GB1602822A - Method for manufacturing ceramic powder materials - Google Patents

Description

(54) METHOD FOR MANUFACTURING CERAMIC POWDER MATERIALS (71) We, TOKYO SHIBAURA ELECTRIC COMPANY LIMITED, a Japanese Corporation, of 72 Horikawa-cho, Saiwai-ku, Kawasaki-shi, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improvements in the treatment of raw ceramic powder materials adapted to provide sintered ceramic products of high mechanical strength and more particularly to the treatment of ceramic powder materials consisting mainly of silicon nitride.

Worldwide studies have been made of sintered silicon nitride which displays prominent mechanical strength at high temperature. Known processes developed to this end include, for example, that which hot-presses silicon nitride powder mixed with magnesia, and that which similarly hot-presses silicon nitride powder mixed with yttria. These processes are found to provide a dense sintered product of silicon nitride.

However, a more dense construction does not offer excellent mechanical strength at high temperature. The reason is that a simply densified sintered product contains glassy material, which gives rise to a decline in the high temperature mechanical strength of the product.

Silicon nitride powder contains various kinds of impurities, and is not formed of silicon and nitrogen alone. For instance, silica is generally deposited on the surface of the particles of silicon nitride powder. Further, iron and calcium, for example, are often present in the silicon nitride powder as unavoidable impurities. Many of these impurities are generally present in the powder material in a form combined with oxygen. The resultant oxides are believed to cause a decline in high temperature mechanical strength of a sintered product. Thus, it has been found that raw ceramic powder material consisting mainly of silicon nitride and containing less than 2.0% by weight (as measured by activation analysis) of oxygen combined with unavoidable impurities, such as those mentioned above, is adapted to provide a sintered product having a good high temperature mechanical strength.It has also been found that it is advisable to decrease the amount of oxygen combined with the unavoidable impurities preferably to less than 1.0% by weight or most preferably to less than 0.5% by weight.

Accordingly it is an object of this invention to provide a process for reducing the oxygen content of a silicon nitride powder (that is a technical silicon nitride powder containing impurities in a form combined with oxygen).

The reduction of the oxygen content of the silicon nitride powder may be accomplished simply by heating the powder, but it has been found, in accordance with the present invention, that the time required to achieve the desired oxygen reduction can be markedly reduced by carrying out the heating in the presence of a non-sintered moulding of a ceramic material and/or a sintered moulding of a ceramic material having a porosity of at least 10%.

According to the invention, therefore, there is provided a process for reducing th oxygen content of a silicon nitride powder which comprises heating the powder in the presence of a heating time reducer which is a non-intered moulding of ceramic material or a sintered moulding of ceramic material having a porosity of at least 10%.

The heating is suitably carried out at a temperature of from 1,400 to 1,9000 C, preferably from 1,500 to 1,8500C, more preferably from 1,5500C to 1,8000C.

The process of the invention can be applied to reduce the oxygen content of silicon nitride alone or of a ceramic powder consisting mainly of silicon nitride.

Experiments indicate that in the case of a powder comprising silicon nitride powder mixed with alumina and yttria, where the content of aluminium ranges between 0.05 to 2.5% by weight, and the content of yttrium ranges from 0.4 to 8.0% by weight, as analyzed by the customary process; the total % by weight WO of oxygen as measured by activation analysis satisfies the following formula indicating the % by weight tl of aluminium and the % by weight Wy of yttrium:: atomic weight atomic weight of oxygen of oxygen Wg < X + 105 x (WAl x ------- + Wy x ) atomic weight atomic weight of aluminium of yttrium and X is smaller than or equal to 2.0, then it is possible to produce a sintered product of excellent property. The term X given in the above formula is preferably less than 1.0, most preferably less than 0.5.

Such a mixed silicon nitride/yttria/alumina powder may be prepared by heating, in accordance with the invention, a starting silicon nitride/yttria/alumina powder containing from 0.1 to 5% by weight of alumina and from 0.5 to 10% by weight of yttria to a temperature ranging between 1,400"C and 1,9000 C, preferably between 1,5000C and 1,8500 C, or more preferably between 1,5500C and 1,800 C.

When a silicon nitride/alumina/yttria powder is heated, it is preferred that part or the whole of yttrium be present in the heated mass in the form of a crystalline compound consisting of silicon nitride and yttrium oxide. The presence of this crystalline compound can be ascertained by the X-ray diffraction analysis. For the growth of said crystalline compound, it is desirable to apply heating at a temperature above 1,6000 C.

Silicon nitride/yttria/alumina powders meeting the above compositional requirements, having the oxygen content indicated by the formula given above and containing crystalline silicon nitride/yttrium oxide compounds are described and claimed in our copending Application No. 1497/78 (Serial No. 1602821).

The heating of the silicon nitride powder is, in accordance with the invention, carried out in the presence of a nonsintered moulding of ceramic material or a sintered moulding of ceramic material having a porosity of at least 10%. Ceramic materials which may be used include aluminium nitride, boron nitride, titanium nitride, silicon nitride and aluminium oxide. Among the above-listed compounds, aluminium nitride displays the most prominent effect of shortening heating time.

Application of the above-mentioned nonsintered or sintered ceramic moulding causes oxygen-containing gas evolved from the heated ceramic powder material to be securely absorbed in said moulding or to be given off to the outside therethrough, thereby supposedly carrying out effective deoxidation.

It is preferred that the heating may be carried out by charging the raw ceramic powder material and the moulding into a vessel made of a material which is nonreactive with the charged mass, desirably aluminium nitride. Heating is carried out in a nonoxidizing atmosphere such as nitrogen gas or any other inert gas.

The nonsintered ceramic moulding acting as a heating time reducer is prepared by mixing the ceramic material with an organic binder, followed by moulding. The nonsintered ceramic moulding acting as a heating time reducer generally has a porosity ranging approximately between 35 and 55%. For the object of this invention, however, the nonsintered moulding can be produced with a porosity-falling outside the above-mentioned range. A sintered ceramic moulding acting as a heating time reducer having a porosity of at least 10% can be prepared by heating such a nonsintered moulding at a high temperature in a nonoxidizing atmosphere.

Table 1 below shows examples of the methods of manufacturing a heating time reducer from nonsintered and sintered mouldings of aluminium nitride with various degrees of porosity.

TABLE I

Form of heating Porosity time reducer (No) Manufacturing method Nonsintered 70 Commercially available aluminium nitride powder is pulverized in a ball mill. An organic binder is added to the pulverized mass, followed by moulding at a pressure of 50 to 100 kg/cm2.

50 Substantially the same pracess as described above is used, except that the pIessure applied ranges between 500 and 700 Kg/cm2. - 30 30 Coarse aluminium nitride particles and fine aluminium nitride particles are mixed in the ratio of 7 : 3. An organic binder is added to the mixture, followed by moulding at a pressure of 5,000 Kg/cm2.

Sintered 50 The above-mentioned nonsintered aluminium nitride having a porosity of 70% is sintered one hour at 1,7000C in an atmosphere of nitrogen.

30 The above-mentioned nonsintered aluminium nitride having a porosity of 50% is sintered two hours at 1,800C in an atmosphere of nitrogen.

10 10 0.5% by weight of yttria is added to fine aluminium nitride powder. After moulding, the mass is sintered two hours at 1,8000C in an atmosphere of nitrogen.

" 8 2% by weight of yttria is added to fine aluminium (Com- nitride powder. After moulding, the mass is sintered parative for two hours at 1,7500C in an atmosphere of Example) nitrogen.

e The nonsintered or sintered moulding acting as a heating time reducer used in this invention need not be formed of a single compound, but may be prepared in the form mixed with, for example, yttria, alumina or silica. However, it is preferred that these ingredients be added in a smaller amount than 20% by weight. The reason is that where the heating time reducer contains a large amount (more than 20%) of the additives, then undesirable reactions will occur between said additives and the raw ceramic powder.

This invention will be more fully understood by reference to the examples which follow.

In the examples there were employed, as starting materials, silicon nitride powders having the elementary analyses given below.

Elementary Analysis (% by weight) Silicon Nitride Powder Silicon Nitrogen Aluminium Iron Calcium Oxygen A 58.6 1 36.1 - 0.25 0.21 4.3 B* 57.8 35.5 0.23 ~L 0.35 0.11 3.52

*CP-85 Grade manufactured by Advanced Engineering Materials Ltd.

(The oxygen content was determined by activation analysis, the nitrogen content by a gas analyzer, and the contents of the other elements by customary wet analytical methods. These methods were used to analyse the products obtained in the Examples).

COMPARATIVE EXAMPLE 1.

(a) Silicon nitride powder A was heated for one hour at 1,7000C in a vessel made of aluminium nitride. As a result, there was obtained a silicon nitride powder (A') formed of 59.2% by weight of silicon, 36.9% by weight of nitrogen, 1.8% by weight of oxygen, 0.28% by weight of iron and 0.25% by weight of calcium.

(b) 2% by weight of alumina and 5% by weight of yttria were added to the nonheated silicon nitride powder A and to the heated silicon nitride powder A' of decreased oxygen content. After moulding, the respective masses were sintered 2 hours at 1,8000C at a pressure of 500 Kg/cm2.

The sintered product from untreated silicon nitride powder A had a flexural strength of 53 Kg/mm2 at 1,200 C. On the other hand, the sintered product from the treated silicon nitride powder A' of reduced oxygen content has a flexural strength of 75 Kg/cm2 at 1,2000 C. The flexural strength test was carried out by the three point bending method under the following conditions: Sample size 3 X 3 X 35 mm Span 20 mm Crosshead speed 0.5 mm/min EXAMPLE 1.

Silicon nitride powder A was heated, mixed in substantially the same manner as described in Comparative Example 1, except that a nonsintered moulding of a heating time reducer of aluminium nitride was mixed with said raw silicon nitride powder in the heating step (a) and heating was applied for 30 minutes, obtaining the same result as in Comparative Example 1. In this case, application of the nonsintered moulding of the heating time reducer of aluminium nitride decreased the heating time by half. When the nonsintered moulding of aluminium nitride was replaced by a sintered moulding of aluminium nitride having a porosity of at least 10%, the same result was obtained.

COMPARATIVE EXAMPLE 2.

A mixture of silicon nitride Powder A, alumina and yttria containing 2.5% by weight of alumina and 4.8% by weight of yttria, was prepared. The mass was placed in a vessel made of aluminium nitride, followed by heating of 2 hours at 1,7500 C.

The heated mass contained 55.7% by weight of silicon, 1.3% by weight of aluminium, 3.8% by weight of yttrium, 0.31% by weight of iron, 0.26% by weight of calcium, 32.6% by weight of nitrogen and 2.6% by weight of oxygen. As analyzed by X-ray diffraction, a crystalline compound of silicon nitride and yttria (Si,N4 . Y2O,) having a molar ratio of 1:1 was grown in the heated mass. It was found that 90 / of the yttrium content in the raw ceramic powder was present in the form of said silicon yttrium oxynitride crystal compound.

After moulding, the raw ceramic powder was sintered for 2 hours at 1,8030C at a pressure of 500 Kg/cm2. The same flexural strength test as applied in Com parative Example 1 proved the sintered product to have a flexural strength of 93 Kg/mm2 at 1,2000 C.

EXAMPLE 2.

The silicon nitride/yttria/alumina mixture of Comparative Example 2 was heated in substantially the same manner as described in Comparative Example 2, except that a nonsintered moulding of a heating time reducer of aluminium nitride was mixed with said mixed powder and heating was applied for 70 minutes, thereby effecting as much deoxidation as in Comparative Example 2. When the nonsintered aluminium nitride was replaced by sintered aluminium nitride having a porosity of at least 10%, the same degree of deoxidation was realized.

COMPARATIVE EXAMPLE 3.

Silicon nitride powder B was used as a starting material X-ray diffraction analysis showed said starting material to contain 87% of a-type silicon nitride, whose particle size was determined to be 1.8 microns by the Fisher Sub-Sieve Sizer.

5% by weight of Y2O, was added to the raw silicon nitride powder followed by crushing and mixing for 100 hours in an alumina pot filled with alumina balls. The abraded portions of the alumina pot and the alumina balls were carried into the crushed silicon nitride powder now mixed with Y2O,, with a resultant increase in the alumina content.The crushed silicon nitride/yttria alumina powder had a particle size of 1.1 microns as measured by the Fisher Sub-Sieve Sizer, and the following analysis:

Element | Silicon Nitrogen Aluminium Iron Calcium Yttrium Oxygen by by weight 56.5 34.7 1.31 0.32 0s 10 3.69 5.31 The above-mentioned mixed powder was then charged into an aluminium nitride vessel, followed by heating for 2 hours at 1,7300 C. The heated mass was analyzed by X-ray diffraction and shown to contain a silicon yttrium oxynitride crystal compound (Si,N4 .Y2O). A calibration curve showed that 80% of the Y203 added to the raw silicon nitride powder was present in the form of said SiN4 . Y2O3 compound. The heat-treated mixed powder had the following analysis.

Element Silicon Nitrogen Aluminium Iron Calcium Yttrium Oxygen % by weight 56.6 35.1 1a26 0.30 0.09 3.71 2.65 The heat-treated mixed powder was sintered for 2 hours by the customary hot press process at a temperature of 1,8000C and a pressure of 400 kg/cm2. Samples were cut out of the sintered mass. The same flexural strength test as applied in ComDarative Example 1 showed the samples to have a flexural strength of 85 Kg/mm at 1,200"C, that is, a prominent heat resistance.

EXAMPLE 3.

The starting silicon nitride/yttria/alumina mixed powder used in Comparative Example 3 was heated with a nonsintered or sintered moulding of heating time reducer of aluminium nitride embedded in the powder. In this case, the porosity of the heating time reducer of aluminium nitride was carried at 8%, 10%, 30%, 50 Ó and 70%. Heating was continued for 2 hours at 1,6500 C. The oxygen content of the samples of the heated raw mixed silicon nitride powder and the sexual strength thereof as measured at 1,2000C by the same process as applied in Comparative Example 3 are set forth in Table 2 below.

TABLE 2

Flexural strength Form of heating Porosity Oxygen content at 1,2000C time reducer % (% by weight) (Kg /mm2) * Sintered 8 3.77 63 10 3.54 75 Nonsintered 1 30 2.60 86 50 2.32 92 70 2.41 90 * Comparative Example.

A heating time reducer of aluminium nitride whose porosity exceeded 70% presented undesirable difficulties in handling. A heating time reduced of aluminium nitride whose porosity was smaller than 10% was unadapted for reuse and indus trially unacceptable and where the oxygen content of raw silicon nitride powder was to be decreased to a desired level, then said powder was to be decreased to a desired level, then said powder had to be heated for a long time. Based on 3û-minute heating, the heating time reducer of aluminium nitride decreased the oxygen content of the starting powder to 4.4% when having a porosity of 8% and to 40 when having a porosity of 10%.

EXAMPLE 4.

A nonsintered moulding of a heating time reducer of aluminium nitride having a porosity of 55% and a moulding of a heating time reducer of finely pulverized aluminium nitride which was sintered at 1,800 C with a porosity of 30% were embedded in the silicon nitride powder B followed by heating of 2 hours at 1,650 C.

The raw silicon nitride powder thus heated had an oxygen content of 2.45% by weight.

A nonsintered moulding of a heating time reducer of aluminium nitride whose porosity was 50% before being used in Example 3, but fell to 45% after said application, and a fresh nonsintered moulding of a heating time reducer of aluminium nitride having a porosity of 50% were jointly embedded in silicon nitride powder B followed by heating of 2 hours at 1,650"C, after which the oxygen content of the silicon nitride powder was 2.35%. Both samples of deoxidized silicon nitride powder were then sintered by hot pressing to provide sintered products of high heat resistance.

WHAT WE CLAIM IS: 1. A process for reducing the oxygen content of a silicon nitride powder which comprises heating the powder in the presence of a heating time reducer which is a nonsintered moulding of ceramic material or a sintered moulding of ceramic material having a porosity of at least 10%.

2. A process as claimed in claim 1 in which heating is effected at a temperature of from 1400 to 1900 C.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 2 Flexural strength Form of heating Porosity Oxygen content at 1,2000C time reducer % (% by weight) (Kg /mm2) * Sintered 8 3.77 63 10 3.54 75 Nonsintered 1 30 2.60 86 50 2.32 92 70 2.41 90 * Comparative Example. A heating time reducer of aluminium nitride whose porosity exceeded 70% presented undesirable difficulties in handling. A heating time reduced of aluminium nitride whose porosity was smaller than 10% was unadapted for reuse and indus trially unacceptable and where the oxygen content of raw silicon nitride powder was to be decreased to a desired level, then said powder was to be decreased to a desired level, then said powder had to be heated for a long time. Based on 3û-minute heating, the heating time reducer of aluminium nitride decreased the oxygen content of the starting powder to 4.4% when having a porosity of 8% and to 40 when having a porosity of 10%. EXAMPLE 4. A nonsintered moulding of a heating time reducer of aluminium nitride having a porosity of 55% and a moulding of a heating time reducer of finely pulverized aluminium nitride which was sintered at 1,800 C with a porosity of 30% were embedded in the silicon nitride powder B followed by heating of 2 hours at 1,650 C. The raw silicon nitride powder thus heated had an oxygen content of 2.45% by weight. A nonsintered moulding of a heating time reducer of aluminium nitride whose porosity was 50% before being used in Example 3, but fell to 45% after said application, and a fresh nonsintered moulding of a heating time reducer of aluminium nitride having a porosity of 50% were jointly embedded in silicon nitride powder B followed by heating of 2 hours at 1,650"C, after which the oxygen content of the silicon nitride powder was 2.35%. Both samples of deoxidized silicon nitride powder were then sintered by hot pressing to provide sintered products of high heat resistance. WHAT WE CLAIM IS:

1. A process for reducing the oxygen content of a silicon nitride powder which comprises heating the powder in the presence of a heating time reducer which is a nonsintered moulding of ceramic material or a sintered moulding of ceramic material having a porosity of at least 10%.

2. A process as claimed in claim 1 in which heating is effected at a temperature of from 1400 to 1900 C.

3. A process as claimed in claim 1 or claim 2 in which the silicon nitride

powder is heated in admixture with yttria and alumina, the mixture containing from 0.5 to 10% by weight of yttria and from 0.1 to 5% of alumina.

4. A process as claimed in claim 1 substantially as hereinbefore with reference to the Examples.

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