Introduction
In order to protect the earth’s environment, the manufacturing industry in the 21st century will have to reduce its consumption of energy. Since the ceramic industry uses sintering processes to make its products, the use of high temperatures requires large quantities of energy. Ceramics are sintered by placing products in a uniform heating environment produced by a gas or electric furnace. Since with this method the temperature of a relatively large volume can be uniformly controlled, it is suitable for mass production. However, a great portion of the energy is consumed in maintaining the temperature of the surrounding furnace material or container rather than being used in product manufacturing. If energy can be efficiently used in the manufacturing of products by improving the sintering process, less energy will be consumed, which will in turn save energy. The microwave sintering process has attracted attention since the 1990’s [1, 2] as a sintering process that uses energy efficiently. The advantages of microwave sintering are:
1. Selective heating can be performed
2. Homogeneous heating is achieved, and
3. The temperature can be raised or lowered rapidly.
By employing these advantages efficiently, this process can be applicable in the production of low volume products of various kinds.
Barium titanate is a well-known material for multilayer ceramics and thick-film capacitors because of its high dielectric constant [3, 4]. Since it is used in large quantities as an electronic material, if low-temperature sintering could be applied the energy-saving effect would be large. This research was aimed at the development of a low-temperature sintering process that combines microwave sintering, which is expected to give an energy-saving effect, with liquid phase sintering, which is currently used as a low-temperature sintering method [5-9]. The component of the liquid phase used is barium borate. It is known that barium borate forms a liquid phase at 924oC when mixed with barium titanate [10] and does not form any other chemical components. Experiments that involved varying the amount of barium borate and modifying the additive method of barium borate were conducted. Also, the validity of combining microwave sintering and liquid phase sintering was examined and the sample were then compared with those produced by conventional additive-free electric furnace methods.
Experimental
Barium titanate (BT-01, Sakai Chemical Industry Co., Japan) was used as the raw material. Barium borate was used in its liquid phase. Barium carbonate (99.9%, Wako pure chemical industries Co., Japan) and boric acid (99.5% (minimum), Kanto Kagaku, Japan) were used in its preparation. The amount of liquid phase added varied between 0 and 5 mol%. The addition was carried out by two methods: 1) barium carbonate and boric acid were mixed with barium titanate, and 2) barium carbonate and boric acid were first mixed and calcined at 1120oC to form barium borate, which was then mixed with barium titanate.
Green pellets with a diameter of 18 mm were formed using a uniaxial pressure of 17 MPa and a cold isostatic pressure of 98 MPa. A magnetron multimode microwave furnace (domestic microwave oven) with a power output of 160 W and 320 W was used for the sintering experiments at 2.45 GHz. Samples were thermally insulated by ceramic fibers as well as embedded in fibers in order to minimize heat loss from the sample surface. Figure 1 shows a schematic illustration of the system. SiC acts as a susceptor. Conventional sintering was also carried out for comparison. The heating rate and sintering temperature were 10oC/min and 1140oC for 2 h, respectively. Density was measured using the Archimedes method. X-Ray diffraction analysis (XRD, RINT-2510, Rigaku. Co. Ltd., Japan) was conducted for phase identification using CuKα radiation (40 kV, 200 mA). Microstructure observations on polished and thermally etched surfaces were made using a scanning electron microscope (SEM, JSM-5600N, JEOL Ltd., Japan). The dielectric constant and dielectric loss were measured by our own evaluation system. A measurement frequency of 1kHz was used with temperatures ranging from room temperature to 280oC.
|
.jpg)
|
|
Figure 1. Illustration of sample setting for microwave sintering.
|
Results and Discussion
Figure 2 shows the relationship between the amount of liquid phase additive and the relative density of the sample sintered at 160 W for 10 min + 320 W for 20 min. Since the relative density was 50%, the additive-free sample could not be densified using these conditions. The relative density increased quickly with the addition of the liquid phase, and when the amount of addition reached 1.5-mol%, the relative density was 94%, a local maximum. From this maximum it decreased as additive level was increased. From this result, the amount of liquid phase addition was set as 1.5-mol% for all future experiments. Next, the influence of microwave irradiation time on sintered body densities was investigated. The irradiation time was increased in increments of 5 min up to 30 min using an irradiation energy of 320 W.
|
.jpg)
|
|
Figure 2. Relative densities of BaTiO3 with liquid phase as a function of BaB2O4 content.
|
The influence of irradiation time on relative density is illustrated in Figure 3. The sample relative density for an irradiation time of 5 min was an average of 65%. The relative density varied for every sample such that the microwave energy is considered not to have been transmitted uniformly to the sample. Relative density became 94% when the irradiation time was 10 min. The variation of relative density between samples was the smallest with an irradiation time of 20 min. However, when irradiation time was increased to 30 min, the relative density showed a declining trend and the variation between samples appeared.
|
.jpg)
|
|
Figure 3. Relative density of BaTiO3 with liquid phase as a function of sintering time.
|
In order to investigate the reason for these results, crystalline phases were identified by powder X-ray diffraction for irradiation times of 10 and 30 min. This result is shown in Figure 4. Although the barium borate phase shown by the circle exists for irradiation times of 10 min, it was not present in samples irradiated for 30 min. We postulate that the barium borate evaporates or the composition of the liquid phase changes as barium ions begin to dissolve from barium titanate, and the amorphous state is maintained under rapid cooling.
|
.jpg)
|
|
Figure 4. XRD patterns of BaTiO3 samples.
|
The microstructure of each sample was observed. SEM micrographs of sintered bodies are shown in Figure 5. At first, the particle size of barium titanate was investigated. Since the relative density of samples was 65% when irradiated for 5 min as shown in Figure 3, the sample was not densified. From the SEM observations, the diameter of barium titanate particles is almost the same as the diameter of the raw material particles indicating that densification has not occurred. When the irradiation time is increased to 10 min, the particle size grows from a submicron size to 3 µm, and the sample becomes densified. When the irradiation time was 10 min, the relative density of samples fluctuated, as shown in Figure 3. When using a rapid heating method like microwave sintering with a short irradiation time, temperature gradients appear in the samples. Therefore, the micrograph of 10 min in Figure 5 shows areas which have different grain growth. Consequently, when the microwave irradiation time was 10 min, the relative density of samples fluctuated. When the irradiation time was increased to 20 min, the particle diameter of barium titanate increased in size, and grain growth increased from 2 μm to about 5 μm. Moreover, since the variation in particle diameter decreased compared with samples where the irradiation time was 10 min, the fluctuation in density also decreased. These results clearly show the relation between grain growth and the relative density fluctuation. This also explains why the barium borate peak shown in Figure 4 disappeared from this microstructure. Since the whole grain consists of very small particles when the irradiation time is 5 min, it is not possible to distinguish the barium titanate and barium borate particles. When the microwave irradiation time is 10 min, many particles adhering to the surroundings of the barium titanate particles are observed. The number of particles observed when the irradiation time was increased to 20 min was less than the number of particles for 10 min. Furthermore, these particles were not observed when irradiation time was increased to 30 min. We postulate that the barium borate peak observed in Figure 4 disappeared when the microwave irradiation time was increased because these particles, considered to be barium borate particles acting as a liquid phase, decreased in number.
|
.jpg)
|
|
Figure 5. SEM photographs of BaTiO3-1.5 mol%BaB2O4 microwave sintered sample.
|
The relative density of 1.5 mol% barium borate samples prepared by each addition method using different sintering conditions is shown in Table 1. The sintering conditions which produced a relative density equivalent to those of the additive-free samples were 1240oC for 2 h when an electric furnace was used, and 800W for 30 min + 960 W for 10 min, when a microwave furnace was used.
Table 1. Relative density of BaTiO3 with 1.5 mol% BaB2O4 sintered under various conditions.
|
|
|
BaCO3 + H3BO3
|
94%
|
94%
|
|
BaB2O4
|
94%
|
97%
|
The dielectric constants and dielectric loss of each sample are shown in Figure 6. The additive-free sample (a) was sintered at 1240°C for 2 h using the electric furnace. Sample (b) was sintered with a microwave furnace at 160 W for 10 min + 320 W for 20 min with barium carbonate and boric acid added separately as a liquid phase. Sample (c) was calcined at 1120°C for 2 h using an electric furnace with barium carbonate and boric acid added separately. Sample (d) was sintered with a microwave furnace at 160 W for 10 min + 320 W for 20 min with barium borate. The result of the dielectric constant characteristic was compared, respectively. The almost same dielectric characteristics were indicated to be the sample (a) and the sample (b). On the other hand, the samples (c) and (d) had lost the dielectric characteristics. First, the difference between sample (a) and sample (b) was compared. Although the dielectric constant of the sample that underwent liquid phase sintering using microwave processing is small, below the Curie point, dielectric constant and dielectric loss showed the almost same curve. The reasons for the decrease in the dielectric constant are as follows: 1) it attributed to the 3% decrease in density of the sintered bodies, 2) the amount of liquid phase that has a dielectric constant of 11 [11] is 1.2 vol%.
|
.jpg)
|
|
Figure 6. Dielectric properties of each sample.
(a) Conventional sintering sample without additive,
(b) Microwave sintering sample with BaCO3 + H3BO3,
(c) Conventional sintering sample withBaCO3 + H3BO3,
(d) Microwave sintering sample with BaB2O4.
|
Second, the effect of sintering method on dielectric properties was investigated to the sample with the same liquid phase ingredient. Sample (b) and sample (c) have the same liquid phase addition method but differ in sintering method only: microwave furnace and electric furnace, respectively. The dielectric constant value of sample (b) was about 4000 at room temperature. On the other hand, the value of sample (c) was much larger, 70000. The resistance value at room temperature for sample (c) was 20 MΩ. Since sample (c) exhibited conductivity with current leakage, the apparent dielectric constant was large. That is, sample (c) lost its dielectric properties. Next we compared sample (b) with sample (d), which were produced using the same microwave sintering methods, but one had barium carbonate and boric acid additions, and the other was produced with pre-formed barium borate. When barium borate is used as a liquid phase additive in the sample, the contact time of the raw material, barium titanate, with barium borate is longer than when the mixture of barium carbonate and boric acid is used as a liquid phase additive. It is possible that some of the barium from barium titanate begins to dissolve into barium borate. Consequently, since the composition of barium titanate changed, it is thought that this affected the dielectric properties of sample (d). The change in the dielectric properties caused by the electric furnace sintering is analyzed in the same way. The electric furnace sintering time is longer than that in the microwave furnace. The contact time of the barium titanate with barium borate, which reacted with the barium carbonate and boric acid during the temperature rise, was enough to cause the change in dielectric properties of sample (c).
Conclusions
The following results were obtained when barium titanate was sintered with the simultaneous application of microwave sintering and liquid phase sintering.
• Although the sintering conditions of 800 W, 30 min + 960 W, 10 min are required for additive-free barium titanate, it was found that samples can be densified under the sintering conditions of 160 W, 10 min + 320 W, 20 min by using barium borate as a liquid phase sintering aid.
• The sample with the barium carbonate and boric acid mixture exhibited dielectric characteristics, whereas the sample with barium borate added directly was not a dielectric.
References
1. J. D. Katz, “Microwave Sintering of Ceramics”, Annual Review of Materials, 22 (1992) 153-170.
2. D. E. Clark and W. H. Sutton, “Microwave Processing of Materials”, Annual Review of Materials, 26 (1996) 299-331.
3. Y. Sakabe, “Recent Progress on Multilayer Ceramic Capacitors”, MRS International Meeting on Advanced Materials, 10 (1989) 119-129.
4. G. Goodman, “Ceramic Capacitor Materials”, in Ceramic Materials for Electronics (Ed. R. C. Buchanan), Marcel Dekker, New York, (1986) pp. 79-138.
5. K. R. Chowdary and E. C. Subbarao, “Liquid phase sintered BaTiO3”, Ferroelectrics, 37 (1981) 689-692
6. L. Burn, “Flux-sintered BaTiO3 dielectrics”, Journal of Materials Science, 17 (1982) 1398-1408.
7. I.C. Ho, “Semiconducting Barium Titanate Ceramics Prepared by Boron-Containing Liquid-Phase Sintering”, Journal of American Ceramic Society, 77 (1994) 829-832.
8. D. Kolar, M. Trintelj and L. Marsel, “Sintering and properties of BaTiO3-BaB2O4 Dielectrics”, Journal de Physique, 47 (1986) C1-447-450.
9. J.H. Lee, J.J. Kim, H.Wang and S.H. Cho, “Observation of Intergranular Films in BaB2O4-added BaTiO3 Ceramics”, Journal of Materials Research, 7 (2000) 1600-1604.
10. Y. Goto and L.E. Cross, “Phase Diagram of the BaTiO3-BaB2O4 System and Growth of BaTiO3 Crystals in the Melt” Yogyo-Kyokai-shi, 77 (1969) 355-356.
11. D. Eimerl, L. Davis, S. Velsko, E. K. Graham and A. Zalkin, “Optical, Mechanical, and Thermal Properies of Barium Borate”, Journal of Applied Physics, 62 (1987) 1968-1983.
Contact Details
|