Title of Invention

"A PROCESS FOR PREPARATION OF STRONTIUM-MODIFIED LEAD ZIRCONATE TITANATE (PZT) CERAMICS"

Abstract

This invention relates to a process for preparation of Strontium Modified lead zirconate titanate (PZT) ceramics comprising of the following steps preparing solutions of pure nitrates of Zirconium (Zr) and Titanium (Ti) and mixing them in stoichiometric proportions, conversion as herein described of nitrates of Zr and Ti into hydroxide coprecipitate (Zrx Ti1-x) (OH)4, decomposition of hydroxides coprecipitate (Zrx Ti1-x) (OHJ4 into oxide (Zrx Ti1-x)O2 is carried out by heating at 200-400°C, preparation of lead (Pb) and strontium (Sr) carbonate co-precipitate (Pbo.94 Sro.oe) CO3 by addition of (NH4)2 CO3 to the stoichiometric Pb(NO3) 2 and Sr(NO3) 2 .ball-milling as herein described and calcinations of the mixture of (Zrx Ti1-x)O2 and (Pbo.94 Sro.oe) CO3 to obtain Pbo.94 Sro.oe (Zrx Ti1-x)O3 is carried out at 500 to 800°C, cold compaction is carried at 150 to 250 Mpa and sintering to form sintered PZT ceramics is carried out at 1000 to 1200°C preferably for 2 to 4 hours, electroding and poling of sintered PZT ceramics is carried out at 30k V/cm to obtain strontium modified lead zirconate titanate (PZT).

Full Text

FIELD OF INVENTION This invention relates to a process for preparation of Strontium-modified Lead Zirconate Titanate(PZT) ceramics for underwater applications such as sonar projectors, hydrophones and ultrasonic imaging. PRIOR ART PZT ceramics are transducer materials which convert mechanical energy into electrical energy and vice-versa and are used in various applications. In Strontium(Sr)-modified PZT, a small amount of Lead(Pb) is substituted by Strontium(Sr). Sr-modified PZT as such or in the form of composites, are widely used as electro¬mechanical transducer material for fabrication of high power sonar projectors and for hydrophones. Such materials are required to possess high piezoelectric strain(d33) co-efficient, high electromechanical coupling coefficient(kp) and moderate value of dielectric constant( Er ) of the order of 1000. In addition, reproducibility in the properties of finished ceramics, is also essential. For PZT based piezo-composites required for hydrophone applications, it is also essential that the ceramics feed ponders must be of high purity, stoichiometric, chemically homogeneous, reactive and of submicron size. Any compromise in these requirements, interferes with the fabrication of piezocomposites and deteriorates the piezoelectric properties of the finished products. The conventional method of producing PZT powders is by solid state thermochemical reaction. The above method has the drawback that it does not provide adequate control on the homogeneity in the composition, powder characteristics and uniform distribution of dopants in bulk of materials and therefore leads to non-reproducibility in properties of the finished product. Yet another disadvantage of the above method is that the process is time-consuming as it requires repeated calcining and grinding cycles to get the PZT powders. A further disadvantage of the above process is that due to excess handling, the undesirable impurities get introduced into the product thereby adversely affecting the properties. Another disadvantage of the above method is that o the process requires high temperature of the order of 1000 C for calcination. According to another process described in US patent no. 5,112,433 modified and unmodified PZT are prepared by mixing the hydroxides of Pb and Sr with the hydroxides of Zr and Ti. This is followed by hydrothermal treatment of the slurry at high temperature and pressure to form the PZT powder. A drawback of the above process is that the PZT powder obtained is not chemically homogenous. This in turn requires high temperature of calcination for the formation of PZT solid solution. Another drawback of the above process is that hydrothermal treatment requires autoclaving with repeated cycles of temperature and pressure. Still another drawback of the above process is that the process is not feasible for bulk production, and does not lead to the reproducibility of the properties in the finished product. Yet further disadvantage of the above process is that the process is time consuming as it requires comparatively longer time for decomposition. OBJECTS OF THE PRESENT INVENTION The'primary object of the present invention is to propose a semi-wet process for preparation of Sr-modified PZT ceramics for underwater applications. Another object of the present invention is to propose a process for preparation of Sr-modified PZT ceramics wherein (he process provides stoichiometric and chemically homogenous Sr-modifled PZT ceramics with reproducible properties. Still another object of the present invention is to propose ii process for preparation of Sr-modified PZT ceramics which provides Sr-modified PZT ceramics of high purity of the order of 99.90 to 99.99%. Further object of the present invention is to propose a process for preparation of Sr-modificd PZT ceramics which is energy-efficient as the synthesis temperature of the present invenlion is around 700°C and sintering temperature is around 1100'C as compared to over 900°C and around 1300°C respectively in the conventional solid state process. Still further object of the present invention is to provide a process for preparation of Sr-modified PZT ceramics which provides PZT powder of submicron size of 0.2 to 0.5 microns which results in higher reactivity for making bulk ceramics and leads to better quality products for hydrophone applications. Further object of the present invention is to propose a process for preparation of Sr-modificd PZT ceramics with uniform microstructure with average grain size of 3.5 microns which lends to better reproducibility. Yet further object of the present invention is to propose a process for preparation of Sr-modified PZT-ceramics wherein (he composition width of the JVlarphotropic Phase Boundary(IYlPK) co- existence region is around 1% as compared to 4 to 8% in the known solid state process. Even further object of the present invention is to propose a process for preparation of Sr-modified PZT ceramics which leads to PZT ceramics Pbo.94 Sro.06 (Zrx Ti1-x)O3 (for x= 0.530) with a unique combination of piezoelectric properties comprising of low dielectric constant (Er) of the order of 1100, high electromechanical coupling coefficient (Kp) of the order of 0.59 and high piezoelectric strain coefficient (d33) of the order of 311. Such combination of piezoelectric properties in PZT ceramics is highly suited for hydrophone application. STATEMENT OF INVENTION According to this invention there is provided a process for preparation of Strontium Modified lead zirconate titanate (PZT) ceramics comprising of the following steps: a) preparing solutions of pure nitrates of Zirconium (Zr) and Titanium (Ti) and mixing them in stoichiometric proportions, b) conversion as herein described of nitrates of Zr and Ti into hydroxide coprecipitate (Zrx Ti1-x) (OH)4, c) decomposition by heating of hydroxides coprecipitate (Zrx Ti1-x) (OH)4 into oxide (Zrx Ti1-x)O2 is carried out at 200-400°C, d) preparation of lead (Pb) and strontium (Sr) carbonate co- precipitate (Pbo.94 Sro.oe) CO3 by addition of (NH4)2 CO3 to the stoichiometric Pb(NO3) 2 and SrfNO3) 2 , e) ball-milling as herein described and calcinations of the mixture of (Zrx Ti1-x)O2 and (Pbo.94 Sro.oe) CO3 to obtain Pbo.94 Sro.oe (Zrx Ti1-x)O3 calcination is carried out at 500 to 800°C, f) cold compaction is carried at 150 to 250 Mpa and sintering to form sintered PZT ceramics is carried out at 1000 to 1200°C preferably for 2 to 4 hours. g) electroding and poling of sintered PZT ceramics is carried out at 30k V/cm to obtain strontium modified lead zirconate titahate (PZT). The present invention provides a novel semi-wet process (as compared to solid state process of the known art) for preparation of strontium (Sr)-modified PZT ceramics Pb0.94 Sr0.06 (ZrxTi1-x )03 with x preferably taking the value between 0.515 to 0.550, which are useful for underwater applications such as hydrophones and ultrasonic imaging. The invention relates to the preparation of PZT powder as well as preparation of sintered ceramics followed by electroding and poling. The process is time-efficient, energy-efficient and at the same time leads to ceramics which is highly pure, stoichiometric, chemically homogenous and reproducible. The homogeneity is particularly of critical importance in PZT powder as non-homogenous powder require relatively much higher temperature for calcination, does not provide reproducibility in properties and leads to products with inferior properties. The novelty in the invention is involved both in preparation of PZT powder and also in compaction and sintering of the powder to form sintered ceramics. In preparation of the PZT powder, the process of the invention does not require hydro-thermal treatment which necessitates expensive autoclaving under repeated cycles of temperatures and is particularly unsuitable for bulk production of PZT powders. The PZT ceramics obtained by the known process lacks reproducibility in essential characteristics desired for PZT ceramics. Further the , known process involves the mixing of hydroxides of Zr and Ti with hydroxide of Pb and Sr. The process of the present invention involves the preparation of (Zrx Ti1-x(()ll)4 from the pure, stoicliiometric solution of nitrates of Zr and Ti which is then decomposed to oxide (ZrxTi1-x)O2 and then mixed (o a coprecipitate of (Pb094Sr006)CO3 and subjected to ball milling and calcination to get chemically homogenous powder of (Pb0.94Sr0.06(Z-rxTi1-x) O3solid solution. The pure solution of the nitrates of the Zr and Ti is prepared starting from their impure chlorides by first precipitating them with dilute ammonia solution, washing them to remove soluble impurities and then dissolving hydoxides in nitric acid to obtain pure nitrate solutions which are then mixed in stoicliiometric proportions.The coprccipitatc (Pb0.94Sr0.06)CO3 used in the above step is prepared by mixing (NH4)2.CO3 in the stoicliiometric solution of Pb(NO3)2 and Si(N()j)2 In preparation of finished ceramics, the cold compaction and sintering parameters of temperature, pressure and time, have been critically evaluated and the preferred range of pa minders have been suggested for optimisation of the desired characteristics of the PZT ceramics. The pressure of 150 to 250mpa, sintering temperature in the range of 1000 to 1200°C for 2 to 4 hours, have been found to produce PZT having unique combination of properties suitable for hydrophones. The process of the present invention enables calcination and sintering at significantly lower temperature in (he ranjg^e of 500 to 80Q!C and 1000 to 1200°C respectively as compared to over 900°C and around 1300°C respectively for known solid state process. The particle size of the PZT powder is in the range of 0.2 to ,0.5 microns as compared to over 1 micron obtained by the known process. The microstructure of the PZT powder is uniform with average grain size of about 3.5 microns. The composition width of the co-existence region is about Wo as compared to 4-8% for (he known process, The process provides Sr-modified PZT ceramics wi(h unique combination of properties comprising of low dielectric constant (d) (for x-0.530) of the order of 1100 and high electromechanical coupling coefficient(kp) of the order of 0.59 and high piezoclectric strain coefficient(d33) of the order of 311. Such a unique combination of propertied, is highly suited for hydrophone applications. The important piezoelectric properties of Sr-modified PZT ceramics Pb0.94Sr0.06(ZrxTi1-x)O3for different values of x, obtained by the process of the present invention have been tabulated in Tahlc-l. The comparative piezoelectric properties of PZT ceramics prepared by the process of the present invention and by the coiivcMilional process, have been give in Table-11. DESCRIPTION OF THE PROCESS According to the present invention the Sr-modified PZT is prepared by the process comprising of the following steps :- (a) Preparation of pure, stoichiometric solution of Nitrates of Zr andTi The chlorides of Zr and Ti are separately dissolved in water and treated with dilute solution of ammonia which results in the precipitation of hydroxides of Zr and Ti. These individual precipitates of the hydroxides of Zr and Ti are then washed with water until free from all traces of soluble impurities. These precipitates are then treated with Nitric acid which dissolves the hydroxides of Zr and Ti while the contaminants are left undissolvcd which are removed by filtration. In this way pure nitrates of Zr and Ti are obtained. Alternately, the nitrates of Zr and Ti can also be taken as starting raw materials which can be purified to obtain the pure nitrates of Zr and Ti. The individual nitrates are then analysed precisely for metal content and mixed in the stoichiometric proportions to get the homogenous, stoichiometric and pure solution of nitrates of Zr and Ti. (b) Preparation of Hydroxides of Zr and Ti The pure stoichionictric solution of nitrates of Zr mid Ti obtained by step(a) is sprayed into a precipitating bath of ummonia solution, at pH preferably from 7.5 to 7.8 and at temperature from 60 to 80°C, to simultaneously precipitate the hydroxides of Zr and Ti so as to form co-precipitate of (ZrxTi1-x)(OH)4. (c) Decomposition of Hydroxides to Oxides The coprecipitate of hydroxides of Zr and Ti as obtained by step (b) is decomposed to oxides by beating at temperature in the range of 200 to 400°C in order to get (ZrxTi1-x) O2. (d) Preparation of co-precipitate of Carbonates of Pb and Sr Co-precipitate of carbonates of Pb and Sr namely (Pb0.94Sr0.06)CO3 is prepared by adding (NH4)2CO, in (he stoichiometric solution of Pb(NO3)2 and Sr(NO3)2. (e) Preparation of PZT powder (Pb0.94Sr0.06)CO3 is mixed with (ZrxTi1-x) O2 by ball milling and calcined at temperature in the range of 500 to 800°Cto get finely divided, chemically homogenous solid solution of (0 Preparation of PZT ceramics Pb0.94Sr0.06(ZrxTi1-x)O3 powder obtained by the step (e) is consolidated into ceramics by cold compaction at pressure of 150 to 250 Mpa and sintering in air at temperature in the range of 1000 to 1200°C for 2 to 4 hours. (g) Electroding and Poling Sintered ceramics obtained by step(f) is then electroded and poled at around 30kV/cm to achieve the desired piezoelectric properties. WORKING EXAMPLE 261.75g of the chloride of Zr and 134.8 g of the chloride of Ti were taken separately and dissolved in dilute solution of ammonia to convert the chloride into the precipitates of their respective hydroxides. These precipitates were individually washed to remove soluble impurities and were then dissolved into 200 g and 60g respectively, of nitric acid to convert hydroxide into nitrate. The impurities which were left undissolved, were removed by filtration. 55g of the nitrate of Zr and 42g of the nitrate of Ti were mixed to get a homogenous and pure solution. This solution was treated with dilute solution of ammonia to reconvert nitrates into hydroxides. This coprecipitate of the hydroxides of Zr and Ti was decomposed to oxide by heating at temperature of 400°C. 96g of nitrate of Lead and O.l5g of nitrate of Strontium(Sr) were taken to which 30g of ammonium carbonate was added to get a co-precipitate of the carbonate of Pb and Sr. To this co-precipitate, the oxide of Zr and Ti obtained earlier was mixed by ball milling and calcination at temperature of 700°C to get a homogenous PZT powder This powder was cold compacted at pressure of 200MPa and sintered in air 1150°C for 2 hours to obtain the sintered PZT ceramics. 'I'llis was electroded and poled at 30kV/cm to get the desired piezoelctric properties. Properties of the Pb0.94Sr0.06 (Zrx Ti,1-x) 03 obtained by the process of the present invention was evaluated for its different piezo¬electric properties, for different values of x varying from 0.515 to 0.550. The piezo-electric properties of different compositions are tabulated in Table-I below. Table - 1 P1EZO-ELECTRIC PROPERTIES OF Pb0.94Sr0.06 Zr, Ti1-x ) ()3 (For Different Values of x) (Table Removed) A comparison of the piezo-electric properties of Sr-modified PZT ceramics prepared by the process of the present invention as compared to the similar properties of PZT ceramics prepared by the conventional process is given in Table - II i1 Table-II COMPARISON OF P1EZO ELECTRIC PROPERTIES OF PZT CERAMICS Pb0.94Sr0.06(ZrxTi1-X)O3( For x=0.530) PREPARED BY THE PROCESS OF PRESENT INVENTION Vs CONVENTIONAL PROCESS (Table Removed) The comparison in Table-II shows the unique combination of the piezoelectric properties the Sr-modified PZT ceramics prepared by the process of the present invention wherein the value of the dielectric constant (Er) is lower and the values of piezo-electric strain co-eff1cient( d33 ) and electro mechanical coupling co-efficient (k,,) are higher as compared to the corresponding piezoelectric properties of PZT ceramics prepared by the conventional process. Such a combination of piezoelectric properties in PZT ceramics is highly suited for hydrophone applications. It is to be understood that the process of the present invention is susceptible to modifications, adaptations, changes by those skilled in the art. Such modifications, changes, adaptations arc intended to be within the scope of the present invention which is set forth under the following claims. WE CLAIM; 1. A process for preparation of Strontium Modified lead zirconate titanate (PZT) ceramics comprising of the following steps: a) preparing solutions of pure nitrates of Zirconium (Zr) and Titanium (Ti) and mixing them in stoichiometric proportions, b) conversion as herein described of nitrates of Zr and Ti into hydroxide coprecipitate (Zrx Ti1-x) (OHJ4, c) decomposition of hydroxides coprecipitate (Zrx Ti1-x) (OH)4 into oxide (Zrx Ti1-x)O2 is carried out by heating at 200-400°C, d) preparation of lead (Pb) and strontium (Sr) carbonate co¬ precipitate (Pbo.94 Sro.o6) CO3 by addition of (NH4)2 CO3 to the stoichiometric Pb(NO3) 2 and Sr(NO3) 2 , e) ball-milling as herein described and calcinations of the mixture of (Zrx Ti1-x)O2 and (Pbo.94 Sro.o6) CO3 to obtain Pbo.94 Sro.oe (Zrx Ti1-x)O3 is carried out at 500 to 800°C, f) cold compaction is carried at 150 to 250 Mpa and sintering to form sintered PZT ceramics is carried out at 1000 to 1200°C preferably for 2 to 4 hours. g) electroding and poling of sintered PZT ceramics is carried out at 30k V/cm to obtain strontium modified lead zirconate titanate (PZT). 2. A process for preparation of strontium modified PZT ceramics as claimed in claim 1 where pure solutions of Zr and Ti are prepared by precipitating their impure chloride solutions with dilute ammonia solution, washing and -dissolving hydroxide thus obtained into nitric acid. 3. A process for preparation of strontium modified PZT ceramics as claimed in claim 1 where said hydroxide coprecipitate (Zrx Ti1-x) (OH)4 of Zr and Ti is prepared by spraying pure and stoichiometric solution of nitrates of Zr and Ti into dilute ammonia solution with pH preferably in the range of 7.5 to 7.8 at the preferred temperature of 60 to 80°C. 4. A process for preparation of strontium modified lead zirconate titanate (PZT) ceramics as substantially described and illustrated herein.

Documents:

199-del-2000-abstract.pdf

199-del-2000-claims.pdf

199-del-2000-correspondence-others.pdf

199-del-2000-correspondence-po.pdf

199-del-2000-description (complete).pdf

199-del-2000-form-1.pdf

199-del-2000-form-19.pdf

199-del-2000-form-2.pdf

199-DEL-2000-Form-3.pdf

199-del-2000-gpa.pdf