Process for producing a dielectric ceramic

Abstract: A process for producing a dielectric ceramic comprising a first heat-treating substep comprising heating said formed product up to a temperature of from 1,500 to 1,700°C at a rate of from 100 to 1,600°C/min. and thereafter maintaining the formed product at the temperature for at least one minute, and a second heat-treating substep comprising maintaining the product at a temperature of from 1,200 to 1,600°C for not less than 10 minutes. This process makes it possible to control the dielectric characteristics, such as resonant frequency temperature property of a dielectric ceramic by changing only the conditions of heat treatment without changing the composition, and the ceramic is useful in high frequency use.

Description: The present invention relates to a process for producing a dielectric ceramic, and in particular to a process for producing a dielectric ceramic of which temperature property is capable of being controlled without change in the composition thereof and which are therefore suitable for high frequency use. With reference to the prior art, a process for the preparation of dielectric ceramics is known from JP-A-6217102 and a process for the preparation of perovskite-type dielectric ceramics is known from GB-A-2166431. Temperature property of dielectric ceramics is generally controlled by changing the composition thereof, that is, changing the kinds or contents of constituent elements thereof. In particular, dielectric ceramics used in the micro wave band are required to have a high relative dielectric constant and unloaded Q and a good resonant frequency temperature property. The dielectric ceramics with a composition that satisfy such requirements include, for example, ceramics of a Ba(Zn,Ni) 1/3 (Ta,Nb) 2/3 O₃ system having a perovskite-type complex structure. In the ceramic of the above composition, a part of Zn ions are replaced with Ni ions for improvement in the temperature property. This replacement acts so as to reduce the relative dielectric constant and unloaded Q. In order to compensate these reduction, Ta ions are partially replaced with Nb ions in attempt to improve the relative dielectric constant. However, this results in deterioration of the temperature property and unloaded Q. As described above, only change in composition can not improve all of the properties required. In addition, it causes the problem that the production unit used needs to be cleaned thoroughly before production, every time the composition is changed. Accordingly, an aim of the present invention is to solve the above problems and to thereby provide a process for producing a dielectric ceramic that can produce a ceramic having desired properties without change in composition of the dielectric ceramic. According to the present invention there is provided a process for producing a dielectric ceramic comprising the steps of finely dividing and thereafter calcinating a raw material, finely dividing the calcinated product, making the size of the particles uniform, forming the powdery product obtained to produce a formed product, and then heat-treating the formed product, wherein said heat-treating step comprises: a first heat-treating substep comprising heating said formed product up to a temperature of from 1,500 to 1,700°C at a rate of from 100 to 1,600°C/min. and thereafter maintaining the formed product at the temperature for not less than one minute, and a second heat-treating substep comprising maintaining the product thus produced through said first heat-treating substep at a temperature of from 1,200 to 1,600°C for not less than 10 minutes, characterised in that the dielectric ceramic has a perovskite-type crystal structure having a composition represented by the general formula (I): Ba x A y B 1-x-y F z O w (I) wherein A represents at least one element selected from the group consisting of Mg, Zn, Ni and Co; B represents at least one element selected from the group consisting of Ta and Nb; x, y and z are numbers represented by 0.48≦x≦0.52, 0.15≦y≦0.19, and 0.00025≦z≦0.05, respectively; and w represents such a number as to neutralize the total electric charge of the cations of Ba, A and B and the anions of F so that said ceramic may become electrically neutral as a whole. According to the production process of the present invention, it is possible to control the dielectric characteristics, i.e., unloaded Q, dielectric constant and resonant frequency temperature property of a dielectric ceramic by changing only the conditions of heat treatment and without changing the composition. Moreover, it is possible to improve the unloaded Q and temperature property (τ f ) with no influence on the dielectric constant, or to control the temperature property, while maintaining its high dielectric constant and unloaded Q. With reference to the drawing: Fig. 1 shows a diffraction pattern of one of the dielectric ceramics produced by the process of the present invention. In the first heat-treating substep of the process of the present invention, temperature for firing is rapidly raised. Heating needs to be carried out at a rate of from 100 to 1,600°C/min., preferably from 300°C to 1,600°C/min. If the heating rate is less than 100°C/min., the ceramic obtained has an insufficient sintered density. If the heating rate is over 1,600°C/min., the ceramic gets brittle. After the temperature is raised, the formed product is maintained at 1,500 to 1,700°C, preferably 1,550°C to 1,650°C. If the temperature is less than 1,500°C, the sintered density of the dielectric ceramic does not increase sufficiently. If the temperature is over 1,700°C, on the other hand, the ceramic obtained is liable to get brittle. The formed product is maintained at the temperature, after the rapid raising of temperature, for one minute or more, preferably for from 2 minutes to 4 hours. Where the formed product is maintained at a higher temperature, a shorter time is enough; whereas where at a lower temperature, a longer time is needed. The first heat-treating substep may be continuously followed by the next firing substep, the second heat-treating substep. Alternatively, after the first heat-treating substep, the formed product may be cooled to room temperature once, and thereafter it may be subjected to the second heat-treating substep. In the second heat-treating substep, firing is carried out at a temperature of from 1,200 to 1,600°C, preferably from 1,400 to 1,500°C. The firing time of 10 minutes or more is enough. Normally, it is fired for from 10 to 50 hours. If the firing temperature is lower than 1,200°C, the temperature property (τ f ) of the ceramic obtained is not improved sufficiently; whereas if the firing temperature is over 1,600°C, components may vigorously evaporates, so that change in the composition is liable to occur. In the case where the formed product is heated up to a given temperature so that the second heat-treating substep may be carried out, a heating rate therefor is not limited. According to the process of the present invention, the controlling of the firing temperature and time in the second heat-treating substep makes it possible to regulate dielectric properties, i.e., unloaded Q, relative dielectric constant and temperature property of resonant frequencies, although the composition is unchanged. There is no limitation on the sort of atmosphere in which the two heat-treating substeps are carried out; any of oxidizing atmospheres, reducing atmospheres and inactive atmospheres may be used. Also, in this invention, there may be added aids for promoting sintering such as manganese and lithium, or a slight amount of metallic elements such as antimony, zirconium, titanium and vanadium or anions such as chlorine as elements for replacement. The process of the present invention is used to produce dielectric ceramics having a pervoskite-type complex crystal structure. With this in mind, having substantially a perovskite-type complex crystal structure means that if a trace amount of a second phase other than a perovskite-type complex crystal structure is found in a ceramic in identification of phases by X-ray diffractometry, the ceramic is regarded as a perovskite-type complex crystal structure ceramic. A process according to the present invention produces dielectric ceramics with a perovskite-type crystal structure having a composition represented by the following general formula (I): Ba x A y B 1-x-y F z O w (I) wherein A represents at least one element selected from the group consisting of Mg, Zn, Ni and Co; B represents at least one element selected from the group consisting of Ta and Nb; x, y and z are numbers represented by 0.48≦x≦0.52, 0.15≦y≦0.19, and 0.00025≦z≦0.05, respectively; and w represents such a number as to neutralize the total electric charge of the cations of Ba, A and B and the anions of F so that said ceramic may become electrically neutral as a whole. In the general formula (I), x is a number of 0.48≦x≦0.52, preferably 0.49≦x≦0.51; y is a number of 0.15≦y≦0.19, preferably 0.16≦y≦0.18; and z is a number of 0.00025≦z≦0.05, preferably 0.0005≦z≦0.01. The w represents a number such that the total electric charge of the cations of Ba, A and B and the anions of F is neutralized and said ceramic thereby become electrically neutral as a whole. The value of w will be automatically fixed if the values of x and y, and optionally the valence of Co, if Co is contained, are fixed. The w is normally a number of from 1.49 to 1.51. When a dielectric ceramic with a perovskite-type complex crystal structure represented by the general formula (I) is produced according to the process of the present invention, powdery raw materials of constituent metallic elements may be weighed depending on a desired composition, blended at a desired proportion and dried, and then the materials may be subjected to the process of the present invention. The formulation of the powdery raw materials may be conducted with consideration of easiness or difficulty in evaporation of the elements, as conducted conventionally. The dielectric ceramic used for high frequencies is generally required to have a relative dielectric constant of 24 or more and an unloaded Q of 6,000 or more. These requirements are met by said ceramics of the general formula (I) produced by the process of the present invention. It is presumed that the firing by rapidly temperature-raising in the first heat-treating substep effectively make the dielectric ceramic dense, and the following firing enhances the regularity of arrangement of the cations as perovskite-type complex crystal structure; hence the unloaded Q and temperature property (τ f ) can be improved while the relative dielectric constant is maintained at an adequate level. EXAMPLES From powders of barium carbonate, magnesium oxide, zinc oxide, nickel oxide, cobalt oxide, tantalum oxide, niobium oxide, barium fluoride and magnesium fluoride each having a purity of 99.9 % by weight or more, necessary powders were selected, and blended to give Test Sample Nos. 1 to 51 which are classified to seven kinds of compositions shown in Table 1 (No. 1 to 36: working examples, No. 37 to 51: comparative examples). The Test Samples were each placed in a ball mill pot together with pure water and wet mixed with resin coated balls for 16 hours. The resulting mixture was taken out of the pot, dried at 150°C for 3 hours, and then calcinated in oxygen at 1,000°C for 2 hours. After the calcination, the mixture was ground into particles, which were passed through a sieve of 354 µm (42 mesh); the particle size thereof were made uniform. The resulting powder was primarily formed into a disc with a diameter of 10 mm and a thickness of about 5 mm under a pressure of 49 MPa (500 kg/cm²) in a mold, and the disc was pressed under a pressure of 196 MPa (2,000 kg/cm²) using a hydrostatic press to give a pressure-formed product. Subsequently, the formed products thus obtained for the examples were each treated in an oxygen atmosphere under conditions given in Table 2 to give dielectric ceramics. For the dielectric ceramics obtained, the composition, and the results of measurement of relative dielectric constant (ε r ), unloaded Q (Qu) and temperature coefficient of resonant frequencies (τ f ) are given in Table 2. The measurements of the above properties were conducted as follows. Relative dielectric constant and unloaded Q Each ceramic obtained was cut into a disc with a diameter of 5.77 mm and a thickness of 2.90 mm, on which relative dielectric constant and unloaded Q at about 13 GHz were measured according to the Dielectric Rod Resonator Method. Temperature coefficient of resonant frequencies For each ceramic, the resonant frequency of the above disc was measured at every 10°C over a range of from 0° to 60°C according to the Dielectric Rod Resonator Method, and the drift of the resonant frequency at each temperature was determined. From the results, the temperature coefficient of the resonant frequency (τ f ) at 20°C was calculated. As is clear from Table 2, in the case of Test Sample Nos. 1 to 36, which include working examples of the present invention, it is possible to make the temperature coefficient (τ f ) of dielectric ceramics with an identical composition to possess varied values, with the ε r and Qu being highly maintained. The dielectric ceramic obtained in Example 5 was ground and the powder obtained was subjected to X-ray diffractometry, resulting in the X-ray diffraction pattern shown in Fig. 1. In the figure, the diffraction lines appended by indices are due to a hexagonal perovskite-type crystal structure. The intensities of the diffraction lines due to other crystal structures are 10/100 or less of those of the strongest diffraction lines (110) or (102) due to the perovskite-type crystal structure; hence the ceramic can be considered to substantially have a perovskite-type complex crystal structure. It was confirmed that the dielectric ceramics produced in the other working examples also had a perovskite-type complex crystal structure.

Claims: A process for producing a dielectric ceramic comprising the steps of finely dividing and thereafter calcinating a raw material, finely dividing the calcinated product, making the size of the particles uniform, forming the powdery product obtained to produce a formed product, and then heat-treating the formed product, wherein said heat-treating step comprises: a first heat-treating substep comprising heating said formed product up to a temperature of from 1,500 to 1,700°C at a rate of from 100 to 1,600°C/min. and thereafter maintaining the formed product at the temperature for not less than one minute, and a second heat-treating substep comprising maintaining the product thus produced through said first heat-treating substep at a temperature of from 1,200 to 1,600°C for not less than 10 minutes, characterised in that the dielectric ceramic has a perovskite-type crystal structure having a composition represented by the general formula (I): Ba x A y B 1-x-y F z O w (I) wherein A represents at least one element selected from the group consisting of Mg, Zn, Ni and Co; B represents at least one element selected from the group consisting of Ta and Nb; x, y and z are numbers represented by 0.48≦x≦0.52, 0.15≦y≦0.19, and 0.00025≦z≦0.05, respectively; and w represents such a number as to neutralize the total electric charge of the cations of Ba, A and B and the anions of F so that said ceramic may become electrically neutral as a whole. A process according to claim 1, wherein in the first heat-treating substep said formed product is heated at a rate of from 300 to 1,600°C/min. up to a temperature of from 1,550 to 1,650°C, and thereafter maintained at the temperature for from 2 minutes to 4 hours. A process according to claim 1 or claim 2, wherein in the second heat-treating substep said formed product is maintained at a temperature of from 1,400 to 1,500°C for from 10 to 50 hours. A process according to any preceding claim, wherein the first and second heat-treating substeps are carried out in an oxidizing atmosphere. A process according to any preceding claim, wherein x, y and z in said general formula (I) are numbers of 0.49≦x≦0.51, 0.16≦y≦0.18 and 0.0005≦z≦0.01, respectively. A process according to any preceding claim, wherein w in said general formula (I) is a number of from 1.49 to 1.51.

Inventors: HYUGA TAKEHIRO, MATSUMOTO KAZUTOSHI, MUKAI TETSUYA, HYUGA, TAKEHIRO, MATSUMOTO, KAZUTOSHI, MUKAI, TETSUYA
Applicants: SUMITOMO METAL MINING CO, SUMITOMO METAL MINING COMPANY LIMITED
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, H01G4, H01B3