Solution coated hydrothermal BaTiO3 for low-temperature firing

[0002] This invention relates to an improved barium titanate based dielectric composition and, more particularly, to a barium titanate composition wherein barium titanate crystallites are coated with a Bismuth metal-organic to achieve both a low-firing temperature and improved dielectric properties.

[0003] Barium titanate (BaTiO₃) based compositions are commercially used as capacitor dielectrics, particularly for multilayer ceramic (MLC) capacitors. Present manufacturing techniques produce MLCs with thickness layers of 5-10 μm. Yet, with decreasing operating voltages of MLCs, thickness layers are expected to be less than 5 μm. In order to achieve these thickness layers and produce reliable MLCs, average grain size of the sintered BaTiO₃ceramics need to be one-tenth the size of the layer thickness (i.e., ˜0.5 μm).

[0004] In addition to producing thinner layers for MLCs, manufacturers are actively pursuing methods for lowering the cost of MLCs. In particular, electrode materials make up a significant portion of the production costs for MLCs. Palladium-silver electrodes (e.g., 70/30 Pd-Ag) are substantially lower in price compared to platinum and pure palladium electrodes. Yet, in order to utilize Pd/Ag electrode materials, BaTiO₃-based dielectric materials must be sintered at or below 1100° C.

[0005] In order to prepare BaTiO₃-based ceramics that can be sintered below 1100° C. and have high densities with average grain sizes of 0.5 μm or less, a combination of hydrothermally-prepared powders with primary crystallites in the nanometer-sized range and a fluxing agent or a low-melting glass powder to promote densification of BaTiO₃by liquid phase sintering below 1100° C. is needed. Fluxing agents and low-melting glass powders have been used to densify BaTiO₃at lower sintering temperatures. Fluxing agents that have been used for flux-sintering BaTiO₃include lithium fluoride, boron oxide, copper oxide, lead germanate, cadmium silicate, and mixtures of cadmium oxide and Bismuth oxide. Also, low-melting glass compounds, such as borosilicate glass powders that contain significant amounts of PbO, BaO, Bi₂O₃, CdO, and ZnO modifiers, have been used to promote densification of BaTiO₃at lower sintering temperatures.

[0006] Kumar et al. describe a low-firing BaTiO₃that is sintered from hydrothermal BaTiO₃mixed with 3-5 wt % Bi₂O₃. (See: Kumar et al., “Preparation of Dense Ultra-Fine Grain Barium Titanate-Based Ceramics”, International Society of Applied Ferroelectrics (ISAF) Conference Proceedings, pp 70-73, 1992). Kumar et al. report that BaTiO₃ceramics can be sintered to high densities as low as 850° C. using this process. On the other hand, Burn (“Flux-Sintered BaTiO₃Dielectrics”, Journal of Materials Science,17, 1398-1408, 1982) indicates that commercially available BaTiO₃can be sintered to high densities only around 1100° C. using the fluxing agents mentioned above. Average grain size of the low-firing BaTiO₃ceramics prepared by Kumar et al. were in the range of 0.15-0.20 μm.

[0007] Kumar et al. also did preliminary work on using a Bismuth-based solution to coat hydrothermal BaTiO₃crystallites so as to form a low-firing BaTiO₃material. (See: Kumar et al., “Densification and Dielectric Properties of Hydrothermal BaTiO₃with Different Bi₂O₃Sources”, Ferroelectrics,Vol. 154, 283-288, 1994). Bismuth oxide powder was also mixed with the hydrothermal BaTiO₃crystallites, and the sintering results from the conventionally mixed batch was compared with the two solution-coated batches. BaTiO₃ceramics with 3.0 wt % equivalent Bi₂O₃addition could be sintered to >90% theoretical density as low as 800° C. for the two solution-coated batches. Yet, even though sintering results for solution-coated batches looked promising, the two solution-coated BaTiO₃powders did not show consistent densification results with each other. No details were given regarding the composition of the Bismuth-based solutions

[0008] Accordingly, it is an object of this invention to provide an improved BaTiO₃ceramic that can be fired at a relatively low firing temperature.

[0009] It is a further object of this invention to provide an improved BaTiO₃ceramic that exhibits consistent densification results.

[0010] It is another object of this invention to provide an improved BaTiO₃ceramic that exhibits improved dielectric properties, even when fired at a low temperature.

[0011] Hydrothermal BaTiO₃crystallites were coated with Bismuth solutions prepared from Bismuth metal-organics and anhydrous solvents. The Bismuth metal-organics were Bi 2-ethylhexanoate and Bi-neodecanoate. Bismuth oxide was also used as a comparison to the Bismuth solutions. BaTiO₃ceramics with either 3.0 wt % equivalent Bismuth oxide or 5.0 wt % equivalent Bismuth oxide were made by sintering the compacts between 700° C. and 1000° C. BaTiO₃ceramics that were coated by Bi-neodecanoate densified >90% theoretical as low as 800° C. for 3.0 wt % equivalent Bi₂O₃. Average grain sizes of 0.2-0.4 μm were observed for Bi-coated BaTiO₃ceramics. Dielectric K versus temperature measurements of Bismuth-coated BaTiO₃ceramics, sintered in the lower temperature ranges, showed consistently superior dielectric characteristics.

[0012] FIG. 1 is a diagram which illustrates the process by which the improved BaTiO₃ceramics were produced.

[0013] FIG. 2 is a plot of dielectric K and tan δ versus temperature for Bi-coated (3.0 wt % Bi₂O₃) BaTiO₃ceramics sintered to 800° C., wherein the Bismuth precursors were: (1) Bi 2-ethylhexanoate (Johnson Matthey Batch 23) and (2) Bi-neodecanoate (Strem, Batch 25). Also, dielectric K and tan δ curves for all figures are plotted at 1.0 kHz frequency.

[0014] FIG. 3 is a plot of dielectric K and tan δ versus temperature for Bi-coated (3.0 wt % Bi₂O₃) BaTiO₃ceramics sintered to 900° C., wherein the Bismuth precursors were: (1) Bi 2-ethylhexanoate (Strem, Batch 14); (2) Bi 2-ethylhexanoate (Johnson Matthey, Batch 23); (3) Bi-neodecanoate (Strem, Batch 25) and (4) Bi₂O₃(Aldrich, Batch 33).

[0015] FIG. 4 is a plot of dielectric K and tan δ versus temperature for Bi-coated (5 wt % Bi₂O₃) BaTiO₃ceramics sintered to 800° C., wherein the Bismuth precursors were: (1) Bi 2-ethylhexanoate (Strem Batch 15); (2) Bi 2-ethylhexanoate (Johnson Matthey, Batch 24); (3) Bi-neodecanoate (Strem, Batch 26); (4) Bi₂O₃(Aldrich, Batch 34); and (5) Bi 2-ethylhexanoate (Strem, Batch 47). Note that for Batch 47, BaTiO₃source is Cabot BT-8, instead of Cabot BT-10.

[0016] FIG. 5 is a plot of dielectric K and tan δ versus temperature for Bi-coated (5 wt % Bi₂O₃) BaTiO₃ceramics sintered to 900° C., wherein the Bismuth precursors were: (1) Bi 2-ethylhexanoate (Strem, Batch 15); (2) Bi 2-ethylhexanoate (Johnson Matthey, Batch 24); (3) Bi-neodecanoate (Strem, Batch 26) and (4) Bi₂O₃(Aldrich, Batch 34); and (5) Bi 2-ethylhexanoate (Strem, Batch 47). Note that for Batch 47, BaTiO₃source is Cabot BT-8, instead of Cabot BT-10.

[0017] FIG. 6 is a plot of dielectric K and tan δ versus temperature for Bi-coated (5 wt % Bi₂O₃) BaTiO₃ceramics sintered to 1000° C., wherein the Bismuth precursors were: (1) Bi 2-ethylhexanoate (Strem, Batch 15); (2) Bi 2-ethylhexanoate (Johnson Matthey, Batch 24); (3) Bi-neodecanoate (Strem, Batch 26) and (4) Bi₂O₃(Aldrich, Batch 34).

[0018] Two types of Bismuth metal-organics, Bi 2-ethylhexanoate (Strem Chemicals and Johnson Matthey) and Bi-neodecanoate (Strem Chemicals) were investigated as coatings for hydrothermal BaTiO₃crystallites. Such crystallites exhibit diameters in the range of 0.1 μm to 0.2 μm. By contrast, Bi₂O₃crystallites exhibit diameters in the range of 1.0μ. Since it is desired that the end result of the process be hydrothermal BaTiO₃crystallites with nanometer-thickness coating of a Bi-containing compound, it was realized that such a result could not be achieved through use of Bi₂O₃crystallites.

[0019] The goals for coating hydrothermal BaTiO₃powders were to (i) minimize the final amount of Bi₂O₃in the densified BaTiO₃ceramics, (ii) minimize the grain size, and (iii) lower the overall sintering temperatures for the Bismuth-coated BaTiO₃ceramics. Three different sources of commercially available hydrothermal BaTiO₃powders were sintered with and without the Bismuth coatings in order to determine if certain powder characteristics (e.g., crystallite size, surface area, Ba/Ti stoichiometry, etc.) in conjunction with the Bismuth coatings enhanced the densification of the hydrothermal BaTiO₃powders.

[0020] BaTiO₃ceramics with either 3.0 wt % equivalent Bismuth oxide or 5.0 wt % equivalent Bismuth oxide were made by sintering the compacts between 700° C. and 1000° C. BaTiO₃ceramics that were coated by Bi-neodecanoate densified >90% theoretical as low as 800° C. for 3.0 wt % equivalent Bi₂O₃. Average grain sizes of 0.2-0.4 μm were observed for Bi-coated BaTiO₃ceramics. Dielectric K versus temperature measurements of Bismuth-coated BaTiO₃ceramics, sintered in the lower temperature ranges, showed consistently superior dielectric characteristics.

[0021] Three hydrothermal BaTiO₃powders were used in the investigation: Cabot BT-10 (i.e. surface area of 10 m²/g), Cabot BT-6 (i.e., 6 m²/g), and Sakai BT-01 (i.e., crystallite size of 0.1 μm). Bismuth-containing chemicals used to coat the hydrothermal BaTiO₃powders were Bismuth 2-ethylhexanoate (Strem Chemicals), Bi 2-ethylhexanoate (Johnson Matthey), and Bi-neodecanoate (Strem Chemicals). Bismuth oxide (Bi₂O₃, Aldrich Chemicals) was used as a comparison to the Bismuth metal organic compounds, since the objective was to determine if the Bismuth precursors were more effective in coating the hydrothermal BaTiO₃crystallites than the Bi₂O₃particles.

[0022] Sources for the various BaTiO₃powders and Bismuth compounds were as follows:

[0023] Anhydrous solvents used to disperse the Bi-containing chemicals were 1-Butanol, 1-Propanol, and Ethanol. A summary of the various experimental parameters is given in Table I below.

[0024] FIG. 1 shows the procedure used for coating and densification of hydrothermal BaTiO₃powders (box 10) in accordance with the invention. The amount of the Bismuth metal-organic materials was varied, using either 3.0 wt % or 5.0 wt % equivalent Bi₂O₃additions to BaTiO₃powders. The Bismuth-containing solutions (box 12) were prepared in a glovebox by mixing the Bismuth metal-organic with an anhydrous solvent in an argon atmosphere.

[0025] The Bismuth-containing solutions were added to hydrothermal BaTiO₃powders and were mixed for six hours (box 14). After drying off the solvent (box 16), the residual organics in the Bismuth-coated hydrothermal BaTiO₃powders were decomposed in air at 500° C. for six hours (box 18). The heat-treated powders were mixed with an acryloid resin binder (box 20) (Rohm and Haas), and then uniaxially pressed at 210 MPa in a Carver press to form green ceramic disks (box 22). The acryloid binder was removed from the samples by a 2-step heating profile, first at 300° C. for 3 hours and then at 550° C. for 5 hours (box 24). Samples were then sintered in closed alumina (Al₂O₃) crucibles for two hours with temperatures ranging from 700° C. to 1000° C. (box 26).

[0026] Uncoated and Bismuth-coated BaTiO₃powders were analyzed (box 28) for surface area (BET) by a Quantachrome Monosorb BET unit. Weight loss and decomposition of the residual organics in the coated BaTiO₃powders were analyzed by a Perkin-Elmer Thermogravimetric Analyzer (TGA-7) and a Perkin-Elmer Differential Thermal Analyzer DTA 1700, respectively. Phase analysis of the uncoated and Bismuth-coated BaTiO₃powders was conducted on a Scintag DMC-105 x-ray diffractometer. Bulk densities of sintered BaTiO₃samples were measured at room temperature by Archimedes principle by immersing the samples in Xylene.

[0027] Microstructures of the sintered samples were observed on fracture surfaces of the samples with an ISI-SKI 130 scanning electron microscope (SEM, Akashi Beam Technology Corporation). Dielectric properties of ceramic disk samples with sputtered gold electrodes were measured on a computer controlled setup consisting of a Hewlett Packard 4274A LCR Bridge and a Delta Design temperature chamber. Measurements were taken on the samples on cooling from 200° C. to −50° C. over a frequency range of 100 to 10,000 Hz, at a cooling rate of 1.0° C./min.

[0028] A. Densification and Properties of Hydrothermal BaTiO₃Powders

[0029] Characteristics of the as-received Cabot BT-10, Cabot BT-6, and Sakai BT-01 hydrothermal BaTiO₃are presented in Table II below.

[0030] The BaTiO₃powders still exhibited a significant surface area after heat treatment at 800° C., ranging from 5.40 m²/g for Cabot BT-6 to 8.80 m²/g for Sakai BT-01, which indicates that these heat-treated powders still had fine crystallite sizes (10²˜10³nm).

[0031] Chemical analysis of the hydrothermal BaTiO₃powders showed the Cabot BT-10 powder to have had a slight excess of Barium (0.002-0.010 moles). This excess Barium in Cabot BT-10 composition is important when analyzing the sintering results of the various hydrothermal BaTiO₃powders.

[0032] Densification of Cabot BT-10 was >96% theoretical density at 1100° C. for a 2-hour soak time, which is 100° C. lower than that for Sakai BT-01 and 200° C. lower than that for Cabot BT-6 (see Table III below). A fine grain-size microstructure was observed for BaTiO₃ceramics of Cabot BT-10 composition that are sintered at 1100° C.; however, large grains (>5.0 μm) were observed in the BaTiO₃ceramics of this composition when sintered above 1200° C.

[0033] Results of densification and dielectric properties of ceramics that were sintered from Bismuth-coated (5.0 wt % equivalent Bi₂O₃) Cabot BT-10, Cabot BT-6, and Sakai BT-01 powders are shown in Table IV below. The Bismuth metal organic used to coat these hydrothermal BaTiO₃powders is a Bismuth 2-ethylhexanoate from Strem Chemicals. Ceramics of Bismuth-coated Cabot BT-10 composition densified as low as 800° C., compared to 900° C. for Bismuth-coated Cabot BT-6 and >1000° C. for Bismuth-coated Sakai BT-01 samples. The reason for Bismuth-coated Cabot BT-10 hydrothermal powder densifying at a lower temperature than the other hydrothermal BaTiO₃powders was the probable formation of a liquid phase due to combination of Bi₂O₃flux and the Barium-rich surface layer of hydrothermal BaTiO₃powder.

[0034] When this liquid phase does occur during sintering, it is likely to cause a glassy grain boundary phase that surrounds the pure BaTiO₃grains, which is typical for core-shell microstructures that are observed in BaTiO₃-based dielectric compositions. Indirect evidence of the formation of this Bismuth-containing glassy phase was illustrated by limited grain growth for Bismuth-coated BaTiO₃ceramics of Cabot BT-10 composition below 1000° C. and

[0035] the lowering and the broadening of cubic to tetragonal transition peak (Table IV). It has been reported that Bismuth does not rapidly diffuse to form a homogeneous composition with BaTiO₃, so that concentration gradients result in an average of localized Curie points broadening the dielectric anomaly, instead of a single transition peak.

[0036] Increasing the sintering temperature to 1000° C. for Bismuth-coated Cabot BT-10 BaTiO₃composition resulted in large grain growth and an increase in the Curie temperature to 129° C. The core-shell microstructure was most likely gone by 1000° C. in the BaTiO₃ceramics of this composition, since higher sintering temperature promoted a more homogeneous distribution of the Bismuth.

[0037] B. Densification and Dielectric Properties of Bismuth-Coated Cabot BT-10 BaTiO₃Using Different Bismuth Sources

[0038] As indicated above, the Bismuth-coated Cabot BT-10 composition exhibited better densification and dielectric properties than the other two Bismuth-coated hydrothermal BaTiO₃powders. Next, two other Bismuth sources, i.e., Bismuth oxide (Bi₂O₃) from Aldrich and Bismuth neodecanoate from Strem Chemicals, were compared to Bismuth 2-ethylhexanoate, in coating the Cabot BT-10 BaTiO₃powder. The solvent used to disperse the Bismuth neodecanoate and the Bismuth 2-ethylhexanoate was 1-Butanol, because Bismuth metal organics readily go into solution and remain so for an extended period of time.

[0039] Thermal gravimetric analysis shows the Bismuth-coated Cabot BT-10 BaTiO₃powders that were coated with either of the Bismuth metal-organics to have small weight losses (5.0˜8.0 wt %) and to have their weight loss finished by 500° C. Differential thermal analysis (DTA) of the Bismuth-coated BaTiO₃compositions also showed the decomposition of the organics to be complete by 500° C.

[0040] Theoretical densities and dielectric properties of Cabot BT-10 hydrothermal BaTiO₃, coated with different Bismuth sources at 3.0 wt % equivalent Bi₂O₃and at 5.0 wt % equivalent Bi₂O₃, are shown in Table V and Table VI (below), respectively.

[0041] Bismuth-coated Cabot BT-10 BaTiO₃coated with Bismuth neodecanoate, sintered to high densities as low as 800° C. for a 2-hour soak time for a 3.0 wt % Bi₂O₃addition, and as low as 700° C. for the same amount of time for a 5.0 wt % Bi₂O₃addition. Comparing the fracture surfaces of 3.0 wt % addition Bismuth-coated BaTiO₃ceramics that were sintered at 800° C., a uniform fine-grain size microstructure (i.e., 0.2˜0.4 μm) was observed for Cabot BT-10 BaTiO₃coated with Bismuth neodecanoate. On the other hand, hydrothermal BaTiO₃crystallites were still being sintered together for Cabot BT-10 BaTiO₃powders that were either coated with Bismuth 2-ethylhexanoate or mixed with Bi₂O₃.

[0042] X-ray dot maps of fracture surfaces of Bismuth-coated BaTiO₃ceramics showed the Bismuth ions to be dispersed throughout the microstructure for these ceramics that were sintered at 800° C. from Cabot BT-10 BaTiO₃powders coated with any one of the Bismuth sources. An X-ray dot map of the fracture surface of a BaTiO₃ceramic that was prepared from Cabot BT-10 BaTiO₃powder, coated with Bismuth neodecanoate and sintered to 1000° C., indicates that the Bismuth had not remained evenly distributed in the microstructure after a 2-hour soak time. Furthermore, the corresponding scanning electron micrograph of the Bismuth-coated BaTiO₃ceramic showed large grain growth. This grain growth was typical for BaTiO₃ceramics that were sintered at 1000° C. from Bismuth-coated Cabot BT-10 BaTiO₃powders. On the other hand, this large grain growth was not observed for other Bismuth-coated BaTiO₃compositions (i.e., Cabot BT-6 and Sakai BT-01) unless they were sintered well above 1000° C.

[0043] Dielectric properties of Bismuth-coated BaTiO₃ceramics that were sintered at 800° C. from Cabot BT-10 BaTiO₃coated with Bismuth neodecanoate were better than those for BaTiO₃ceramics that were sintered from Cabot BT-10 BaTiO₃powders coated with the other two Bismuth sources. Bismuth-coated BaTiO₃ceramics that were sintered at 800° C. from powders coated with Bismuth neodecanoate have a room temperature dielectric K value of 2115 and a tan δ value of 0.026 for 3.0 wt % equivalent Bi₂O₃addition, and also a room temperature dielectric K value of 1990 and a tan δ value of 0.053 for 5.0 wt % equivalent Bi₂O₃addition.

[0044] Dielectric K and tan δ vs. temperature curves for Bismuth-coated BaTiO₃ceramics at 3.0 wt % addition and at 5.0 wt % addition are shown in FIGS. 2, 3 and 4, 5, 6, respectively. For Bismuth-coated BaTiO₃ceramics that were sintered at 900° C. and 1000° C. from BaTiO₃powders that were coated with Bi-2-ethylhexanoate, the loss (tan δ) curves show a sharp decrease at the Curie temperature and then sharply increase above the Curie temperature.

[0045] Bismuth-coated hydrothermal Cabot BT-10 BaTiO₃densifies to high densities as low as 800° C. for a 2-hour soak for a 3.0 wt % equivalent Bi₂O₃addition. Bismuth-coated BaTiO₃ceramics densify at lower temperatures than those for non-coated BaTiO₃ceramics, due to the formation of a Bismuth glassy phase surrounding the hydrothermal BaTiO₃crystallites, which is similar to formation of core-shell microstructures in BaTiO₃-based dielectric ceramics. Indirect evidence of this formation of core-shell microstructures in Bismuth-coated BaTiO₃ceramics is observed in the lowering and the broadening of the cubic to tetragonal transition peak for these BaTiO₃ceramics, which indicates a distribution of local Curie temperatures resulting from localized distribution of Bismuth. Bismuth neodecanoate results in BaTiO₃ceramics densifying as low as 800° C. for 3.0 wt % equivalent Bi₂O₃addition and as low as 700° C. for 5.0 wt % equivalent Bi₂O₃addition. Uniform fine-grain size microstructure (0.2˜0.4 μm) was observed in these Bismuth-coated BaTiO₃ceramic materials. Above sintering temperatures of 900° C., large grain growth occurs in BaTiO₃ceramics that were sintered from Cabot BT-10 hydrothermal BaTiO₃powders which were coated with Bismuth neodecanoate. Hydrothermal BaTiO₃powders which were coated with Bismuth 2-ethylhexanoate also sintered to high densities, while exhibiting fine grain size microstructures.

[0046] It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For instance, while the above discussion has considered the pre-fired shapes (i.e., masses) as preforms, it is to be understood that such masses can also take the form of a paste or a liquid. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.