|Title of Invention||
AM IMPROVED PROCESS FOR THE PREPARATION OF ZIRCONIUM OXIDE FIBERS FROM NOVEL SOL OF ZIRCONIUM OXIDE
|Abstract||The present invention provides an improved process for the preparation of zirconium oxide fibers from novel sol of zirconium oxide which comprises drawing gel fibers by any conventional method at temperature ranging from 25-35°C from zirconium oxide sol having been prepared using hydrogen peroxide followed by drying the gel fibres at a temperature in the range of 1000-200°C for a period in the range of 2-8 hrs, heating the resulting gel fibers at a temperature in the range of 800 - 1200°C for a period in the range of 0.5 - 3.0 hrs. for obtaining the zirconium oxide fibres. The zirconium oxide fibre is useful as insulating material in furnace and as reinforcement material in composite in ceramic and cement industries .|
|Full Text||This invention relates to an improved process for the preparation of fibres from the novel sol of zirconium oxide. The fibres prepared by the process of the present invention are used mainly in two areas: (1) as insulating material in furnaces etc. employing high temperature after processing them into mats, boards etc.; (ii) as reinforcement materials in composites with ceramic, cement and metal. The invention thus broadly relates to ceramic and cement industry.
Starting materials for the preparation of zirconium oxide (or briefly "zirconia") fibres are normally prepared by the following techniques:
(a) Melting of high purity oxide powders:
In this method high purity raw materials are mixed and melted at 1800°C. Fibres are produced after blowing the melt. Fibres obtained are of 2 to 4 micron diameter and lengths up to 20 cm.
The method is described by M. Natrajan in Proc. Inter-nat. Symp. Refractories held at Jamshedpur during February 27-29, 1992; pp.169-171.
The above method suffers from the following disadvantages: (i) Processing temperature is very high, making handling of materials extremely difficult and expensive;
(ii) At such a high temperature, compositional non-homogeneity arises.
(b) Impregnation of cellulose fibres with a solution
prepared from zirconium and dopant cation salts:
The method is described in P.A. Vityaz, I.L. Fyodorova, I.N. Yermolenko and T.M. Ulyanova, Ceramics International, 9_ (1983) 46-48.
In this method, the cellulose fibres whose external surfaces and internal pore surfaces are coated with the solution, are then calcined at 550°C to burn out cellulose fibre, and the remaining zirconium-containing part is finally calcined up to 1000°C to 1200°C to make oxide fibre. The diameters of the fibres are 8-10 micron.
The above impregnation method suffers from the following main drawbacks :
(i) The fibres are generally hollow as also porous because of the burning of cellulose fibres (core) during calcination, when a large amount of gas evolution also takes place. This makes the fibres fragile and low-strength.
(ii) Because of the high weight ratio of cellulose/zirconium salt, the probability of carbon formation is extremely high, which causes the above damages in a high degree. The weight loss is also extremely high, i;e. more than 60% at 80°C.
(c) Hydrolysis of zirconium alkoxides :
This process is described in detail in K.Kamiya, T. Yoko, K.Tanaka and H. Itoh, Yogyo-Kyokai-Shi, £5 (1987) 1157-1163.
In this method, zirconium n-propoxide [ZrfCXLHL) ] is allowed to dissolve in alcohol and react with water. The process of hydrolysis leads to fully or partially hydrolyzed molecules, which gives rise to ultrafine polymeric particles via condensation in an organic liquid.
This state, In which zirconium-containing ultrafine particles are floating about in an alcoholic liquid, is connoily called a "zirconia" sol. This sol is heated at 60°-90°C for promoting the condensation reaction and removing the liquid slowly, in which process the viscosity of the sol increases. A stage is arrived at by continuing this heating process when the sol attains such a viscosity level that fibres can be drawn by hand from it. Such fibres are now in the form of gel. On heating, the gel fibres are converted to zirconia fibres. Cations required to be present in the zirconia are added in the form of soluble salts in the sol.
The alkoxide route described above suffers from the following drawbacks :
(i) The rate of hydrolysis of zirconium alkoxide is very high and any contact with atmospheric moisture leads to rapid hydrolysis and hydroxide precipitation, making the preparation of. fibres extremely difficult. Therefore, prepration of sols from alkoxides requires very slow addition of water in the alkoxide solution and makes the process extremely slow;
(ii) If no chemical agent is added to slow down the rate of hydrolysis of the alkoxide, the trace water dissolved In the solvent (usually alcohols) may also create the same problem of uncontrolled hydrolysis.
(ill) If a chemical agent is added to slow down the rate of hydrolysis, such an agent prevents rapid hydrolysis by chelating action, but creates other- problems during heating of the gel fibres. Acetylacetone, CH COCH-COCH is a typical example of
O Z 3
chemical agents reducing the rate of hydrolysis of alkoxides used by various workers. This compound has the ability of chelate formation with the alkoxide, thus making some of the hydrolysable, functional groups of the alkoxide molecule inactive. The chemical reactions involved have been described by J.C. Debsikdar in J. Non-Cryst. Solids, 86 (1986)231-240.
The use of acetylacetone in preparing zirconia fibres has been described by T. Yogo in J; Mat; Sci., 25_ (1990) 2394-2398. The fibres obtained in this work have diameter ranging from 3 to 100 micron.
This method is disadvantageous because it produces residual carbon in the oxide fibres even after calcination at 800°C.
(d) Fibres from zirconium acetate sol :
Zirconium acetate in aqueous medium exists as diacetato zirconic acid and is assumed to have a formula of Zr(OH)IOCOCH_)2
xH . Different methods are followed for fibre preparation using z
(1) The zirconium acetate solution (containing 22 wt% ZrO ) is
concentrated under vacuum at 50°C in an evaporator. The concentration is monitored by measuring the volume of evaporated wateri After concentration at the required level, the concentrated liquid is allowed to age at 20°C to attain the proper fiberizability. Fibres are obtained by extrusion and drawn by air jet. The oxide fibres are produced at 750°C. In this method, fibres with about 10 micron diameter and 20 mm maximum length are obtained. The details have been described in E. Leroy, C. Robin-Brosse and J.P. Torre, Ultrastructure Processing of Ceramics, Glasses and Composites, Edited by L.L. Hench and D.R. Ulrich, John Wiley 6 Sons, New York, 1984 ; pp;219-231.
(2) In this method water-soluble fibre precursor granules are
prepared from pH-modified zirconium acetate solutions (containing
20-22 wt% ZrCL). Fibres are produced by a rotary fiberization z
process. The gel fibres are calcined up to 1250°C. The diameter of the oxide fibres is about 3 micron. The method is described in detail in S.S. Jada and J;F. Bauer, Ceram. Eng. Sci. Proc., 11(1990) 1480-1499;
The above methods using zirconium acetate suffer from the following disadvantages :
(i) In method (1), a complicated step of vacuum distillation is required, which makes the fiberizable sol preparation cumbersome and less economic (additional equipment and time are necessary).
(ii) In method (2), fibre precursor granules are first prepared from the" acetate, which means an additional step, and hence, increase in preparative time.
(e) Fibres from colloidal sols containing ultrafine ZrO particles and a polymeric binder :
In this process, ultrafine powders are dispersed in a liquid medium to obtain a soli The viscosity is increased by the addition of an organic polymer (polyethylene oxide). This sol is then further concentrated by vacuum evaporation before fibre drawing. Fibres are drawn by centrifugal spinning. Oxide fibres are obtained after calcination up to 1500°C. The diameters of the fibres are The disadvantages of the process are :
(i) Addition of an organic polymer causes carbon deposition and formation of pores during calcination of gel fibres. Such fibres also crack easily;
(ii) An ultrafine powder batch, suitable for the preparation of a homogeneous colloidal sol without aggregation/flocculation, has to be first prepared or used. This itself is a time-taking and inconvenient step.
(ill) The vacuum evaporation step makes the process further time-consuming and requires additional equipment.
The necessity of dopant cations in the above processes may be explained as follows:
When a pure zirconia ceramic is cooled from its sintering or service temperature to room temperature, then due to tetragonal monoclinic polymorphic transformation of crystalline ziconia, which is associated with 3-5% volume expansion, the ceramic suffers catastrophic cracking and fails to show its performance. To overcome this difficulty, cubic stabilizing oxide, e.g. CaO, MgO, CeC^ etc. is added to the system, whereby cubic ZrC>2 exists at room temperature up to its melting point. Depending on the concentration of the dopant, partially or fully stabilized zirconia is obtained. Such stabilized zirconia is used with full potential (i.e. no cracking, increase in strength and/or toughness etc.) in ceramic industry.
The object of the present invention is to provide an improved process for the preparation of a starting sol for making thin, crystalline zirconia fibres with doping of other cations which include, but are not confined to Ca2+, La3+ and Ce4+.
Accordingly, the present invention provides an improved process for the preparation of fibres from the novel sol of zirconium oxide. The preparation of sol has been made a subject matter of an copending application No.805/DEL/92 from which this application has been divided out. The preparation of sol comprises:
(i) dissolving zirconyl chloride (ZrOCl2.8H20) in water at ambient temperature for obtaining a clear solution; (ii) adding ammonia to zirconyl chloride solution at ambient temperature for precipitating zirconium as hydrous zirconium oxide;
(iii) filtering and washing the precipitate with water to remove the electrolytes;
(iv) adding glacial acetic acid (99.8%) to the filtered cake and heating the resulting material at a temperature in the range of 60°-95°C for a period ranging from 12 to 30 hrs. to obtain an acetate solution;
(v) mixing dopant cations in the form of their soluble salts with the zirconium acetate solution;
(vi) adding hydrogen peroxide (M) to the resultant solution at ice-cold temperature and ageing the sol thus obtained at ambient temperature and
(vii) further ageing the sol at a temperature in the range of 40°-60°C to attain the proper fiberizability.
According to the present invention there is provided a process for the preparation of zirconium oxide fibres from novel sol of zirconium oxide which comprises drawing gel fibres by any conventional method at ambient temperature from the novel sol, prepared by the process as here in
followed by drying the gel fibres at a temperature in the range of 100-200°C fora period in the range of 2-8 hrs, heating the resulting gel fibers at a temperature in the range of 800-1200°C for a period in the range of 0.5 - 3.0 hrs. for obtaining the zirconium oxide fibres.
Zirconyl chloride is dissolved in deionized water so as to obtain a clear solution of ZrC>2, preferably in a concentration of 0.1 to 0.5 M. Precipitation of hydrous zirconium oxide is effected from this solution by raising the pH in the range of 8-12 by the addition of dilute ammonia solution under stirring at ambient temperature following direct strike process (addition of ammonia solution to the zirconium-containing solution). The precipitate is filtered, washed with deionized water for 3-4 times till the foreign ions (e.g. Cl~, NH4+) are virtually removed.
Zirconium acetate solution is prepared by adding glacial acetic acid to the filtered precipitate preferably at a range of mole ratios of ZrC^acetic acid = 0.01-0.20 and finally heating in the temperature range of 60°-S5°C for a period in the range of 12-30 hrs.
The dopant cation such as Ca2+, La3+, Ce4+ is added in the form of cheir water-soluble salts like Ca(N03)2. 4H20, La(NO3)3. 6H20 and (NH4)2[Ce(N03)g]. The dopant concentration may be kept in the range of 2-20 mol% depending on the zirconium content of the
solution. Hydrogen peroxide Is added into the above solution under ice-cold condition at a mole ratio which may be selected from the range 10 - 20. The sol thus obtained is aged at ambient temperature for a period which may range from 12 - 25 hrs.
Fiberizability. is attained after ageing the above sol at a temperature which may range from 40° - 60°C.
Fibres may be drawn from the viscous sol at room temperature. The drawn ffbres are dried at a temperature which may range from 100° - 200°C. Soaking period is selected from the range 2-8 hrs.
The same sol can also be used for preparing zirconia fibres by spinning.
Crystallization temperature may be in the range of 800° -1200°C with a soaking period in the range of 0.5 - 3.0 hrsi
The preferred conditions for carrying out of the present invention are :
(i) Concentration of ZrCL in the zirconyl =0.25 - 0.4M
(li) pH of precipitation =8-10
(iii) ZrCL : acetic acid =0.05 - 0.10
(iv) Temperature of preparation of =75° - 90°C
zirconium acetate solution
(v) Ageing period of acetate =15 - 25 hrs.
solution at 75°-90°C
(vi) Dopant concentration =3-11 equivalent
(vii) H202/Zr02 =12-17(mole ratio)
(viii) Ageing period of sol at =16-20 hrs.
(ix) Temperature of further ageing of =45°-55°C the sol for attaining fiberizability
Ihe above sol can be used for fibre drawing. Preferred conditions for processing of drawn gel fibres are :
(I) Tenperature for gel fibre drying =150° - 160°C
(ii) Soaking period (at the drying =4-6 hrs.
(iii) Crystallization tenperature =900°C - 1000°C
(iv) Soaking period =1-2 hrs.
The invention is described in detail in the following exanples which are given by way of illustration only and therefore should not be construed to limit the scope of the invention.
Example 1 :
500 gm of zirconyl chloride (ZrOCl2.8H2O)was dissolved in 6207 ml of deionized water so as to obtain a solution of ZrCL
content 0.25 M and filtered to get a clear solution. Hydrous zirconium oxide was precipitated at pH 9 by adding 1 : 1 amnonia solution to the zirconyl chloride solution under stirring at ambient tenperature (30°±2°C). The precipitate was filtered and washed thoroughly four times with deionized water till the foreign ions, e.gi C1~NH, etci were virtually removed. The ZrO, content of the precipitate was determined by assaying it to 1000°C with 1 hr. soaking and was found to be 12.11 wt%.
1578 gm of the freshly precipitated hydrous zirconium oxide
was dissolved in 2225 ml of glacial acetic acid under stirring at 00°C for 20 hrs. to obtain a clear solution. The concentrations of ZrO« and acetic acid of this solution were found to be 0;50 M and
10.04 M respectively. This solution was designated as 'ZA1'.
To 50 ml of ZA1, Oi3760 gm of calcium nitrate, Ca(NO ) ,4H 0
o / 2*
(equivalent to 6 mol% CaO) was added. 42.5 ml of HO was then mixed with the above solution (dropwise) at ice-cold temperature under stirring for 15 rnin. The sol thus obtained was allowed to age at ambient temperature for 16 hrs.
The sol was then aged at 50°C to attain the proper fiberizability. About 90-100 an long gel fibres were hand-drawn at ambient temperature by dipping a glass rod of 0.3 an diameter into the viscous sol and pulling it up. The gel fibres were dried at 150°C for 6 hrs; and finally calcined at 1000°C with 1 hr. soak and furnace cooling.
The diameter of the gel fibres was 25-40 micron. After calcination, the diameter reduced to 4-25 micron and lenght became 4-5 on. The calcined fibres crystallized as tetragonal ZrCL with a
crystallite size of 21.36 run. Example 2 :
In this case, the same zirconium acetate solution, ZA1, as described in Example 1 was usedi
To 50 ml of ZA1, 1.4759 gm of Ca(NO,)9.4H90 (equivalent to 20
O /J £•*
mol% CaO) was mixed. 42.5 ml of HO was then added to the above
solution (dropwise) at ice-cold temperature under stirring for 15 min. The sol was then allowed to age at ambient temperature for 16 hrs.
'Ihe sol was further aged at 50°C to attain the proper fiberizobiIIty. About 20-30 cm long gel fibres were hand-drawn
from the viscous sol at ambient temperature. The gel fibres were dried at 150°C for 6 hrs. and finally calcined at 1000°C with 1 hr. soak and furnace cooling.
The diameter of the gel fibres was 15-30 micron. After calcination, the diameter reduced to 5-20 micron and length became 4-
6 on. The calcined fibres crystallized as cubic ZrCL with a
crystallite size ol 20.59 nm. Example 3 :
800 gm of zirconyl chloride (ZrCCl ;8R.O) was dissolved in 4965 ml of deionized water to make the ZrCL content of the solution 0.5 M and filtered to obtain a clear solution. Hydrous zirconium oxide was precipitated at pH 8 by adding 1:1 ammonia solution to the zirconyl chloride solution under stirring at ambient temperaturei The precipitate was washed thoroughly four times with deionized water till the foreign ions, e.g. Cl",NH. etc. were virtually removed and filtered;
The ZrCL content of the precipitate was determined after calcining it to 1000°C with 1 hr. soaking. The value was found to be 10.70 wt%.
2843il6 gm of the freshly precipitated hydrous zirconium oxide was dissolved in 975 ml of glacial acetic acid at 90°C for 25 hrs. to obtain a clear solution. The concentrations of ZrCL and acetic acid of this solution were found to be 0.78 M and 4.95 M respectively. This solution was designated as 'ZA21.
To 50 ml of ZA2, 1.0444 gm of lanthanum nitrate, La(NCL) ,6H 0
(equivalent to 3 mol% La_CL) was added. 48.65 ml of H_CL was then
£t O Z £t
mixed with the above solution (dropwise) at ice-cold temperature under stirring for 20 min. The sol thus obtained was allowed to age at ambient temperature for 18 hrs.
The sol was further aged at 55°C to attain the proper fiberizabilityi About 100-110 on long gel fibres were hand-drawn from the viscous sol at ambient temperature. The gel fibres were dried at 200°C for 4 hrs., finally calcined at 1000°C with 1 hr. soak and furnace cooling.
The diameter of the gel fibres was 10-30 micron. After calcination, the diameter reduced to 3-15 micron and length becane 2-3 on. The calcined fibres crystallized as tetragonal ZrO2 with a crystallite size of 19.56 nm.
Example 4 :
In this case, the same zirconium acetate solution, ZA2, as described in Exanple 3 was used.
To 50 ml of ZA2, 2.1145 gm of ammonium eerie nitrate, (NH ) [Ce(NO3 ) ] (equivalent to 9 mol% CeOJ was added. 48.65 ml
of HO was then added to the above solution (dropwise) at
ice-cold temperature under stirring for 15 min. The sol thus obtained was allowed to stand at ambient temperature for 16 hrs.
line sol was further aged at 60°C to attain the proper fiberizability. About 100-120 cm long gel fibres were hand-drawn from the viscous sol at ambient tanperature. The gel fibres were dried at 175°C for 8 hrs., finally calcined at 1000°C with 1 hr. soak and furnace cooling.
The diameter of the gel fibres was 20-35 micron. After calcination, the diameter reduced to 4-12 micron and length became 3-5 on. Tetragonal ZrO2 was identified in the oxide fibre and crystallite size obtained was 28.04 run.
1. An improved process for the preparation of zirconium oxide fibers from
novel sol of zirconium oxide which comprises drawing gel fibers by any
conventional method at temperature ranging 25-35°C from zirconium
oxide sol having been prepared using hydrogen peroxide as herein
described, followed by drying the said gel fibres at a temperature in the
range of 100-200°C for a period in the range of 2-8 hrs, heating the
resulting gel fibers at a temperature in the range of 800 - 1200°C for a
period in the range of 0.5 - 3.0 hrs. for obtaining the zirconium oxide
2. An improved process for the preparation of zirconium oxide fibres from the
novel sol of ziroconium oxide substantially as herein described with
reference to the example.
|Indian Patent Application Number||1511/DEL/1998|
|PG Journal Number||12/2008|
|Date of Filing||03-Jun-1998|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001.|
|PCT International Classification Number||C04B 35/005|
|PCT International Application Number||N/A|
|PCT International Filing date|