Title of Invention

A PROCESS FOR MAKING PRECURSOR POWDER USEFUL FOR PREPARING DENSE HEXAGONAL BORON NITRIDE - MULLITE COMPOSITE.

Abstract The invention provides a process for preparation of precursor powder comprising boron nitride, silicon and aluminium through hydrogel route to impart greater homogeneity in the said precursor powder as well as to the dense hexagonal boron nitride-mulite composite prepared from the said powder. The process also reduced the sintering temperature and eliminate the use of inert atmosphere for sintering of the product as against the known process.
Full Text The present invention relates to a process for making precursor powders useful for preparing dense hexagonal boron nitride - mullite and a process for preparing dense hexagonal boron nitride - mullite composite useful for making industrial components.
The main usages of the hexagonal boron nitride-mullite composite is in the field of engineering ceramics such as producing parts of gas jets, bushes of muffle furnace, as a material for absorbing thermal neutron, for bio-protection in nuclear equipment and refractory items such as heat insulating and refractory materials for producing refractory ceramic items, for metallurgical and chemical industries.
There is no known method of making precursor powder useful for making dense boron nitride-mullite composite, though dense hexagonal boron nitride products are known where boron oxide or different boron containing glasses are used as compositing material. The present day processes of making such products essentially consists of mixing additives with hexagonal boron nitride by wet or dry mixing, forming shapes by conventional shaping processes, sintering in inert atmosphere mostly by hot pressing, references for which may be made to "Special Ceramics" Edited by P. Popper ( Heywood, London, 1960 ) pp 144, by T. A. Ingles and P. Popper, "Sci Ceram", Vol. 11, 1981, pp 251, by D. Launary and Thevenot, "Ind. Engg. Chem", Vol. 47, 1955, pp 2506, by K. M. Taylor, "Izv. Akad. Nauk SSSRNeorg. Mater", Vol 11, 1975, pp 1778, by I. G. Kuzenetsova et al., "Ceram Engg. Sci. Proc.", Vol 6, 1985, pp 1305, by D. Fister, or by mechanochemical treatment followed by pressureless sintering references for which may be made to "Sintering and crystallisation of ground hexagonal boron nitride powders", "J. Mat. Sc. Let.", Vol 13,1994, pp 653-55, by T. Hagio and H. Yoshida.

The main draw back of the above processes may be listed as below.
1. The process require inert atmosphere for sintering.
2. Hot pressing is frequently required for adequate densification.
3. Sintering temperature is higher.
4. Bonding or composite phases are of comparatively low fusion range.
5. Homogeneity of the product is difficult to maintain.
The main object of the present invention is to provide a process of making precursor powders
useful for preparing dense hexagonal boron nitride - mullite composite and a process for
preparing dense hexagonal boron nitride mullite composite, which obviates the draw back as
detailed above.
Another object of the present invention is to process the raw materials thorough hydrogel route to
impart greater homogenity to the fired products.
Still another object of the present invention is to eliminate the use of a inert atmosphere for
sintering of the products.
Yet another object of the present invention is to reduce the sintering temperature of the products.
Still another object of the present invention is to increase the fusion-range of the compositing
phases present in the product.

Accordingly the present invention provides, a process for making precursor powder, useful for preparing dense hexagonal boron nitride-mullite composite and comprising boron nitride, silicon and aluminium , which comprises making a slurry of fine silica and boron nitride in aluminium salt solution such as herein described, at a concentration ranging 5-40%, converting the slurry into a gel-like mass by ammonia at a pH ranging 10-11 , ageing the gel-like mass for a period in the range of 24 - 48 hours , washing the resultant solid followed by drying at a temperature range 110 + 5°C , for a period in the range of 24 - 36 hours, calcining the dried mass at temperature in the range of 800° - 950°C for a period in the range of 1 - 5 hours, grinding the calcined mass to pass 100 British Standard mesh to get the desired precursor powder
I an embodiment hexagonal boron nitride-mullite composite may be prepared using obtained powder by conventional manner at a temperature in the range of 1250-1550 ° C in air atmosphere.
In another embodiment of the present invention aluminium salt used may be such as nitrate, chloride, sulphate.
In still another embodiment of the present invention concentration of aluminium salt solution may be maintained in the range of 0.5 - 1.5 (M).
In yet another embodiment of the present invention ammonia may be used as gaseous or in water solution. The details of the process of the present invention are given below.
- Boron nitride powder is mixed with aluminium nitrate solution with constant stirring.
- Very fine silica powder is added in the boron nitride suspension prepared in step 'a'.
- The entire mixture is strried for 2 - 12 hours.
- Ammonium hydroxide solution is slowly added in the mixture produced by step 'a' -
' c'till pH is 10.5-11.0.
- The entire mass converts to semisolid gel-like mass which is allowed to age for 24
- 48 hrs.

f) Product formed in step 'e' is filtered and washed by deionised water.
g) The washed material is dried at 110 + 5°C for 24 hrs.
h) The dried material obtained in step 'g' is heat treated at 800-850°C for 2-4 hrs in air
environment.
i) The heat treated product is ground and sieved through 100 mesh B.S.
j) The sieved powder is formed into green shape by cold isostatic pressing.
k) The green pressed product is fired in air environment at 1200-1400°C for 1-5 hrs.
The process of the present invention can be used for making precurser powders useful for
preparing dense Hexagonal Boron Notride - Mullite & a process for preparing Dense Hexagonal
Boron Nitride Mullite Composite, of various shapes and sizes required for application as
engineering and refractory material.
Hexagonal boron nitride is essentially a non-sinterable covalent compound with flaky structure.
It is frequently converted to a dense body by incorporating low melting boron containing glasses.
High level of grinding of HBN was also claimed to be another effective means of making the
powder sinterable. However, a second compatible phase i.e. mullite may also be used for
consolidation of HBN where mullite and HBN may very well be connected by a boron
containing glass. Mullite, at the same time, may provide a layer on the surface of the compacted
mass thus protecting it from oxidation. However, during heat treatment in air, oxidation of BN
and formation of mullite occurs simultaneously. If mullite formation is accelerated by taking
recourse to active ingredients for the formation of mullite which is accomplished by using fine
SiO2 and gel-alumina in the present invention, formation of mullite will be predominant and
oxidation BN will be controlled.

The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention. Example -1
5 gm boron nitride powder and 13.34 gm silica fume are added to 1386 ml 0.5 (M) aluminium nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 295.5 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking Example - 2
10 gm boron nitride powder and 11.86 gm silica fume are added to 1231 ml 0.5 (M) aluminium nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 250.4 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa . The pellets are fired in ambient atmosphere at 1500°C in a crucible using boron nitride as packing material with 1 hour soaking.

Example - 3
15 gm boron nitride powder and 10.38 gm silica fume are added to 718.34 ml 0.75 (M) aluminiun nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. Ammonia gas is then slowly passed with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1450°C in a crucible using boron nitride as packing material with 1 hour soaking. Example - 4
20 gm boron nitride powder and 8.895 gm silica fume are added to 615.7 ml 0.75 (M) aluminium nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. Ammonia gas is then slowly passed with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Articles of desired shapes are prepared from this powder by slip castinga. The shapes are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking.

Example - 5
25 gm boron nitride powder and 7.412 gm silica fume are added to 385 ml 1 (M) aluminiun nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 158.65 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Articles of desired shapes are prepared from this powder by slip casting. The shapes are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking. Example - 6
30 gm boron nitride powder and 5.93 gm silica fume are added to 205 ml 1.5 (M) aluminiun nitrate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 126.7 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking.

Example - 7
35 gm boron nitride powder and 4.447 gm silica fume are added to 206 ml 0.5 (M) aluminium sulphate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 94.6 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa. The pellets are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking. Example - 8
40 gm boron nitride powder and 2.965 gm silica fume are added to 92 ml 0.75 (M) aluminium sulphate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 65.0 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by seiving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa. The pellets are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking.

Example - 9
45 gm boron nitride powder and 1.482 gm silica fume are added to 34.5 ml 1 (M) aluminium sulphate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 33.2ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by seiving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1400°C in a crucible using boron nitride as packing material with 1 hour soaking. Example -10
5 gm boron nitride powder and 13.34 gm silica fume are added to 1268 ml 0.5 (M) aluminium chloride solution. The mixture is then shaken in a mechanical shaker for 12 hours. Ammonia gas is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1350°C in a crucible using boron nitride as packing material with 1 hour soaking.

Example-11
10 gm boron nitride powder and 11.86 gm silica fume are added to 735.7 ml 0.75 (M) aluminium chloride solution. The mixture is then shaken in a mechanical shaker for 12 hours. 250.4 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Articles of desired shapes are prepared from this powder by slip casting. The shapes are fired in ambient atmosphere at 1350°C in a crucible using boron nitride as packing material with 2 hour soaking. Example -12
15 gm boron nitride powder and 10.38 gm fine silica powder are added to 483 ml 1(M) aluminium chloride solution. The mixture is then shaken in a mechanical shaker for 12 hours. 220.34 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1550°C in a crucible using boron nitride as packing material with 4 hour soaking.

Example - 13
30 gm boron nitride powder and 5.93 gm fine silica powder are added to 184 ml 1.5(M) aluminium chloride solution. The mixture is then shaken in a mechanical shaker for 12 hours. Ammonia gas then slowly passed with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa. The pellets are fired in ambient atmosphere at 1500°C in a crucible using boron nitride as packing material with 3 hour soaking. Example -14
15 gm boron nitride powder and 10.38 gm fine silica powder are added to 161 ml 1.5 (M) aluminium sulphate solution. The mixture is then shaken in a mechanical shaker for 12 hours. 220.34 ml ammonium hydroxide solution is then slowly added with constant stirring. The whole mixture is then allowed to settle for overnight. The clear liquid over gel-like mass is decanted followed by filtration and washing with deionised water. The filtered mass is then air dried. The air dried mass is heat treated at 800°C for 12 hours. The heat treated mass is ground followed by sieving through 100 mesh B.S. Pellets are prepared from this powder by pressing uniaxially at 25 Mpa followed by isostatic pressing at 250 Mpa. The pellets are fired in ambient atmosphere at 1450°C in a crucible using boron nitride as packing material with 2 hour soaking. The main advantages of the present invention are : 1) The process of sintering does not required any inert atmosphere to be maintained.

2) The sintered product is made by pressureless sintering thus eliminating hot pressing.
3) Sintering of products is possible at lower temperature.
4) Homogeneity of the product is ensured



We Claim:
1. A process for making precursor powder, useful for preparing dense hexagonal
boron nitride-mullite composite and comprising boron nitride, silicon and
aluminium , which comprises making a slurry of fine silica and boron nitride in
aluminium salt solution such as herein described, at a concentration ranging 5 -
40%, converting the slurry into a gel-like mass by ammonia at a pH ranging 10 -
11 , ageing the gel-like mass for a period in the range of 24 - 48 hours , washing
the resultant solid followed by drying at a temperature range 110+ 5°C , for a
period in the range of 24 - 36 hours, calcining the dried mass at temperature in
the range of 800° - 950°C for a period in the range of 1 - 5 hours, grinding the
calcined mass to pass 100 British Standard mesh to get the desired precursor
powder.
2. A process as claimed in claim 1, wherein the aluminium salt used is selected
from aluminium nitrate, chloride and sulphate.
3. A process as claimed in claim 1-2 wherein ammonia is used as gaseous or in
water solution.
4. A process for making precursor powder useful for preparing dense hexagonal
boron nitride-mullite composite and comprising boron nitride, silicon and
aluminium substantially as herein described with reference to the examples.

Documents:

1456-del-1999-abstract.pdf

1456-del-1999-claims.pdf

1456-del-1999-correspondence-others.pdf

1456-del-1999-correspondence-po.pdf

1456-del-1999-description (complete).pdf

1456-del-1999-form-1.pdf

1456-del-1999-form-19.pdf

1456-del-1999-form-2.pdf

1456-del-1999-form-3.pdf


Patent Number 233352
Indian Patent Application Number 1456/DEL/1999
PG Journal Number 13/2009
Publication Date 27-Mar-2009
Grant Date 29-Mar-2009
Date of Filing 05-Nov-1999
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 ASIS KUMAR BASU CENTER GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
2 ARUP KUMAR SAMANTA CENTER GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
3 KAKALI DAS AND SANKAR GHATAK CENTER GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
PCT International Classification Number C04B 35/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA