Heat resistive phosphate sintered bodies and a process for producing the same

Abstract: A heat resistive phosphate sintered body comprises a solid solution of R y Zr₄Si x P 6-x O₂₄ in which 0 ≦ x ≦ 2, 2 / 3 ≦ y ≦ 2, R is a combination of one or more of cations having 2-3 valences, and x and y meet an electrically neutral condition, wherein a thermal expansion hysteresis loss is not more than 0.3%, and a dimensional change after a heat cycling in which heating and cooling are repeated between 100°C and 1,200°C at 100 times is not more than 1%. A process for producing such a heat resistive phosphate is also disclosed. The process comprises the steps of preparing a batch mixture, as a starting material, of (ZrO)₂P₂O₇ ZrP₂O₇, RO and/or a phosphate of R in which R is a combination of one or more of cations having two or three valences, and if necessary further SiO₂, calcining, milling and shaping the thus prepared mixture, and firing the thus shaped body, wherein a calcining temperature is not less than 1,400°C and equal to or higher than a firing temperature, the starting material is a powder milled to the average particle diameter of not more than 7 µm, and the firing temperature is not less than 1,300°C.

Description: The present invention relates to heat resistive phosphate sintered bodies and a process for producing the same. More particularly, the invention relates to heat resistive phosphate sintered bodies which have excellent heat cycling stability in that their dimensional change after heat cycling is not more than 1%, although the sintered bodies contain microcracks. The invention also relates to a process for producing such heat resistive phosphate sintered bodies. Recently, demands for materials having excellent heat resistance and low expansion have been increasing with advance in industrial technologies. Among such demands, it has been made clear that zirconyl phosphate (ZrO)₂P₂O₇ is promising as a material having excellent heat resistance and low expansion. Further, zirconyl phosphates of alkali metals such as sodium have recently been proposed as materials having good heat resistance and low coefficients of thermal expansion (Mat. Res. Bull., Vol. 19, pp 1451-1456 (1984), Journal of Materials Science 16, 1633-1642 (1981) and Yogyo Association Report, No. 95 [5], pp. 531-537 (1987)). Further, phosphate compounds of alkaline earth metals, which have specific compositions, have been proposed as compounds having low expansion [Mat. Res. Bull., Vol. 20, pp. 99-106, 1985, J. Am. Ceram. Soc., 70 [10] C232-C236 (1987), and U.S. Patent 4,801,566]. U.S. Patent 4,675,302 proposes ceramic materials having a fundamental composition of Ca 0.5 Ti₂P₃O₁₂ as having low expansion. Further, trials have been effected to combine a specific phosphate compound with zircon to obtain a composite material (Journal of Materials Science, Vol. 20, 4617-4623 (1985) and EP-0 260 893A2. In addition, EP-0 30 6242 A2 discloses solid solution of R y Zr₄Si x P 6-x O₂₄, sintered bodies containing such a solid solution, and a process for producing the same. However, although a phosphate compound such as zirconyl phosphate advantageously has low expansion, it is pointed out that such a phosphate is thermally decomposed at high temperatures of not less than 1,200°C, and a phosphorus component (P) evaporates. For example, when thermally treated at 1,400°C for 100 hours, zirconyl phosphate and sodium zirconyl phosphate exhibit as much as 19% and 36% in weight reduction, respectively. Further, ceramic materials proposed in U.S. Patent 4,675,302 are mainly used as substrates for low expansion optical reflectors which are free from deformations due to temperature changes and which have low expansion. Those optical reflectors are to be used in artificial satellites. As shown in Fig. 2, it is presumed that the temperature change is considered to be up to around 500°C at the maximum. However, no attention is paid to heat stability and wear resistance at high temperatures of not less than, for example, 1,200°C. Incidentally, as a process for producing phosphate compounds, a process is known, which uses Na₂CO₃, ZrO₂, ZrOCℓ₂·8H₂O, SiO₂, (NH₂)₂HPO₄, H₃PO₄, Nb₂O₅, Y₂O₃, SrCO₃, K₂CO₃, CaCO₃ and the like in combination [T. Oota and I. Yamai, Journal of the American Ceramic Society, 69 , 1 (1986)]. However, since a P₂O₅ component is singly produced in a decomposing step of ammonium phosphate or H₃PO₄ in this process, the concentration of phosphorus locally becomes higher so that a compound having a low melting point is formed during sintering. Consequently, large pores (voids) are formed around the low melting point compound in the sintered body to cause grave defects. Further, sintered bodies of specific phosphates suffer dimensional increase or decreased strength in a heat cycling deterioration test by repeatedly heating and cooling for instance between 100°C to 1,260°C. As the case may be, the sintered bodies may be broken. Having made various investigations to solve the above-mentioned conventional problems, the present inventors have found that heat resistive phosphate sintered bodies having excellent heat cycling resistance can be obtained by controlling a calcining step under specific conditions and setting a firing temperature at not less than a given temperature in a process for mixing, calcining and milling raw materials, shaping the mixture, and firing the shaped bodies. By a first aspect of the present invention there is provided a heat resistive phosphate sintered body as set out in Claim I. By a second aspect of the present invention there is provided a process for producing a heat resistive phosphate sintered body as set out in Claim II. The sintered body according to the present invention is fundamentally composed of a solid solution of R y Zr₄Si x P 6-x O₂₄. In this solid solution, a combination of one or more kinds of elements capable of exhibiting two or three valences is selected as R, x is a value more than 0 to not more than 2, and y is a value not less than 2 3 not more than 2. In the above composition, it is necessary that x and y meet an electrically neutral condition. If monovalent cations are present as R, high temperature stability becomes poorer, while if x exceeds 2, heat resistance decreases. R denotes a combination of one or more of elements capable of exhibiting two-valent or three-valent cations, and ordinarily denotes elements belonging to Group IIa of Periodic Table. It is preferable that R is constituted by at least one of barium (Ba), strontium (Sr) and calcium (Ca). The sintered body according to the present invention has the thermal expansion hysteresis loss of not more than 0.3%, preferably, 0.03 to 0.25%. Thus, the sintered body has excellent dimensional stability. With the sintered body of the present invention, the weight-reduction percentage due to evaporation of phosphorus can be suppressed to a low value of not more than 5% even after heat treatment at 1,400°C for 100 hours. In addition, the four point flexural strength after heat cycling in which heating and cooling are repeated between 100°C and 1,260°C at 100 times is not less than 100 kgf/cm², more preferably not less than 200 kgf/cm². Furthermore, the self-weight softening percentage after heat treatment at 1,400°C for 5 hours can be not more than 1%. Thus, the sintered body according to the present invention meets the requirements for heat resistive materials. The sintered body according to the present invention also has a small dimensional change rate. When the sintered body is thermally heated at 1,400°C for 100 hours, the dimensional change rate can be not more than 3.5%. Moreover, in the sintered body, the coefficient of thermal expansion in a temperature range from room temperature to 900°C may be as small as more than 25 x 10⁻⁷/°C, preferably not more than 10 x 10⁻⁷/°C. This low coefficient of thermal expansion is maintained even when the sintered body is subjected to the heat cycling in which heating and cooling are repeated between 100°C and 1,260°C at 100 times. Thus, it is seen that the sintered body according to the present invention has excellent thermal shock resistance. Therefore, the sintered bodies according to the invention, which have the above-mentioned characteristics, are favorably used as materials required to have heat resistance and thermal stability at high temperatures, for example, ceramic honeycomb structural bodies such as automobile exhaust gas purification catalyst carriers, rotary regeneration type ceramic heat exchangers, heat conductive type heat exchangers, turbocharger rotor housings, engine manifold-insulating materials and diesel particulate filters. In the process for producing the heat resistive sintered bodies in the present invention in which the starting materials are mixed, calcined, ground, and shaped, followed by firing, the shaped body is calcined at temperatures not less than 1,400°C. If the calcining temperature is less than 1,400°C, the heat resistance and the dimension stability in the heat cycling of the sintered bodies become poorer. Further, the calcining temperature should fundamentally be higher than the use temperature for the sintered bodies. As the starting materials, it is preferable to use a batch material composed of (ZrO)₂P₂O₇, ZrP₂O₇, RO and/or a phosphate of R, and if necessary SiO₂. The above compositions are stable compounds, which are difficult to make non-uniform during the shaping and firing steps and thus can be fired at high temperatures. On the other hand, when a conventionally used phosphate is used as a P₂O₅ source for the starting material, the mixture becomes non-uniform because phosphoric acid is liquid. Consequently, as mentioned above, the concentration of phosphorus becomes locally higher, so that a compound having a low melting point is formed during the calcining step. Moreover, it is important to mill the calcined powder to the average particle diameter of not more than 7 µm, preferably not more than 2.5 µm. The reason why the average particle diameter is set at not more than 7 µm is that the sintered body having the thermal expansion hysteresis loss of not more than 0.3% with excellent dimensional stability can be obtained. With respect to RO as the component for the starting material, a stable compound which is converted to RO during firing, for example, a hydroxide, a carbonate and a sulfate can selectively be used therefor. The starting material having the average particle diameter of not more than 50 µm, preferably not more than 10 µm is used. In the producing process according to the present invention, after the calcined powder is milled, the ground material is plasticized with use of a plasticizer. The plasticized material is shaped by extrusion, followed by drying and firing. This process is preferred from the standpoint of easy shaping. As the firing conditions for the fired products according to the present invention, the firing temperature is not less than 1,300°C, preferably 1,400 to 1,600°C, and the firing time is preferably 1-24 hours, more preferably 2-10 hours. When the firing temperature is set at not less than 1,300°C, the calcined and milled powder is fully sintered so that the fired products according to the present invention can be obtained. If the firing time is less than 1 hour, sintering is insufficient. On the other hand, if it is more than 24 hours, the strength deceases due to abnormal grain growth, and a different phase is precipitated owing to evaporation of the phosphorus component. Furthermore, the calcining temperature should be not less than the firing temperature, and preferably 1,500 to 1,600°C. If the calcining temperature is less than the firing temperature, abnormal grain growth occurs during the firing, or the microstructure of the fired body is insufficiently stabilized. Consequently, the dimensional increase due to the heat cycling becomes greater. The calcining time is preferably 1-24 hours, and more preferably 2-10 hours. Preferred embodiments of the present invention mentioned above are summarized below. a) The heat resistive phosphate sintered body has the thermal expansion hysteresis loss of 0.03-0.25%. b) The heat resistive phosphate sintered body has the coefficient of thermal expansion of not more than 10 × 10⁻⁷/°C. c) The heat resistive phosphate sintered body has the weight reduction rate of not more than 5% in the heat treatment of 1,400°C × 100 hours. d) The heat resistive phosphate sintered body has the self-weight softening percentage of not more than 1% in the heat treatment of 1,400°C × 5 hours. e) The heat resistive phosphate sintered body has the dimensional change rate of not more than 3.5% in the heat treatment of 1,400°C × 100 hours. f) The heat resistive phosphate sintered body has the four point flexural strength of not less than 100 kg/cm², preferably not less than 200 kg/cm². g) The heat resistive phosphate sintered body has the coefficient of thermal expansion of not more than 10 × 10⁻⁷/°C after the heat cycling. h) The heat resistive phosphate sintered body has a honeycomb structure. i) The heat resistive phosphate sintered body comprises a combination of one or more of elements of Group IIa in the Periodic Table. j) The process for producing the heat resistive phosphate sintered body comprises the step of milling the calcined powder to the average grain size of not more than 2.5 µm. k) The process for producing the heat resistive phosphate sintered body comprises the steps of plasticizing the batch mixture of the raw materials, shaping the mixture through extrusion, and drying and firing the shaped body. l) The process for producing the heat resistive phosphate sintered body comprises the keeping time in each of the steps of the calcining and the firing being 1-24 hours, preferably 2-10 hours. m) The process for producing the heat resistive phosphate sintered body comprises the calcining temperature of 1,500-1,600°C. n) The process for producing the heat resistive phosphate sintered body comprises the firing temperature of 1,400-1,600°C. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a graph illustrating a thermal expansion-shrinkage curve (thermal expansion hysteresis) of Example 1; Fig. 2 is a graph illustrating a heating-cooling schedule in heat cycling tests; and Fig. 3 is a view illustrating a method for measuring a self-weight softening percentage in a heat treatment of 1,400°C for 5 hours. In the following, the present invention will be explained based on particular examples, but the invention is not limited to them. Examples 1-12 and Comparative Examples 21-24 According to the formulating recipes shown in Table 1, zirconyl phosphate [(ZrO)₂P₂O₇], zirconium phosphate (ZrP₂O₇), calcium carbonate, strontium carbonate, barium carbonate, yttria, and silica were mixed. The grain size of the zirconia phosphate had preliminarily been adjusted by using a vibration mill filled with ZrO₂ sintered media having the diameter of about 5 mm. The grain size may be adjusted by using a pot mill or an attritor mill. As the ZrO₂ sintered media, ZrO₂ sintered media stabilized with MgO or those stabilized with Y₂O₃ was used. The chemical composition of the media used is shown in Table 2. The analyzed chemical compositions of the raw materials used are shown in Table 3. The formulated mixture shown in Table 1 was filled into a crucible, and calcined in an electric furnace in air under conditions shown in Table 1. The heating rate was 5-1,700°C/hr. After the calcining, the calcined body was crushed, and milled to the average particle size shown in Table 1 by using the vibration mill with the media. As mentioned above, the mixture might be milled with the pot mill or the attrition mill. After drying, 5 parts by weight of a 10% PVA aqueous solution was added to and fully mixed with 100 parts by weight of the milled powder. The mixture was press shaped into a shape of 25 x 80 x 6 mm by a mold under pressure of 100 kg/cm², and further rubber pressed under a pressure of 2 ton/cm². After the shaped body was dried, and dried body was fired in the electric furnace in air under conditions shown in Table 1. The heating rate was 5-1,700°C/hr. After the firing, the sintered body was worked into a shape of 3 × 4 × 40 mm as a flexural test piece prescribed in JIS R 1601 (1981). The thus obtained test pieces were subjected to measurements for the weight reduction rate and the dimensional change rate in a heat treatment of 1,400°C for 100 hours, the coefficient of thermal expansion in a range from 40-900°C, the four point flexural strength, the self-weight softening percentage, the open porosity, and the dimensional change in the heat cycling. For the measurement of the coefficient of thermal expansion, a push rod type differential thermal expansion meter employing a quartz glass was used. As shown in Fig. 1, the maximum value among differences in the coefficient of thermal expansion between during the heating and during the cooling was taken as the thermal expansion hysteresis. The temperature range in the measurement was 40-900°C. The four point flexural strength was measured according to the method prescribed in JIS R 1601. The self-weight softening percentage was determined as follows: First, a flexural test piece of 3 × 4 × 40 mm was placed on supports spaced by 30 mm as shown in Fig. 3, and heated at 1,400°C for 5 hours in air, a self-weight change rate Δx was measured, and the self-weight softening percentage was determined by filling the measured weight change rate Δx in the following equation: Self-weight softening percentage = Δx/ℓ x 100(%). The open porosity was measured by the Archimedean method. The melting point was visually judged by observing whether a sintered body cut in a shape of 3 × 4 × 5 mm was melted or not in a thermal treatment at 1,700°C for 10 minutes in the electric furnace. The heat cycling test was effected by repeatedly placing a test piece into the electric furnace held at a given temperature and taking it out therefrom. Fig. 2 shows a graph illustrating a heating-cooling schedule in the heat cycling test. The temperature is a temperature which a thermocouple set near the test piece exhibited. As to the solid solution of R y Zr₄Si x P 6-x O₂₄, it was judged that this solid solution was produced when the phase satisfied the indexes of CaZr₄(PO₄)₆ in JCPDS 33-321, SrZr₄(PO₄)₆ in JCPDS 33-1360 or BaZr₄(PO₄)₆ in JCPDS 34-95. As to the other crystalline phases, only whether the crystalline phases were present or not were determined by X ray diffraction patterns. The lattice constants of the solid solutions were calculated by determining a diffraction angle 2 ϑ at a (018) plane of R y Zr₄Si x P 6-x O₂₄ and that at a (208) plane thereof, determining d-values d₀₁₈ and d₂₀₈ and calculating "a" and "c" based on the following simultaneous equations: 1/d²₀₁₈ = 4/3a² + 64/c² and 1/d²₂₀₈ = 16/3a² + 64/c². As is clear from results of Examples 1-12 and Comparative Examples 21-24 shown in Table 1, the heat resistive sintered bodies according to the present invention could be obtained by setting the calcining temperature to not more than 1,400°C and milling the calcined powder to the average particle diameter of not more than 7 µm. Further, the sintered bodies were obtained by calcining, milling and sintering the batch mixtures selected from ZrP₂O₇, (ZrO)₂P₂O₇, SiO₂, the phosphate of R and RO under the conditions given in Table 1, in which R denotes an element capable of exhibiting two- or three-valence cations. As explained above, according to the process for producing the heat resistive phosphate sintered bodies in the present invention, the heat resistive phosphate sintered body having the solid solution composed of R y Zr₄Si x P 6-x O₂₄, the thermal expansion hysteresis loss of not more than 0.3%, and the dimensional change of not more than 1% after the heat cycling in which heating and cooling are repeated between 100°C and 1,260°C at 100 times can be obtained. Therefore, when the heat resistive phosphate sintered body according to the present invention is shaped as articles required to exhibit heat resistance and dimensional stability in heat cycling, for instance, in the form of honeycomb structural bodies as shaped by extrusion, the sintered body can be applied as a rotary regeneration type ceramic heat exchanger, a heat conductive heat exchanger, a diesel engine particulate filter, an automobile exhaust gas purification catalyst carrier, a catalyst combustion carrier, insulating materials for housing for ceramic turbocharger rotors, engine manifold, etc. The housing and the engine manifold may be shaped by slip casting, press molding, or injection molding. Example 13 Zr₂P₂O₉, ZrP₂O₇, SrCO₃, and SiO₂ shown in Table 3 were measured in amounts of 52.5 wt%, 22.8 wt%, 19.5 wt% and 5.2 wt%, respectively, and mixed together. The resultant mixture was charged into an alumina crucible, and calcined in air inside the electric furnace at 1,500°C for 5 hours. The calcined product was milled by a vibration mill for 12 hours to the average grain diameter of 0.8 µm. As grinding media, grinding media made of zirconia stabilized with Y₂O₃ shown in Table 2 were used. The resultant milled powder was kneaded with water and methylcellulose in the kneader, and a bullet was produced from the kneaded matter by a de-airing auger machine, and extruded with a plunger type extruder. After drying, an extrudate was fired in air in the electric furnace at 1,420°C for 5 hours. The characteristics of the thus obtained honeycomb fired body are given below. The mean pore size and the total pore volume were measured by a Mercury porosimeter. Honeycomb size : 45 mm in diameter × 100 mm long Cell structure : Square cell (thickness of 0.15 mm (6 mil), density of 62 cell/cm² (400 cell/inch²)) Mean pore size : 0.4 µm Total pore volume : 0.06 cc/g Coefficient of thermal expansion: -6 × 10⁻⁶/°C (40 to 900°C) Example 14 (ZrO)₂P₂O₇, ZrP₂O₇, BaCO₃ and SiO₂ shown in Table 3 were measured in amounts of 49.2 wt%, 21.3 wt%, 24.6 wt% and 4.9 wt%, respectively, and mixed together. The resultant mixture was charged into an alumina crucible, and calcined in air inside the electric furnace at 1,500°C for 5 hours. The calcined product was milled by a vibration mill for 12 hours to the average grain diameter of 1.6 µm. As the grinding media, grinding media of zirconia stabilized with Y₂O₃ shown in Table 2 were used. The resultant milled powder was kneaded with water and methylcellulose in the kneader, and a bullet was produced from the kneaded matter by the de-airing auger machine, and extruded with the plunger type extruder. After drying, an extrudate was fired in air in the electric furnace at 1,650°C for 5 hours. The characteristics of the thus obtained honeycomb fired body are given below. The pore diameter and the pore volume were measured by the Mercury porosimeter. Honeycomb size : 45 mm in diameter × 100 mm long Cell structure : Square cell (thickness of 0.15 mm (6 mil), density of 62 cell/cm² (400 cell/inch²) Mean pore size : 0.37 µm Total pore volume : 0.07 cc/g Coefficient of thermal expansion: 9 × 10⁻⁶/°C (40 to 900°C)

Claims: A heat-resistive phosphate sintered body comprising a solid solution of R y Zr₄Si x P 6-x O₂₄ in which 0 < x ≦ 2, 2/3 ≦ y ≦ 2, R represents one or more cations having 2-3 valency, and x and y meet a condition of electrial neutrality, wherein the thermal expansion hysteresis is not more than 0.3%, the dimensional change in a heat cycling test in which a heating and cooling cycle between 100°C and 1,200°C is repeated 100 times is not more than 1%, and the four point flexural strength of the sintered body after said heat cycling test is not less than 100 kg/cm². A heat-resistive phosphate sintered body according to Claim 1, which has microcracks and wherein the thermal expansion hysteresis of the sintered body is in the range 0.03-0.25%. A heat-resistive phosphate sintered body according to Claim 1 or Claim 2, wherein the coefficient of thermal expansion of the sintered body is not more than 10 x 10⁻⁷/°C. A heat-resistive phosphate sintered body according to any one of Claims 1 to 3, wherein the weight reduction of the sintered body after heat treatment at 1,400°C for 100 hours is not more than 5%. A heat-resistive phosphate sintered body according to any one of Claims 1 to 4, wherein the self-weight softening percentage of the sintered body after heat treatment at 1,400°C for 5 hours is not more than 1%. A heat-resistive phosphate sintered body according to any one of Claims 1 to 5, wherein the dimensional change of the sintered body after heat treatment at 1,400°C for 100 hours is not more than 3.5%. A heat-resistive phosphate sintered body according to Claim 6, having a four point flexural strength of not less than 200 kg/cm². A heat-resistive phosphate sintered body according to any one of Claims 1 to 7, which has a honeycomb structure. A heat-resistive phosphate sintered body according to any one of Claims 1 to 8, wherein R comprises one or more elements of Group IIa in the Periodic Table. A heat-resistive phosphate sintered body according to Claim 10, wherein R consists of at least one of Ca, Sr and Ba. A process for producing a heat-resistive phosphate body comprising the steps of preparing a batch mixture, as a starting material, of (ZrO)₂P₂O₇, ZrP₂O₇, RO and/or a phosphate of R in which R represents one or more of cations having valency of two or three, and optionally SiO₂, calcining, milling and shaping the thus prepared mixture, and firing the thus shaped body, wherein the calcining temperature is not less than 1,400°C and equal to or higher than the firing temperature, said mixture to be shaped is a powder milled to an average particle diameter of not more than 7 µm, and the firing temperature is not less than 1,300°C. A process for producing a heat-resistive phosphate sintered body according to Claim 11, wherein the calcined powder is milled to an average particle diameter of not more than 2.5 µm. A process for producing a heat-resistive phosphate sintered body according to Claim 11 or Claim 12, comprising the step of plasticizing the batch mixture before the shaping step. A process for producing a heat-resistive phosphate sintered body according to any one of Claims 11 to 13, wherein the duration of each of the steps of calcining and firing is in the range 1-24 hours. A process for producing a heat-resistive phosphate sintered body according to any one of Claims 11 to 14, wherein the calcining temperature is in the range 1,500-1,600°C. A process for producing a heat-resistive phosphate sintered body according to any one of Claims 11 to 15, wherein the firing temperature is in the range 1,400°C-1,600°C. A process for producing a heat-resistive phosphate sintered body according to any one of Claims 11 to 16, wherein the shaping step is effected by extrusion.

Inventors: MATSUHIRO KEIJI, OHASHI TSUNEAKI, MATSUHIRO, KEIJI, OHASHI, TSUNEAKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35