiferroelectric (AFE) and provides enhanced energy efficiency and increased dielectric breakdown strength compared to conventional AFE ceramics. In this way, the disclosed AFE ceramic can be used in capacitors to replace traditional ferroelectric ceramic materials, thereby providing higher energy density while maintaining a high power density.
As will be described more fully below, the energy efficiency (η) of such AFE capacitors is 90% or higher, in particular 94% or higher. Further, the energy density (Wrec) is at least 3.0 J/cm3. In this way, high energy-density capacitors with high efficiency can be produced at large scale using current MLCC technologies.
As such, AFE ceramic materials can replace current linear or ferroelectric ceramics as the dielectric layer in capacitors, improving the energy storage density by more than 10-fold. AFE capacitors maintain the high power density of current dielectric capacitors, allowing the AFE capacitors to release stored electric energy at least 100× faster than electrochemical devices, such as batteries and hydrogen fuel cells. Moreover, it is believed that the disclosed AFE ceramic material could also have applications in other contexts, including as an electrocaloric medium for dielectric refrigeration and as an active material in large stroke actuators.
In a first aspect, embodiments of the disclosure relate to an antiferroelectric material. The antiferroelectric material is made from a PbZrO3-based ceramic in which at least 1 at % and up to 16 at % of Pb is substituted with a combination of Li and Bi.
In a second aspect, embodiments of the disclosure relate to an antiferroelectric material according to the first aspect in which a ratio of Li to Bi is about 1:1.
In a third aspect, embodiments of the disclosure relate to an antiferroelectric material according to the first aspect or the second aspect in which the antiferroelectric material has an energy density of at least 3.0 J/cm2.
In a fourth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the first aspect to the third aspect in which the antiferroelectric material has an energy efficiency of at least 90%.
In a fifth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the first aspect to the fourth aspect in which the antiferroelectric material has a general formula of [Pb1-x-y-z-1.5w(Li1/2Bi1/2)xSryBazLaw](Zr1-u-vSnuTiv)O3.
In a sixth aspect, embodiments of the disclosure relate to an antiferroelectric material according to the fifth aspect in which 0.01≤x≤0.12.
In a seventh aspect, embodiments of the disclosure relate to an antiferroelectric material according to the fifth aspect or the sixth aspect in which 1-x-y-z-1.5w≥0.85.
In an eighth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the fifth aspect to the seventh aspect in which 1-u-v≥0.50.
In a ninth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the fifth aspect to the eighth aspect in which v≤0.12.
In a tenth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the first aspect to the ninth aspect in which the antiferroelectric material has an average grain size of 3 μm or less.
In an eleventh aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the first aspect to the tenth aspect in which the antiferroelectric material has a dielectric breakdown strength of 180 kV/cm or higher.
In a twelfth aspect, embodiments of the disclosure relate to an antiferroelectric material according to any of the first aspect to the eleventh aspect in which a temperature at which a maximum dielectric constant is reached is at least 140° C.
In a thirteenth aspect, embodiments of the disclosure relate to a capacitor including the antiferroelectric material according to any of the first aspect to the twelfth aspect.
In a fourteenth aspect, embodiments of the disclosure relate to the capacitor according to the thirteenth aspect in which the capacitor is a multilayer ceramic capacitor.
In a fifteenth aspect, embodiments of the disclosure relate to a method of preparing an antiferroelectric material made of a PbZrO3-based ceramic in which at least 1 at % and up to 16 at % of Pb is substituted with Li and Bi. In the method, a plurality of powdered raw materials is mixed. The plurality of powdered raw materials includes cations of lead, zirconium, lithium, and bismuth. The plurality of powdered raw materials is sintered at a temperature of 1300° C. or less to form the antiferroelectric material.
In a sixteenth aspect, embodiments of the present disclosure relate to the method according to the fifteenth aspect in which mixing further involves mixing the plurality of powdered ceramics with a binder. The method further includes pressing the plurality of powdered raw materials into a green body and heating the green body to a temperature sufficient to burn off the binder. Sintering further involves sintering the plurality of powdered ceramics in the green body.
In a seventeenth aspect, embodiments of the present disclosure relate to the method of the fifteenth aspect or the sixteenth aspect in which the plurality of powdered raw materials includes PbO, Li2CO3, Bi2O3, ZrO2, and at least two of La2O3, SrCO3, SnO2, or BaO.
In an eighteenth aspect, embodiments of the present disclosure relate to the method of the seventeenth aspect in which excess PbO powder is provided to account for evaporation during sintering.
In a nineteenth aspect, embodiments of the present disclosure relate to the method of any of the fifteenth aspect to the eighteenth aspect in which the antiferroelectric material has a density of at least 95% after sintering.
In a twentieth aspect, embodiments of the present disclosure relate to the method of any of the fifteenth aspect to the nineteenth aspect in which the temperature during sintering is 1100° C. or less.
These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:
While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.
Reference will now be made in detail to various embodiments of an antiferroelectric (AFE) ceramic material having an improved dielectric breakdown strength, improved energy efficiency, and decreased sintering temperature. According to the present disclosure, the AFE ceramic material is based on lead zirconate (PbZrO3) in which lead is substituted with lithium (Li) and bismuth (Bi). In one or more embodiments, the lead may also be substituted with strontium (Sr), lanthanum (La), and barium (Ba), and the zirconium (Zr) may be substituted with tin (Sn) and titanium (Ti). The AFE ceramic material according to the present disclosure can be used in capacitors, particularly capacitors for power electronics of electric vehicles and windmills.
Advantageously, the disclosed AFE ceramic material has an increased energy density of 3.0 J/cm3 or greater and an energy efficiency of over 90%. Conventionally, AFE ceramic materials have been limited to energy densities of about 2 J/cm3 and an energy efficiency of about 70%. In addition to the improved performance, the disclosed composition of the AFE ceramic material also provides the advantage of a lower sintering temperature, which decreases energy consumption and costs associated with fabrication.
These and other aspects and advantages will be described more fully below in relation to the disclosed embodiments, and such embodiments are presented by way of illustration and not by way of limitation.
AFE materials have the potential for use in many different applications, and many uses of AFE materials take advantage of the AFE-to-ferroelectric (FE) phase transition, where certain properties abruptly change at the FE critical field (EF) and the AFE critical field (EA). At the transition field, conventional AFE materials have a large AFE/FE phase incompatibility, and a large hysteresis is formed, causing a large strain. In certain applications, the strain is advantageous for use in actuators, transducers, shape memory devices, etc. However, the large hysteresis is disadvantageous for capacitor applications because the large hysteresis decreases energy efficiency (causing energy to dissipate through heat) and decreases the number of cycles for capacitor lifetime.
In a crystal of an AFE ceramic, the polarization is in such a manner that neighboring ions are polarized into the antiparallel direction resulting in zero macroscopic polarization. In many AFE ceramic materials, when an external stimulus, such as an electric field, is applied, the antiparallel polarization can be aligned to a long-range polar order, which leads to the field-induced AFE/FE phase transition and the peculiar signature of the AFE, which is a double hysteresis loop as shown in
From
In a capacitor, the working mechanism of an AFE ceramic material is through a diffusionless, displacive phase transition.
Typically, a relaxor material has complicated compositions with multiple cations of different valences sharing the same lattice site. The random occupancy of these cations in the lattices disrupts the long-range ferroelectric order, which creates easily polarized polar nanoregions, leading to a nearly zero electric hysteresis. Based on this phenomenon, the inventors propose a “relaxor AFE” material as shown in
According to the present disclosure, the AFE ceramic material is based on PbZrO3, which is a perovskite compound. The perovskite structure 10 can be represented as shown in
In conventional PbZrO3 materials, a single crystal material provides better properties than polycrystalline materials. In particular, polycrystalline PbZrO3 ceramic at room temperature has a critical field for the phase transition, EF, that exceeds the breakdown strength of the ceramic. However, the synthesis of PbZrO3 single crystal is much pricier and more challenging, and because of the high phase transition field, the merits of pure polycrystalline PbZrO3 have not been fully exploited. Thus, PbZrO3-based oxides are modified for practical utilization. Such modifications include doping the PbZrO3 crystal with various dopants to decrease the phase transition field, boost the polarization, and lower the sintering temperature, among other property enhancements.
According to the present disclosure, one strategy is doping of the A-site cations 20 with, for example, Li+, Bi3+, La3+, Ba2+, and Sr2+. Another strategy is to substitute B-site cations 30 with, for example, Ti4+, Sn4+, and Nb5+. Multiple A-site cation 20 and B-site cation 30 dopants can be used to tune the electric and dielectric properties as needed for a particular application. The addition of such dopants directly influences the tolerance factor, which affects antiferroelectricity (i.e., EF, EA, electric hysteresis, and energy storage properties). The dopant Sr2+ is effective in reducing the hysteresis and producing a higher transition field. The dopant La3+ produces A-site vacancies to preserve charge neutrality. Low La3+ doping concentration (usually less than 3 mol %) increases the antiferroelectric-to-ferroelectric transition field, and the inventors expect that a higher concentration may lead to a relaxor behavior.
The disclosed AFE oxide has the general formula of:
[Pb1-x-y-z-1.5w(Li1/2Bi1/2)xSryBazLaw](Zr1-u-vSnuTiv)O3 (1)
Thus, the base PbZrO3 crystal may be doped with Sr2+, Ba2+, La3+ for the A-site cations 20, and B-site cation 30 dopants include Sn4+ and Ti4+. According to the present disclosure, the advantages of increased dielectric breakdown strength, increased energy efficiency, and decreased sintering temperature result from the inclusion of Li+ and Bi3+ ions as A-site substitutions in the perovskite crystal structure.
In one or more embodiments, x (Li+ and Bi3+) is in a range from 0.01 to 0.16, in particular from 0.04 to 0.12). In one or more embodiments, the atomic ratio of Li+ and Bi3+ is about 1:1. In one or more embodiments, y (Sr2+) is in a range from 0 to 0.10, in particular in a range from 0.03 to 0.06. In one or more embodiments, z (Ba2+) is in a range from 0 to 0.10, in particular in a range from 0.03 to 0.06. In one or more embodiments, w (La3+) is in a range from 0 to 0.05, in particular in 0.02 to 0.03. In one or more embodiments, u (Sn2+) is in a range from 0 to 0.50, in particular 0.30 to 0.45. In one or more embodiments, v (Ti4+) is in a range from 0 to 0.12, in particular from 0.05 to 0.10.
According to the embodiments of the present disclosure, the PbZrO3-based AFE material can be prepared by substituting lead oxide (PbO) with lithium oxide (Li2O) and/or lithium carbonate (Li2CO3) and bismuth oxide (Bi2O3) during sintering.
(Li1/2Bi1,2)O was substituted for PbO in the formation of PbZrO3-rich AFE compositions starting with the base composition of (Pb0.93Sr0.04La0.02)(Zr0.57Sn0.34Ti0.09)O3. In particular, substitutions were made according to the formula:
[Pb0.93-x(Li1/2Bi1/2)xSr0.04La0.02)](Zr0.57Sn0.34Ti0.09)O3 (2)
in which x was 0.04, 0.08, 0.12, and 0.16.
To prepare the AFE base compositions, high-purity raw powders (>99.9%) of PbO, La2O3, Li2CO3, Bi2O3, SrCO3, ZrO2, SnO2, TiO2 were baked first to remove the moisture. Because PbO evaporates during high temperature processing, an extra 5 wt % PbO was added to preserve the designed stoichiometry. The dry powder was batched according to chemical formula (2) and mixed in a vibratory mill for 6 hours with zirconia media in ethanol. The mixed slurry was dried for 24 hours, and then the powder was calcined at 935° C. for 4 hours. The calcined powder was re-milled for another 6 hours and dried for 24 hours. The dry powder was mixed with 10 wt % polyvinyl alcohol binder and uniaxially pressed into green pellets at a pressure of 150 MPa. The pellets were de-bindered at 600° C. for 3 hours and then sintered at temperatures in the range of 1075-1350° C. During sintering, the pellets were buried in the calcined powder of the same composition, and a double crucible setup was applied to further minimize the PbO evaporation loss.
X-ray diffraction (XRD) was performed to examine the crystal structure and phase purity of the sintered ceramics on a Siemens D500 X-ray diffractometer using a Cu-Kα radiation in a 2θ range from 20° to 80°. The fracture surfaces of the ceramics were examined with FEI Inspect 50 scanning electron microscope (SEM) to reveal the grain morphology and size. The grain size was quantified using the line intercept method.
To characterize the dielectric and ferroelectric properties, ceramic pellets were polished to a thickness of about 0.25 mm and electroded with a Pd-target sputter coater. The polarization vs. electric field loops were measured at room temperature at a frequency of 1 Hz using a standardized ferroelectric test system (Precision LC II, Radiant Technologies). The dielectric property of the AFE ceramic material was measured using a Novocontrol system at a heating rate of 1° C./min at 1 KHz frequency.
Sintered ceramic pellets of all compositions exhibit a relative density of greater than 95%. To achieve the same relative density, the sintering temperature was almost linearly reduced from 1350° C. for the base composition (x=0) to 1075° C. for the composition of x=0.16. The main phase of all compositions was determined to be perovskite, and for the high-dopant composition x=0.16, a second phase, La2Sn2O7, was observed. La2Sn2O7 might be an intermediate phase during the formation of the final perovskite phase, and the lower sintering temperature of 1075° C. is not high enough to achieve the full reaction. As a comparison experiment, x=0.16 was also sintered at a high temperature (1300° C.), and XRD was performed. At the higher sintering temperature, the La2Sn2O7 phase disappeared.
The radii of all A-site substitutions [Sr2+ (1.44 Å), Bi3+ (1.35 Å), Li+ (0.92 Å), and La3+ (1.36 Å)] are smaller than that of Pb2+ (1.49 Å). Further, the radii of the B-site substitutions [Sn4+ (0.69 Å) and Ti4+ (0.605 Å)] are smaller than that of Zr4+ (0.72 Å). The incorporated smaller ions lead to a decrease in the interplanar spacings because of the more compact unit cell, which results in the XRD peaks shifting to high 2θ angles. The peak shift is accompanied with the double-peak merging, which is a sign of the reduction of tetragonality, and the crystal structure gradually transforms into the cubic phase with the increase of the (Li,Bi)-co-doping amount. Interestingly, the splitting of (200)/(020) peak of the composition x=0.16 does not occur at low sintering temperature but can be resolved when sintered at 1300° C., which is a sign of weaker tetragonal distortion in the pellet sintered at 1075° C.
The detailed values for the grain size and sintering temperature are provided in Table 1 and graphed in
As mentioned above, the grain size and sintering temperature decreased almost linearly with increasing (Li1/2B1/2)O content. In particular, the average grain size was 3 μm or less for each of the compositions including 4 at % of (Li1/2B1/2)O or more, and the sintering temperature decreased to 1300° C. or less, including 1100° C. or less at 12 at % and higher.
The dielectric constant and loss tangent of the modified AFE ceramic materials were measured and are provided in Table 1 (at 25° C.) and are graphed in
All P-E loops were replotted in in graph (f) of
(Li1/2Bi1/2)O was substituted for PbO in the formation of PbZrO3-rich AFE compositions starting with the base composition of (Pb0.87Ba0.05Sr0.05La0.02)(Zr0.52Sn0.40Ti0.08)O3. In particular, substitutions were made according to the formula:
[Pb0.87-x(Li1/2Bi1/2)xBa0.05Sr0.05La0.02)](Zr0.52Sn0.40Ti0.08)O3 (3)
in which x was 0.04, 0.06, and 0.08.
The samples of the AFE ceramic material prepared according to Equation 3 were sintered for 3 hours at 1050° C., which is about 300° C. less than the sintering temperature of a conventional PbZrO3-based ceramic material.
The phase purity of the AFE ceramic samples prepared according to Equation 3 was confirmed by XRD spectra as shown in
The dielectric breakdown strength was measured on multiple samples of each composition and analyzed using Weibull statistics as shown in
As discussed above, the disclosed AFE ceramic material is particularly suitable for use in capacitors, such as the capacitor 100 shown in
Advantageously, a capacitor 100 made from the AFE ceramic material 110 provides a fast discharge rate (<1 μs), making the capacitor 100 particularly suitable for systems requiring high-frequency (1˜100 kHz) power conditioning and fast-discharge pulse power. With the fast growth of clean energy from renewable sources, where power inverters (DC to AC) are important to connecting electricity to a regional grid, and millions of electric cars projected for production, where power conditioning is key to energy efficiency, the disclosed AFE ceramic material is expected to satisfy the urgent demand for reliable capacitors having a large energy density while maintaining the desired level of power density.
All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
This patent application claims the benefit of U.S. Provisional Patent Application No. 63/503,302, filed May 19, 2023.
This invention was made in part with Government support under Contract Number DE-EE0009105 awarded by the Department of Energy. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63503302 | May 2023 | US |
