DETAILED DESCRIPTION

[0015] Barium titanate-based compositions having a high tetragonality and methods of forming the same are provided, as well as devices formed from the compositions. The barium titanate-based compositions advantageously have a high tetragonality and small particle sizes. For example, in some embodiments, the barium titanate-based compositions have a tetragonality of equal to or greater than about 2.0. and an average particle size of equal to or less than about 0.3 micron. As described further below, some methods involve achieving high tetragonality by limiting the concentration of certain metals (other than barium or titanium) in the compositions and/or heat treating the compositions at relatively high temperatures. In some methods, the A/B ratio of the composition may be adjusted prior to heat treatment to ensure that a small particle size is maintained during heat treatment. The barium titanate-based compositions may be processed to form dielectric layers in electronic devices, such as MLCCs, having excellent electrical properties as a result of the high tetragonality and relatively small particle sizes.

[0016] As used herein, the “tetragonality” of the barium titanate-based composition refers to the ratio (I₂₀₀/I_b). The ratio may be determined from an x-ray diffraction pattern that plots Intensity (counts) versus 2-Theta (degrees). I₂₀₀ is the value of the intensity at the (200) peak on an x-ray diffraction pattern obtained from the composition. When a clear valley exists between the (002) and (200) peaks on the x-ray diffraction pattern (FIG. 1A), I_b is defined as the intensity at the minima of the valley defined as d(I)/d(2-Theta)=0. A clear valley exists when d(I)/d(2-Theta) has a minimum of less than 0. When a clear valley does not exist between the (002) and (200) peaks on the x-ray diffraction pattern (FIG. 1B), I_b is defined as the intensity where d(I)/d(2-Theta) has a minimum value of greater than 45 degrees and less than the I₂₀₀ peak. The x-ray diffraction pattern may be obtained using standard x-ray powder diffraction techniques. One suitable technique involves using a copper target at a voltage of 40 kV and a current of 35 mA to provide a copper K-alpha monochromatic x-ray source, a sample rotation speed of 60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta. FIGS. 1A-1C show schematic x-ray diffraction patterns including I₂₀₀ and I_b.

[0017] As described further below, methods of the present invention may be used to produce barium titanate-based compositions having a desirably high tetragonality. Generally, the barium titanate-based compositions have a tetragonality of equal to or greater than about 1.5. In some cases, the compositions have a tetragonality of equal to or greater than about 2.0. or equal to or greater than about 2.5. In some cases, when a very high tetragonality is desired, the compositions may have a tetragonality of equal to or greater than about 3.0. The desired tetragonality of the barium titanate-based composition depends, in part, on the application and performance requirements in which the composition is used.

[0018] As used herein, “barium titanate-based” compositions refer to barium titanate, solid solutions thereof, or other oxides based on barium and titanium having the general structure ABO₃, where A represents one or more divalent metals such as barium, calcium, lead, strontium, magnesium and zinc and B represents one or more tetravalent metals such as titanium, tin, zirconium and hafnium. One type of barium titanate-based composition has the structure Ba_(1-x)A_xTi_(1-y)B_yO₃, where x and y can be in the range of 0 to 1, where A represents one or more divalent metal other than barium such as lead, calcium, strontium, magnesium and zinc and B represents one or more tetravalent metals other than titanium such as tin, zirconium and hafnium. Where the divalent or tetravalent metals are present as impurities, the value of x and y may be small, for example less than 0.1. In other cases, the divalent or tetravalent metals may be introduced at higher levels to provide a significantly identifiable compound such as barium-calcium titanate, barium-strontium titanate, barium titanate-zirconate and the like. In still other cases, where x or y is 1.0, barium or titanium may be completely replaced by the alternative metal of appropriate valence to provide a compound such as lead titanate or barium zirconate. In other cases, the compound may have multiple partial substitutions of barium or titanium. An example of such a multiple partial substituted composition is represented by the structural formula Ba_{(1-x-x′-x″)}Pb_xCa_x′Sr_x″O.Ti_{(1-y-y′-y″)}Sn_yZr_y′Hf_y″O₂, where x, x′, x″, y, y′, and y″ are each greater than or equal to 0. In many cases, the barium titanate-based material will have a perovskite crystal structure, though in other cases it may not.

[0019] Barium titanate (i.e., BaTiO₃) particles may be preferred in some embodiments of the invention. In particular, relatively pure barium titanate particles may be preferred because of their high tetragonality. It has been discovered that limiting concentrations of certain contaminant metals (other than barium or titanium) in the barium titanate particle composition can increase the tetragonality of the composition. It is believed that the presence of such contaminants can substitute on barium or titanium lattice sites and, thus, disrupt the formation of tetragonal unit cells. For example, as described further below, limiting the presence of divalent metals other than barium, such as strontium and/or calcium, which are common contaminants in barium titanate particle compositions may increase tetragonality. In some cases, the barium titanate particles have less than 500 parts per million of strontium and/or calcium. In other cases, the barium titanate particles have less than 200 parts per million, or even less than 50 parts per million, of strontium and/or calcium. In some cases, the calcium and/or strontium concentration may be substantially zero. Techniques for limiting strontium and/or calcium concentration are described below.

[0020] However, it should also be understood that not all particles produced in accordance with the invention have low contamination levels (i.e., less than 500 parts per million) when other techniques are used to increase the tetragonality of the composition to the desired level.

[0021] The barium titanate-based particles may have a variety of different particle characteristics. As noted above, the barium titanate-based particles typically have a small particle sizes, such as an average particle size of equal to or less than about 0.5 micron. As used herein, the average particle size refers to the average size of the primary particles of the composition. The average particle size of a composition may be determined using SEM image analysis by measuring the size of a representative number of particles (e.g., 300). In some cases, the average particle size is equal to or less than about 0.30 micron; in some cases, the average particle size may be equal to or less than about 0.20 micron; in some cases, the average particle size is equal to or less than about 0.15 micron; and, in some cases, the average particle size is equal to or less than about 0.10 micron. Particle size may be controlled by the processing technique used and processing conditions, as described further below. The desired particle size depends, in part, on the application and desired characteristics of the resulting device.

[0022] As noted above, compositions of the present invention may have high tetragonalities and small particle sizes. Accordingly, the compositions may have any of the above-described tetragonalities coupled with any of the above-described particle sizes. For example, the compositions may have a particle size of equal to or less than about 0.3 micron and a tetragonality of equal to or greater than about 2.0, 2.5, or 3.0. The compositions may also have a particle size of equal to or less than about 0.2 micron and a tetragonality of equal to or greater than about 2.0, 2.5, or 3.0. The compositions may also have a particle size of equal to or less than about 0.15 micron and a tetragonality of equal to or greater than about 1.5, 2.0, 2.5, or 3.0. The particular combination of particle size and tetragonality of the composition may be selected based on the particular application and achieved using the methods described herein.

[0023] The barium titanate-based particles may have a variety of shapes which may depend, in part, upon the process used to produce the particles. The barium titanate-based particles may be equiaxed and/or substantially spherical, in particular, if the particles are hydrothermally produced as described further below. In some cases, the particles may have an irregular, non-equiaxed shape.

[0024] The barium titanate-based particles may be produced in a hydrothermal process. Hydrothermal processes generally involve mixing a barium source with a titanium source in an aqueous environment to form a hydrothermal reaction mixture which is maintained at an elevated temperature. A suitable barium source is barium hydroxide solution which may be heated to an elevated temperature. A suitable titanium source is a hydrous titania gel. The titania gel may be produced by mixing titanium oxychloride (TiOCl₂) with water, and then adding ammonia hydroxide to increase the pH of the solution, thus, precipitating the titania gel. The titania gel may be washed to remove excess chloride and dispersed in water.

[0025] In the reaction mixture, the barium source reacts with the titanium source to produce barium titanate particles which remain dispersed in the aqueous environment to form a slurry. The particles may be washed to remove excess barium ions from the hydrothermal process while being maintained in the slurry. Suitable hydrothermal processes for forming barium titanate-based particles have been described, for example, in commonly-owned U.S. Pat. Nos. 4,829,033, 4,832,939, and 4,863,883, which are incorporated herein by reference in their entireties. Hydrothermal processes are particularly well-suited to produce particles having a small average particle size (e.g., equal to or less than 0.5 micron) and/or a substantially spherical shape. Particle size may be controlled, for example, by adjusting the hydrothermal reaction conditions such as reaction time.

[0026] It should also be understood that the particles may be produced according to other suitable techniques known in the art including solid-state reaction processes, sol-gel processes, as well as precipitation and subsequent calcination processes, such as oxalate-based processes.

[0027] As noted above, limiting the presence of metals other than barium or titanium (e.g., strontium and/or calcium) in barium titanate particle compositions may increase the tetragonality of the compositions. Contaminants (e.g., strontium and/or calcium) can be introduced into hydrothermally-produced barium titanate because such contaminants may be present in the barium source. Some hydrothermal methods of the present invention use barium sources that have low concentrations of contaminant metals, such as strontium and/or calcium, to limit the presence of such metals in the resulting composition. For example, in some cases, sources having a concentration of less than about 500 parts per million of strontium and/or calcium are used. In other cases, sources having a concentration of less than about 200 parts per million, or even less than about 50 parts per million, of strontium and/or calcium are used.

[0028] The strontium and/or calcium concentration in the barium source depends, in part, upon the raw material source. The methods of the present invention typically involve selecting barium sources having the desired contaminant metal concentrations. However, it should be understood that purification techniques may also be used to reduce the contaminant metal concentration to the desired level.

[0029] In some embodiments, methods of the invention subject the barium titanate-based particles to a heat treatment step to increase the tetragonality of the composition. For example, the particles may be heated to a temperature of between about 850° C. and about 1150° C. to increase the tetragonality to the values described. In some cases, the temperature is between about 950° C. and about 1050° C. Generally, tetragonality increases with increasing temperature within these ranges. Different temperatures may be used to achieve different tetragonalities. However, heat treatment temperatures of greater than about 1150° C. are typically unsuitable because the particles begin to sinter at such temperatures.

[0030] When hydrothermally-produced barium titanate-based particles are subjected to a heat treatment step, the water in the slurry may be removed (e.g., by filtering or decanting) and the particles may be dried at a lower temperature prior to heat treatment. General heat treatment processes have been described in commonly-owned, co-pending U.S. patent application Ser. No. 09/689,093, which was filed on Sep. 12, 2000, and is incorporated herein by reference in its entirety.

[0031] It should be understood that the tetragonality of the composition may be increased even when the heat treatment step does not substantially, if at all, increase the average particle size of the composition. In some embodiments, the A/B ratio of the barium titanate-based composition may be adjusted prior to the step of heat treating the coated particles in order to reduce the amount of particle growth during heat treatment. As used herein, A/B ratio is defined as the ratio of divalent metals (e.g., Ba.) to tetravalent metals (e.g., Ti) in the barium titanate-based particle composition. The A/B ratio may be measured by determining the concentration of divalent metals and tetravalent metals in a composition using an XRF compositional measurement technique.

[0032] It has been discovered, for example, that the amount of particle growth during heat treatment may be suitably controlled by adjusting the A/B ratio to a value other than 1.000. For example, the A/B ratio may be adjusted to values below 1.000, such as between about 0.975 and about 1.000, or between about 0.982 and about 0.988. The A/B ratio may also be adjusted to values above 1.000, such as between about 1.000 and about 1.025, or between about 1.008 and about 1.012. In certain MLCC applications, it may be preferred to adjust the A/B ratio to a value of greater than 1.000 to increase the compatibility of the composition with base metal electrodes.

[0033] The A/B ratio may be adjusted using any suitable technique. The A/B ratio may be adjusted to values of greater than 1.000 by adding a compound comprising an A group element. In some embodiments, the compound comprising an A group element (e.g., BaCO₃ or BaSiO₃) is coated on the barium titanate-based particles using a precipitation technique described further below. In other embodiments, the A group element compound may be added in particulate form to the composition. The A/B ratio may be adjusted to values of less than 1.000 by washing the composition with a fluid (e.g., water) in which a small percentage of the A group element compound is dissolved.

[0034] In some methods of the present invention, the barium titanate-based particles may be deagglomerated prior to the heat treatment step to reduce particle growth during heat treatment. The deagglomeration step may involve mechanically milling the particles to break up agglomerates.

[0035] It should also be understood that not all methods of the invention include an A/B ratio adjustment step and/or deagglomeration step.

[0036] The methods of the present invention may involve adding one or more dopant metal to the barium titanate-based composition. The dopant metal(s) may be added either before or after heat treatment. In some methods, one or more of the dopant metals may be added before heat treatment and one or more of the dopant may be added after heat treatment. The dopant metal(s) are selected to impart the resulting composition with the desired properties (e.g., electrical properties such as dielectric constant and dissipation factor). Any dopant metal known in the art may be used including Mg, Mn, W, Mo, V, Cr, Si, Y, Ho, Dy, Ce, Nb, Bi, Co, Ta, Zn, Al, Ca, Nd, and Sm. For some MLCC applications, Y, Mg and Mn may be preferred dopant metals. Silicon compounds (e.g., SiO₂, BaSiO₃) added as dopant metals may function as sintering aids which reduce sintering temperatures.

[0037] In some cases, the dopant metals are coated on surfaces of the barium titanate-based particles. The coating comprises at least one, but oftentimes more than one, dopant metal. The dopant metals in the coating are typically in the form of metal oxides, hydroxides, or hydrous oxides. The form of the dopant metal compounds depends, in part, on the particular dopant metal and the coating process.

[0038] The dopant metal coatings may be formed using any suitable coating process. For example, the dopant metal coating may be formed by precipitating the dopant metal compound(s) from an aqueous solution. One suitable precipitation technique involves forming a mixture of barium titanate-based particles and appropriate dopant metal solutions. A base is added to the mixture to cause the dopant metal solutions to precipitate on surfaces of the barium titanate-based particles. In some cases, the base may be added to the mixture in a manner that causes the dopant metals to sequentially precipitate onto surfaces of the particles. The resulting particles are coated with respective layers having different dopant metal compositions. This coating process and other suitable coating processes are described in U.S. patent application Ser. No. 10/194,936, filed Jul. 12, 2002, and entitled “Process for Coating Ceramic Particles and Compositions Formed From the Same,” by Venigalla et al, which is incorporated herein by reference in its entirety. Other suitable dopant coating processes have been described, for example, in commonly-owned U.S. Pat. No. 6,268,054, which is incorporated herein by reference in its entirety.

[0039] As described above, in some cases, the coating includes a series of chemically distinct layers. Each layer may comprise a different dopant metal compound. It should be understood that the respective layers of the coating may not be entirely chemically distinct. That is, there may be a small percentage of other dopant metal compounds within each layer and, in particular, near interfaces between adjacent layers. There may also be one or more layers that do not completely cover the particle. These small amounts of inhomogeneity within the layers do not significantly effect the overall uniformity of the composition.

[0040] In some cases, the coatings are homogeneous with each dopant metal distributed relatively uniformly throughout the coating. However, it should be understood that the homogeneous coatings may not include a perfectly homogeneous distribution of dopants.

[0041] The coatings may cover the entire particle surface, or only over a portion of the particle surface. In some embodiments, the coating may have a uniform thickness such that the thickness of the coating varies by less than 20% across the surface of an individual particle. In other cases, the thickness may vary by larger amounts. It is possible that some barium titanate-based particles may not be coated at all.

[0042] It should be understood that in some applications dopant metals are not added to the barium titanate-based composition.

[0043] As noted above, the barium titanate-based particles may be processed to form dielectric layers in MLCC devices. In some processes for forming MLCC devices, the particles may be mixed with a liquid (aqueous or non-aqueous) to form a slurry. Dispersants and/or binders may be added to the slurry to form a castable slip. The slip may be cast to form a green layer. To form an MLCC, additional electrode layers and green layers may be deposited on top of one another. The structure is sintered at elevated temperatures (e.g., between about 1200° C. and about 1300° C.) to convert individual particles into a plurality of grains fused together to form a densifled dielectric layer. The grain are of a similar, or slightly greater, size than the particles. The resulting MLCC includes alternating dielectric and electrode layers.

[0044] MLCCs produced according to the invention may include dielectric layers having grain sizes equal, or similar, to the sizes of particles from which the grains are formed. Similarly, MLCCs produced according to the invention may include dielectric layers having tetragonalities equal, or similar, to the tetragonalities of the particles from which the grains are formed. For example, the MLCCs may include dielectric layers having a grain size of equal to or less than about 0.3 micron and a tetragonality of equal to or greater than about 2.0, 2.5, or 3.0. The dielectric layers may also have a particle size of equal to or less than about 0.2 micron or 0.1 micron, and a tetragonality of equal to or greater than about 1.5, 2.0, 2.5, or 3.0. The particular combination of grain size and tetragonality may be selected based on the particular application and achieved using the methods described herein.

[0045] It should also be understood that the barium titanate-based particles of the present invention may be processed to form dielectric materials in other electronic devices, or otherwise as desired.

[0046] The following are examples that illustrates certain embodiments of the invention. It should be understood that these are not limiting and do not illustrate all embodiments of the invention.

EXAMPLE 1

[0047] This example shows the effect of heat treatment temperature on the tetragonality of barium titanate compositions.

[0048] Barium titanate particles were produced in a hydrothermal process. The barium source had a strontium concentration of greater than 200 ppm. The average particle size of the barium titanate was about 0.15 micron. The A/B ratio of the barium titanate composition was adjusted to a value of about 1.004. The barium titanate particles were divided into three samples.

[0049] The first sample was heat treated at a temperature of about 1000° C. The second sample was heat treated at a temperature of about 960° C. The third sample was heat treated at a temperature of about 920° C. Each sample had an average particle size of about 0.30 micron after heat treatment. The heat treatment times were selected to achieve the same average particle size for each sample.

[0050] Each sample was analyzed using an x-ray powder diffraction technique to determine the tetragonality. The diffraction technique used a copper target at a voltage of 40 kV and a current of 35 mA to provide a copper K-alpha monochromatic x-ray source, a sample rotation speed of 60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0051] FIG. 2 shows the x-ray diffraction pattern for the three samples. The tetragonalities were calculated from the diffraction pattern. The sample heat treated at 1000° C. had a tetragonality of about 3.00. The sample heat treated at 960° C. had a tetragonality of about 2.71. The sample heat treated at 920° C. had a tetragonality of about 2.18.

[0052] The results show that heat treating barium titanate particles according to a method of the present invention can produce barium titanate particles having high tetragonalities (e.g., greater than 2.0) and small particle sizes (e.g., 0.3 micron). The results also show that tetragonality increased with increasing heat treatment temperature within the range considered.

EXAMPLE 2

[0053] This example shows the effect of strontium concentration on the tetragonality of barium titanate compositions.

[0054] Barium titanate particle samples were produced in three different hydrothermal processes. In the first process, the barium source had a strontium concentration of 439 ppm. In the second process, the barium source had a strontium concentration of 387 ppm. In the third process, the barium source had a strontium concentration of 222 ppm. The strontium concentration was measured using an inductively coupled plasma (ICP) spectroscopy technique.

[0055] The average particle size for each sample was about 0.15 micron. The A/B ratios of the samples were not adjusted. Each sample was heat treated at 1000° C. The average particle size for each sample was about 0.40 micron after heat treatment.

[0056] Each sample was analyzed using an x-ray powder diffraction technique to determine the tetragonality. The diffraction technique used a copper target at a voltage of 40 kV and a current of 35 mA to provide a copper K-alpha monochromatic x-ray source, a sample rotation speed of 60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0057] FIG. 3 shows the x-ray diffraction pattern for the three samples. The tetragonalities were calculated from the diffraction pattern. The sample formed using a barium source that had a strontium concentration of 439 ppm had tetragonality of about 4.06. The sample formed using a barium source that had a strontium concentration of 387 ppm had tetragonality of about 4.64. The sample formed using a barium source that had a strontium concentration of 222 ppm had tetragonality of about 5.53.

[0058] The results show that heat treating barium titanate particles according to a method of the present invention can produce barium titanate particles having high tetragonalities (e.g., greater than 4.0) and small particle sizes (e.g., 0.4 micron). The results also show that tetragonality increased with decreasing strontium concentrations.

EXAMPLE 3

[0059] This example shows the effect of A/B ratio on particle size growth during heat treatment of barium titanate compositions.

[0060] Barium titanate particles were produced in a hydrothermal process. The barium source had a strontium concentration of 50 ppm. The average particle size of the barium titanate was about 0.060 micron. Three samples were produced by adjusting the A/B ratio to different values. The first sample was washed with deionized water to reduce the A/B ratio to 0.983. The second sample was washed with deionized water to reduce the A/B ratio to 0.995. The third sample was processed to coat barium carbonate on particle surfaces (using a precipitation technique involving adding barium hydroxide and ammonium carbonate to a slurry of the particles) to increase the A/B ratio to 1.013. The A/B ratio of a fourth sample was not adjusted from 1.000.

[0061] The samples were heat treated at 1000° C. The average particle size of each sample was measured. The sample having an A/B ratio of 0.983 had an average particle size of about 0.18 micron. The sample having an A/B ratio of 0.995 had an average particle size of about 0.22 micron. The sample having an A/B ratio of 1.013 had an average particle size of about 0.24 micron. The sample having an A/B ratio that was not adjusted (1.000) had an average particle size of about 0.35 micron.

[0062] Each sample was analyzed using an x-ray powder diffraction technique to determine the tetragonality. The diffraction technique used a copper target at a voltage of 40 kV and a current of 35 mA to provide a copper K-alpha monochromatic x-ray source, a sample rotation speed of 60 rpm, a scan rate of 0.1°/minute and a scan range of 44°-46° 2-Theta.

[0063] The sample having an A/B ratio of 0.983 had a tetragonality of about 2.29. The sample having an A/B ratio of 0.995 had a tetragonality of about 2.98. The sample having an A/B ratio of 1.013 had a tetragonality of about 3.75. The sample having an A/B ratio that was not adjusted (1.000) had a tetragonality of about 5.66.

[0064] The results show that adjusting the A/B ratio of samples to values of greater than or less than 1.000 can reduce particle size growth during heat treatment. The results also show that small barium titanate particles can be produced having high tetragonalities according to methods of the present invention.

[0065] Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.