Title of Invention

A PROCESS FOR THE DENSIFICATION OF YTTRIUM ALUMINUM GARNET (YAG) MATERIAL

Abstract

A method for the densification of Yttrium Aluminum Garnet (YAG) material which comprises the following steps: 0 fabrication of a porous compact of yttria and alumina mixture by pot milling in alcohol and calclmlning the pressed component at a suitable temperature ti) addition of Cobalt to the porous compact by a cost-effective and novel solution infiltration processing method, III) Sintering of the dried compact at a temperature range of 1850 - 1900 K for a duration not exceeding 3 hours.

Full Text

FIELD OF THE INVENTION This invention relates to a process for the densification of Yttrium Aluminum Garnet (YAG) material. This invention further relates to a method of densifiying YAG material by introducing suitable transition metal oxide additives through solution infiltration route. BACKGROUND Of THE INVENTION Yttrium aluminiun garnet (YAG, Y3 Al5 O12) itan important ceramic material became of its wide applications. It is used as a rare-earth based phosphorus material in Iwers and as a refractory coating in electronic devices. Its compatibility with sapphire leads to development of composites for structural applications. As a structural material, it has lot of applications due to its excellent creep resistance and high chemical and thermal stability. The sintering of YAG requires very high temperature of the order of 1873-2073 K and longer sintering times of the order of several hours. As a result, several, liquid-phase routes have been discussed in the literature to lower the sintering temperature. Sinterability of the YAG power obtained by the chemical methods is known in the art Recently, Kong and Huang have reported the low temperature formation of YAQ from oxides via a high-energy ball milling process, Panoerselvam et al have reported the denfiiftcation of YAG by microwave assisted route. Efforts have also been made to density YAG with suitable additives in order to reduce the sintering temperature. De With sintered YAG to almost theoretical density and obtained a translucent product at a temperature of 2023 K using SiO2 and MgO as sintering aids. The effect of metal oxide on the densification of ceramics plays an important role. There are however, no published patents available till date for the method of densification of YAG material. Also, no reports or patents are available on the effect of transition metal oxides on the densification of YAG. In most of the work mentioned here in before, mere are some shortcomings. Often, the addition of sintering aids by simple mixing leads to inhomogeneous mixing which may eventually leads to differential densification of the material. Further, the preparation of powders by solution route has been found economical for bulk production. The properties may also vary from batch to batch preparation and it is also difficult to handle huge amount of very fine powder, which is health-hazardous. Densification using microwaves is also known and which is costly proposition at this stage. In a number of order studies, the duration of sintering at high temperature are large thus making it ineffective for bulk production. Further it is important to form a single phase YAG without any intermediate formation (i.e. any second phase formation) originating from the reaction of the matrix with the infiltrated solution to obtain the desired properties OBJECTS OF THE INVENTION It is therefore an object of this invention to propose for the densification of Yttrium Aluminum Garnet (YAG) material, which is economical. It is a further object of this invention to propose a process for the densification of Yttrium Aluminum Garnet (YAG) material, which requires a nominal quantity of additive for densification. Another object of this invention to propose a process for the densification of Yttrium Aluminum Garnet (YAG) material which ensures homogeneous distribution of the additive into the matrix. Yet another object of this invention to propose for the densification of Yttrium Aluminum Garnet (YAG) material where the sintering of YAG is carried out at lower soaking duration at high temperature DESCRITPtON OF THE INVETNION According to this invention, there is provided a process for the densification of Yttrium Aluminum Garnet (YAG) material by infiltration of additives comprising the steps of- i) fabrication of a porous compact of yttria (40-45g) and alumina (50-60g) mixture by a pot milling in alcohol (70-80 wt%) and calcining the compact at a temperature of 1200-1300K for 1.5-2 hours to obtain a porous compact, ii) preparation of cobalt nitrate solution in distilled water followed by infiltration of the porous compact in the solution in a vacuum dessicator, iii) sintering of the dried compact at a temperature range of 1850-1973 K for a duration not exceeding 3 hours. More precisely the process for densification of YAG comprises the steps of: i. intimate mixing of calculated quantity of high purity raw materials of Yttria and Alumina in a pot mill using an organic solvent and Alumina grinding media; ii. removal of solvent, addition of an organic binder and sieving by known methods; iii. fabrication of cylinders by uniaxial pressing iv. calcinations of the compact at a temperature range of 1000-1300K to obtain a porous compact; v. preparation of cobalt nitrate solution of 1 -25% in distilled water; vi. infiltration of the porous compact in the solution in a vacuum dessicator; vii. drying at temperature range of 350-500K for overnight in an electrical oven; viii. sintering for a duration of less than 3hrs at peak temperature. The alumina oxide (Al2 O3) used has a purity of >99.9%, the average particle size-0.2 micron (0.1 to 0.3 micron in general). The other impurities should be in parts per million (ppm) level. The yttrium oxide (YjQj) used has a purity of >99.9% the average particle size-0.2 micron (0.1-0.3 micron in general). The cobalt nitrate (Co(NOj)j oTfeO) is analytical reagent grade with a purity >99.0%. The alumina and yttria powders are used in a ratio in the range of 57 to 43. Intimate wet mixing in pot mill for mill for 3-6hrs is required Solvents may be dried out by electrical heating in oven or using infrared lamp. The binder can be polyethylene glycol (PEG-4000) or polyvinyl alcohol (PVA). The binder may be in the range of 2-5% of the total mix Distilled water should be used for making solution. The infiltration time can from 15mins to 60mins. The forming pressure during compaction may be in the range of 300-400 Kgs/cm2 . Sintering of dried and infiltrated components is effected in an electric / gas. operated furnace at a temperature in the range of 1850-1973K for 1.5-3 hrs. It has been seen that different colors can be imparted to YAG by infiltrating different transition metal oxides, eg. nickel, copper, chromium, etc. The density was measured by water displacement method. The beat yttrium aluminum garnet (YAG) material produced by the process of the present invention has the following properties . Density : 4.5g/cc Apparent porosity : 0% Linear shrinkage : 19 Vickers hardness 1250HV1.0 Major phases: only : YAG Average grain size : 5.5 micron The invention will now be explained in. greater detail with the help of the following non-limiting examples and illustrated with the help of the accompanying drawings. However, the examples are given as a way of illustration of the invention and should not be construed to limit the scope of the present invention. Example 1 57g high purity alumina and 43 g of high purity yttria are intimately mixed in 80 ml analytical reagent grade of isopropyl alcohol for 6 hrs. in a plastic pot rsing a roller mill. High purity alumina grinding media was used for this purpose. Heating under infrared lamp dried the solvent, 2-4% polyethylene glycol was used in water as binder to the dried powder and the mixture was sieved by a conventional method. The sieved powder was fabricated into cylinders by uniaxial pressing at a pressure, range of 300-400 kgs/cm2 and dried for 4-6 hrs, in an electrical oven at a temperature range of 350-400 K The dried compact was calcined at a temper at ui a of 1273K For 2. hrs. duration. The dried compact, was infiltrated in a vacuum dessicaior for 15 min. with 1% cobalt nitrate solution in water. The infiltrated compact was dried in an electrical oven for 3-6 hrs. prior to sintering in air at a temperature of 1873K for a duration of 2 hrs, at the peak temperature, The density, porosity of the sintered compacts are given in Table 1. Example 2 57g high purity alumina and 43g of high purity yttria are intimately mixed in 80ml analytical reagent trade of isopropyl alcohol for 6 hrs. in a plastic pot using a roller mill. High purity alumina grinding media was used for this purpose. Heating under infrared lamp dried the solvent. 2-4% polyethylene glycol was used in water as binder to the dried powder and the mixture was sieved by a conventional method. The sieved powder was fabricated into cylinders by uniaxial pressing at a pressure range of 300-400 kgs/cm2 and dried for 4-6 hrs. in an electrical oven at a temperature range of 350-400K. The dried compact was calcined at a temperature of 1273K for 2 hrs. duration. The dried compact was infiltrated in a vacuum dessicaior for 15 mins. with 5% cobalt nitrate solution in water. The infiltrated compact was dried in an electrical oven for 3-6 hrs. prior to sintering in air at a temperature of 1873K for a duration of 2 hrs. at the peak temperature. The density, porosity of the sintered compacts are given in Table 1, Fig,l is the x-ray diffraction of pure YAG infiltrated with 5% cobalt nitrate solution. Fig.2 is the energy dispersive x-ray analysis (EDAX) of sintered YAG and the scanning electron micrograph of the sintered YAG is given in Fig. 3, (Table Removed)#: As sintered at 1873K for 2 hre in air (Green density 2.60 g/cc) As can be seen from Table 1, YAG can be fully sintered by infiltrating with 5% cobalt, nitrate solution by a simple, novel and cost-effective- method. Further., addition of cobalt nitrate has no significant effect on the densification). There was no evidence of cobalt in the x-ray diffraction pattern (Fig. 1). The EDAX pattern also could not detect significant amount of cobalt in YAG (Fig.2), This observation implies that cobalt is present in a very small quantity which cannot be detected by the analytical techniques used in this study. The scanning electron micrographs revealed equiaxed grains with no porosity confirming the densification of the material (Fig.3). The present invention is advantageous as it uses a simple, low-cost and infiltration technique to introduce a very small amount of additive typically less than 1 mol %) uniformly in the matrix and is very effective in densification of yttrium aluminum garnet (YAG) material. Further, the Soking period at the peak temperature can kept minimum to make it a cost- effective route. This also reduces the unnecessary grain growth in the material, which is a negative factor for higher structural characteristics of the material. WE CLAIM 1. A process for the densification of Yttrium Aluminum Garnet (YAG) material by infiltration of additives comprising the steps of:- i) fabrication of a porous compact of yttria (40-45g) and alumina (50-60g) mixture by a pot milling in alcohol (70-80 wt%) and calcining the compact at a temperature of 1200-13 OOK for 1.5-2 hours to obtain a porous compact, ii) preparation of cobalt nitrate solution in distilled water followed by infiltration of the porous compact in the solution in a vacuum dessicator, iii) sintering of the dried compact at a temperature range of 1850-1973 K for a duration not exceeding 3 hours. 2. A process as claimed in claim 1 wherein both alumina (0.1-0.3 Um) and yttria (01-0.3 Um) are of high purity and posses sub-micro average particle size. 3. A process as claimed in claim 1 wherein a porous compact has been fabricated to provide sufficient strength (15-20 MPa) to the body for infiltration of liquid into it. 4. A process as claimed in claim 1 wherein Cobalt is chosed as an additive for its improved role over other metals for enhancing the densification of YAG. 5. A process as claimed in claim 1 wherein Cobalt is introduced to the porous matrix by infiltration of Cobalt nitrate solution in water resulting in homogenous distribution of Cobalt in the porous compact. 6. A process as claimed in claim 1 wherein the amount of additive used in enchancing the densification of YAG is kept at a minimum of less than 1 mole percent. 7. A process as claimed in claim 1 wherein the sintering of the compact with the infiltrated component is carried out at a temperature range of 1850-1900K for a duration of 2-3 hours at the peak temperature. 8. A process as claimed in claim 1 wherein after densification, a single phase YAG is developed excluding any second phase development originating from the reaction of the matrix with the infiltrated solution. 9. A process as claimed in claim 1 wherein different colours are imparted to YAG by infiltrating different transition metal oxides e.g. Nickel, Copper, Chromium etc. 10. A process for densification of Yttrium Alumina Garnet (YAG) material by infiltration of additives substantially as herein described.

Documents:

1197-del-2003-abstract.pdf

1197-del-2003-claims.pdf

1197-del-2003-correspondence-others.pdf

1197-del-2003-correspondence-po.pdf

1197-del-2003-description (complete).pdf

1197-del-2003-drawings.pdf

1197-del-2003-form-1.pdf

1197-del-2003-form-19.pdf

1197-del-2003-form-2.pdf

1197-del-2003-form-3.pdf

1197-del-2003-gpa.pdf