Title of Invention

"A PROCESS FOR PREPARATION OF TETRAGONAL ZIRCONIA POLYCRYSTALLINE (TZP)"

Abstract

A Process for the preparation of Tetragonal zirconia polycrystalline (TZP) This invention relates to a Process for the preparation of Tetragonal zirconia polycrystalline (TZP) comprising the steps of Purifying zircornium oxchloride by recrystallization and precipitating hydroxide with dilute solution ammonia, Dissolving the precipitate obtained by step (a) into nitric acid, Dissolving yttrium oxide in nitric acid, Preparing the stoichiometric composite solution of zirconium nitrate and yttrium nitrate obtained by steps (a) and (b) respectively, Spray drying the composite solution, Subjecting the spray dried powder obtained by steps (e) to a step of heating at a temperature of 450° C, Calcining the spray dried powder at a temperature of 550°C.

Full Text

The major problem of this process is that it is that it is difficult to attain the homogeneous distribution of yttria in zirconium oxide (Zro2) grains for preparation of dense TZP ceramics Another disadvantage of the above process is that the powders prepared by the above process have higher particle size due to which the powder is inhomogeneous at sintering temperature. Still another disdvantage of the above process is that TZP powder obtained by this process has inferior quality. Still another disadvantage of the above process is that this process does not provide adequate control on phase composition and microstructure. Consequently the powder has poor reproducibility and so the properties of the finished ceramics are not reproducible. Further disadvantage of the above process is that the Zirconium powder obtained by this process requires sintering at higher sintering temperatures at which it is nor possible to retain high amount of tetragonal phase which is necessary for imparting transformation toughening. Still further disadvantage of the above process is that the powder prepared by this process is contaminated with undesirable impurities. Still further disadvantage of the above process is contaminated with undesirable impurities. Still further disadvantage of the above process is that the zirconium oxide is required to be calcined at high temperature at 1100 to 1300°C which leads to increase in particle size and longer processing time. Another process known in the art for preparation of TZP powders is sol-gel process. In this process sols of Zirconia are prepared and converted into gels. These gels are dried to get the powders of desired characteristics. The disadvantage of the above process is that handling of large volume of gelateneous precipitates and drying of gel is a cumbersome and time consuming process. The primary object of this invention is to propos a process for preparation of Tetragonal Zirconium polyc ystalline (TZP) powder and ceramics. Another object of the present invention is to propose a process for preparation of TZP powder and ceramics which provides high purity, ultrafine and homogeneous TZP powder and ceramics which provides high purity, ultrafine and homogeneous TXP powder. Still another object of the present invention is to propose a propose for preparation of TZP powder which provides control on phase composition and micro-structure thereby enabling achievement of reproducibility of TZP powder and consequently reproducibility in the properties of the finished ceramics. A Further object of the present invention is to propose a process for preparation of TZP powder and ceramics with relatively high mechanical properties like hardness, flexural strength and fracture toughness as compared to the TXP powder obtained by conventional process. An even further object of the present invention is to propose a process for preparation of TZP powder which sinter at much lower temperature and retain high amount of tetragonal phase necessary for imparting transformation toughening in this material as compared to conventionally prepared TZP powder which sinter at high temperature at which it is not possible to retain high amount of tetragonal phase. A still further object of the present invention is to propose a process for preparation of TZP powder which is energy efficient as it requires calcinations at lower temperatures of 550°C for 4 hours to form a solid solution of Yattria and Zirconia as compared to the conventional process which require calcinations at higher temperatures of 1100 to 1300°C. Yet another object of the present invention is to propose a process for preparation of TZP powder, which provides Zirconia powders suitable for providing thermal barrier ceramic coating on metallic components. Still further object of the present invention is to propose spray-drying process for preparation of TZP powder, which are suitable for the synthesis of artificial diamond and fabrication of gas sensors. According to the present invention there is provided a Process for the preparation of tetragonal zirconium polycrystalline (TZP) comprising the steps of: - (a) Purifying zirconium oxchloride by recrystallisation and precipitating hydroxide with dilute solution ammonia; (b) Dissolving the precipitate obtained by step (a) into nitric acid; (c) Dissolving yttrium oxide in nitric acid; (d) Preparing the stoichiometric composite solution of zirconium nitrate and yttrium nitrate obtained by steps (a) and (b) respectively; (e) Spray drying the composite solution; (f) Subjecting the spray dried powder obtained by steps (e) to a step of heating at a temperature of 450° C; (g) Calcining the spray dried powder at a temperature of 550°C, Further in accordance with the present invention, zirconium oxichloride is purified by recrystallisation and is then precipitated as hydroxide with dilute solution of ammonia at controlled pH. The precipitable is washed till it is free from all traces. The precipitate is dissolved in dilute nitric acid to get zirconium nitrate. Shows that powders contain considerable amount of transferable tetragonal phase and by suitable heat treatment powders of desired phase compositions can be prepared. The Williamson Hall plot of the TZP powder obtained establishes the compositional homogeneity of the powder. The Vickers hardness, flexural strength and fracture toughness of sintered ceramics prepared using these powders were measured by microintendation 3 point bend test and Single Edge Notch Beam (SENB) techniques respectively. The range of the properties of TZP powders and ceramics prepared using different chemical compositions and processing parameters is given below: - TZP Powders 0.1-0.2um 2 - 40um 90 - lOOm/g 0-75 15 - 100 0-50 0-30 5 - 20nm Particle Size Agglomerate Size Sp. Surface Area Phase composition Monoclinic (%) Tetragonal (%) (Transformable) Tetragonal (Non Transformable) Cubic (%) Crystallite Size Variation in the yttria concentration in the range of 3% troughout Homogeneity Zirconia grains TZP Ceramics Grain Size : 0.2 - O.Sum Phase composition (%) Monoclinic : 0-70 Tetragonal : 20-90 (Transformable) Tetragonal : 0-50 (Non-transformable) CLAIM 1. A Process for the preparation of Tetragonal zirconia polycrystalline (TZP) comprising the steps of: - (a) Purifying zircornium oxchloride by recrystallization and precipitating hydroxide with dilute solution ammonia, (b) Dissolving the precipitate obtained by step (a) into nitric acid; (c) Dissolving yttrium oxide in nitric acid; (d) Preparing the stoichiometric composite solution of zirconium nitrate and yttrium nitrate obtained by steps (a) and (b) respectively; (e) Spray drying the composite solution; (f) Subjecting the spray dried powder obtained by steps (e) to a step of Xheating at a temperature of 450° C; (g) Calcining the spray dried powder at a temperature of 550°C; 2. A process as claimed in claim 4 wherein said spray dried powder is subjected to heating a temperature of 450°C for 2 hours. 3. A process as claimed in claim 5 wherein the step of calcinations is carried out for 4 hours. 4. A process for the preparation of Tetragonal Zirconia polycrystalline (TZP) substantially as herein describaed and illustrated.

Documents:

2873-del-1996-abstract.pdf

2873-del-1996-claims.pdf

2873-del-1996-correspondence-others.pdf

2873-del-1996-correspondence-po.pdf

2873-del-1996-description (complete).pdf

2873-del-1996-form-1.pdf

2873-del-1996-form-19.pdf

2873-del-1996-form-2.pdf

2873-del-1996-form-26.pdf