Title of Invention

LOW TEMPERATURE SYNTHESIS OF NANOCRYSTALLINE SPINEL POWDER

Abstract The present invention provides a large scale process for the synthesis of nanocrystalline spinel powder at low temperature. The spinel powder is produced by preparing a transparent resin by mixing constituent metal salts and organic precursor molecules such as methylol urea which is then subjected to drying and self sustained combustion to form homogeneous precursors that form spinel phase during calcinations at temperature in the range of 700°C to 1200°C. The calcined combustion product on milling produces the spinel powder that could be subjected to uni-axial pressing and sintering at a temperature in the range of 1500 to 1700°C resulting in a ceramic with more than 99% of theoretical density.
Full Text FIELD OF INVENTION
The present disclosure relates to the development of a process for low temperature synthesis of nanocrystalline spinel powder.
BACKGROUND OF THE INVENTION
Magnesium aluminate (MgAhO4) spinel powder is a strategic material for making transparent ceramics for transparent armour and infrared windows as they provide ballistic, thermal and mechanical protection in equipments that require transmission of light or other electromagnetic radiations owing to its high mechanical strength, high hardness, low density and excellent transmittance for ultraviolet to infrared light. Direct fabrication of spinel ceramics from the constituent oxides by solids state mixing and sintering is difficult as the spinel formation is accompanied by nearly 5% volume expansion. In actual practice, a two-stage firing process is used for preparation of MgAl2O4 spinel. By this solid state route, it is difficult to prepare fine MgAl2O4 powders which can achieve high density during sintering. Moreover, the spinel ceramic fabricated by this method is quite expensive.
Various wet-chemical methods, such as co-precipitation, sol-gel, mechano-chemical synthesis and combustion synthesis have been used for the preparation of MgAl2O¬ spinel. The co-precipitation method described by Bratton et al.,Am.Ceram.Soc. Bull. 48, 759-62 (1968), Li et al., Ceram. Int. 27,481-89(2001), and Katanic-Popovic et al, Ceram. Int. 17, 49-52 (1991). US4,400,431 describes preparation of magnesium aluminate spinel from water soluble compounds of aluminium and magnesium. In this process, the hydroxide precipitate prepared form aqueous solution of aluminum and magnesium salts was calcined at temperature of 1100°C and above for preparation of spinel. US4,492,678 discloses spinel preparation from sodium aluminates and magnesium nitrate. The process requires rigorous washing to remove the sodium ions. 1186,306,360 describes a method for preparation of spinel powder by co-precipitation from aqueous solution containing magnesium nitrate and aluminium nitrate. All the above reported co-precipitation processes require high temperatures for spinel formation.
The sol-gel method for the preparation of spinel has been described by Shiono et al. J.Am. Ceram. Soc., 83, 235-57 (2000), Lepkova et al., J. Mater. Sci., 26, 4861-64 (1991), and Naskar et al, J. Am. Ceram. Soc., 88,38-44 (2005). US 4,474,745 discloses method of producing magnesium aluminate spinel which comprises reacting magnesium oxide or hydroxide with an aluminum chlorohydrate-glycol complex to form a magnesium aluminum salt-containing composite in the form of a gel followed by reacting the composite with an alkali to hydrolyze the salt to the hydroxide and heating the hydroxide at elevated temperatures to form magnesium aluminate spinel powder. The method suffers from likely contamination of the product with alkali metal ions. In sol-gel process, a large volume of the sol produce small quantity of powder and therefore the process is only suitable for low volume products. More over the process uses costly precursor materials.
The mechano-chemical synthesis reported by Domanski et al.., J.Am.Ceram.Soc. 87,2020-24 (2004), and Kim et al, Powder Technol, 113, 109-13 (2000) describe the method of spinel formation from heterogeneous sol-gel precursors A1(OH)3 & Mg(OH)2. at low temperature with the help of high intensity mechanical milling. However, the process is only at demonstration level.
Bhaduri et al, J. Mater. Res., 14, 3571-80 (1999), and Ganesh et al, Brit. Ceram. Trans., 101, 247-54 (2002) have reported the preparation of pure MgAl2O4 powders using combustion synthesis. Among the wet chemical methods combustion synthesis is a simpler and faster production method. However, the redox reaction between the oxidant (metal salts) and fuel (urea, glycine, citric acid etc.) is so vigorous and fast that a large volume of gases are generated in a small interval of time. This results in foaming and overflow of the contents from the vessel during combustion which limits high volume production of spinel powders by these methods.
Therefore, there exists a need for development of a low temperature combustion synthesis process having a moderate combustion rate that could be used for large volume production of nanocrystalline spinel powder.
UMMARY OF THE INVENTION
The present invention relates to the development of a process for low temperature combustion synthesis of nanocrystalline spinel powder comprising the steps of forming methylol urea solution by mixing urea and formaldehyde solution in a ratio of 1:4 at pH ranging from 8 to 10, dissolving a mixture of metal salts, adding further urea to obtain a resultant methylol urea solution having urea and formaldehyde in the ratio of 1:2, heating the resultant methylol urea solution to form a transparent resin, drying and combusting the resin to obtain a combustion product, calcining the combustion product to obtain a calcined product and milling the calcined product to obtain spinel powder.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 XRD spectrum of the combustion product calcined at various temperatures showing formation of spinel at 800°C.
Figure 2 TEM photograph of spinel powder showing crystallite size of nearly lOnm. Figure 3 Variation of surface area of the spinel powder with calcination temperature.
DETAILED DESCRIPTION
The present disclosure relates to a process for synthesis of nanocrystalline spinel powder, said method comprising;
(a) preparing methylol urea solution by mixing urea and formaldehyde solution in
a ratio of 1:4 at a pH ranging from 8 to 10.
(b) dissolving a mixture of metal salts in the methylol urea solution and adding
further urea to obtain a resultant methylol urea solution having ratio of urea and
formaldehyde 1:2
(c) heating the resultant methylol urea solution to form a transparent resin
(d) drying and combusting the resin to obtain a combustion product
(e) calcining the combustion product at a temperature in the range of 700°C to
900°C for a period of 1-6 h (s) to obtain a calcined product; and
(f) milling the calcined product to obtain spinel powder.
One embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder wherein the metal salts that are dissolved in
the methylol urea solution are selected from a group consisting of aluminium nitrate, aluminium chloride, aluminium acetate, magnesium nitrate, magnesium chloride, and magnesium acetate.
Another embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder wherein the resultant methylol urea solution is heated in a temperature range of 70°C to 120°C.
Further embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder wherein the dried resin is combusted in a temperature range of 175°C to 250°C.
Yet another embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder wherein the combustion product is calcined in a temperature range of 700°C to 900°C for a period of 1-6 hour(s).
Still another embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder that further comprises heating the transparent resin for a duration 30-45 mins to obtain transparent hybrid gel. The heating could be carried out on a hot plate.
Another embodiment of the present disclosure provides a process for low temperature synthesis of nanocrystalline spinel powder wherein the nanocrystalline spinel powder is further subjected to uni-axial pressing and sintering in a temperature range of 1500 to 1700°C resulting in a ceramic with more than 99% of theoretical density.
Another embodiment of the present disclosure provides a process wherein large amount of gaseous byproducts are formed during the combustion process which makes the product highly porous with high surface area. However the surface area decreases with increase in calcination temperature due to grain growth and particle coarsening.
Still another embodiment of the present disclosure provides a process resulting in the nanocrystalline spinel powder with a specific surface area ranging from 10 m /g to 130m2/g.
Yet another embodiment of the present disclosure provides a process resulting in the nanocrystalline spinel powder characterized as having particle size ranging from 0.1 -l0µm.
Further embodiment of the present disclosure provides a process resulting in the nanocrystalline spinel powder characterized as having crystallites of size ranging from 5 to 20 nm.
Another embodiment of the present disclosure provides a process for the development of pure, reactive spinel powder useful in the preparation of transparent ceramics by sintering and hot isostatic pressing.
Yet another embodiment of the present disclosure provides a mild gel combustion process to be used for large volume production of spinel powder.
Yet another embodiment of the present disclosure provides a process for the combustion synthesis of nanocrystalline spinel powder that uses urea-formaldehyde as gel forming agent and fuel for combustion synthesis of spinel powder. The reaction of urea with formaldehyde solution in a ratio of 1:4 leads to the formation of methylol urea that remains in homogeneous solution. The complete urea-formaldehyde polymerization reaction requires mole ratio 1:2. However, if the urea and formaldehyde are mixed in the mole ratio 1:2 at pH 8.5, methylol urea precipitates out. Additional urea is dissolved in the methylol urea solution to make the urea:formaldehyde mole ratio 1:2 in order to avoid the precipitation.
Yet another embodiment of the present disclosure provides a process for the combustion synthesis of nanocrystalline spinel powder that uses metal salts, preferably nitrates, of aluminum and magnesium as oxidant.
One embodiment of the present disclosure provides a process wherein the formation of an organic-inorganic gel takes place with the metal nitrates. When the metal nitrate is dissolved in the methyl urea-solution, the pH of the solution comes down to the acidic range due to hydrolysis of the nitrates. At this acidic pH, methylol urea reacts with the urea added in the second stage to form urea-formaldehyde polymer. The metal ions form co-ordination complex with the urea-formaldehyde (UF) polymer through NH, OH and CO groups to formal transparent organic-inorganic hybrid gel. The dried gel showed a strong exothermic peak at temperature nearly 200°C corresponding to the combustion of the gel. The dried gel undergoes self sustained combustion due to the nitrate oxidant when initiated with help of a glowing splinter in a set up fabricated in the laboratory.
Another embodiment of the present disclosure provides a process wherein a base preferably ammonium hydroxide and an acid preferably nitric acid is used for pH adjustments.
Still another embodiment of the present disclosure provides the set-up used for combustion of the organic-inorganic gel. The combustion set up is an open cylindrical vessel whose walls are insulated with glass wool insulation. Alternatively, the organic-inorganic hybrid gel in a glass beaker can be heated on a hot plate for drying and subsequent combustion. The combustion process is not as vigorous as the combustion processes with the common fuels such as urea, glycine and citric acid. Therefore, large volume production could be possible by continuous combustion by feeding the dried gel in to the combustion set up in which the combustion process has already been initiated. Since the metal ions are homogeneously distributed through out the gel matrix due to co¬ordinate complex formation, the combustion product formed is a homogeneous mixture of the spinel precursors.
The precursors further react during calcinations at low temperature to form the crystalline spinel phase. Formation of large volume of gases during the combustion process and formation of the spinel at low temperature keeps the crystallites in the nanocrystalline range and surface of the powder relatively high. These nanocrystalline spinel powders are highly reactive and sintered to near to 99% of its theoretical density by heating at temperature in the range of 1500°C to 1700°C and expected to produce transparency by hot pressing or hot isostatic pressing.
The present disclosure is more particularly described with reference to the following non-limiting examples.
EXAMPLES Example 1
500 ml of formaldehyde solution (37g/100mL) was prepared and its pH was adjusted to 8.5 using ammonium hydroxide. To this solution, 92.5g of urea was added resulting in the formation of a methylol urea solution. The solution obtained was permitted to age for 24 hours after which the metal salts, aluminum nitrate (385.55g) and magnesium nitrate (131.766g), were dissolved. Another 92.5g of urea was dissolved in the reaction mixture
to produce a resultant methylol urea solution with a urea: formaldehyde ratio of 1:2. The solution obtained was then heated in a boiling water bath for 30 to 45 minutes to form a transparent resin which was then dried in an oven at 120° C. The dried resin was ignited inside a 5L glass beaker with glass wool insulated walls. The ignition was initiated using a glowing splinter. The product obtained from combustion was calcined at a temperature of 850° C for 5 hours. The calcined product was ball milled in isopropanol medium using a planetary ball mill and zirconia grinding media for 12 hours to obtain the spinel powder.
Example 2
500 ml of formaldehyde solution (37g/100mL) was prepared and its pH was adjusted to 8.5 using ammonium hydroxide. To this solution, 92.5g of urea was added resulting in the formation of a methylol urea solution. The solution obtained was permitted to age for 24 hours after which the metal salts, aluminum nitrate (385.55g) and magnesium nitrate (131.766g), were dissolved. Another 92.5g of urea was dissolved in the reaction mixture to produce a resultant methylol urea solution with a urea : formaldehyde ratio of 1:2. The solution obtained was then heated on a hot plate to form a transparent resin. The resin was continuously heated on the hot plate to transform the resin into a transparent gel and to initiate combustion of the gel. The product obtained from combustion was calcined at a temperature of 850° C for 5 hours. The calcined product was ball milled in isopropanol medium using a planetary ball mill and zirconia grinding media for 12 hours.
Example 3
The spinel powder samples prepared by the process described in Example 1 and Example 2 were further subjected to uni-axial pressing and sintering at a temperature ranging from 1500°C to 1700°C resulting in a ceramic with more than 99% theoretical density. Hot pressing of the samples or sintering followed by hot pressing by lead to transparency of the resulting ceramic.
Example 4
X-Ray Diffraction study
Figure 1 shows the XRD spectrum of the combustion product calcined at various temperatures. It shows the formation of spinel at 800°C. Large amount of gaseous byproducts are formed during the combustion process which make the product highly porous with high surface area. However, the surface area decreases with increase in calcination temperature due to grain growth and particle coarsening. Figure 3 shows the variation of surface area of the spinel powder with calcination temperature.
Example 5 Transmission Electron Microscopy (TEM)
Transmission Electron Microscopy photograph of spinel powder was taken to determine the crystallite size. Figure 2 shows the Transmission Electron Microscopy photograph of spinel powder showing crystallite size of nearly lOnm.



We claim,
1. A process for synthesis of nanocrystalline spinel powder, said method
comprising:
(a) preparing methylol urea solution by mixing urea and formaldehyde solution in
a ratio of 1:4 at a pH ranging from 8 to 10;
(b) dissolving a mixture of metal salts in the methylol urea solution and adding
further urea to obtain a resultant methylol urea solution having ratio of urea
and formaldehyde 1:2;
(c) heating the resultant methylol urea solution to form a transparent resin;
(d) drying and combusting the resin to obtain a combustion product;
(e) calcining the combustion product at a temperature in the range of 700°C to
900°C for a period of 1 -6 h (s) to obtain a calcined product; and
(f) milling the calcined product to obtain spinel powder.

2. The process as claimed in claim 1, wherein the metal salts are selected from a
group consisting of aluminium nitrate, aluminium chloride, aluminium acetate,
magnesium nitrate, magnesium chloride, and magnesium acetate
3. The process as claimed in claim 1, wherein the heating is carried out at
temperature ranging from 70°C to 120°C.
4. The process as claimed in claim 1, wherein the combusting is carried out at
temperature ranging from 175°C to 250°C.
5. The process as claimed in claim 1, optionally comprising heating the transparent
spinel powder resin for a time period in the range of 30-45 min to obtain
transparent hybrid gel.
6. The spinel powder prepared according to the process as claimed in claim 1, that
is further subjected to uni-axial pressing and sintering at a temperature in the
range of 1500 to 1700°C resulting in a ceramic with more than 99% of theoretical
density.
7. The spinel powder as claimed in claim 1, characterized in that having a specific
surface area ranging from 10 m2/g to 1302/g.

8.
The spinel powder as claimed in claim 1, characterized in that having particle size ranging from 0.1-lOum.
9.
The spinel powder as claimed in claim 1, characterized in that having crystallites of size ranging from 5 to 20nm

Documents:

368-del-2008-abstract.pdf

368-del-2008-claims.pdf

368-DEL-2008-Correspondence-Others (09-10-2009).pdf

368-del-2008-Correspondence-Others-(28-04-2008).pdf

368-del-2008-correspondence-others.pdf

368-del-2008-description (complete).pdf

368-del-2008-drawings.pdf

368-del-2008-form-1.pdf

368-DEL-2008-Form-18.pdf

368-del-2008-form-2.pdf

368-del-2008-form-3.pdf

368-del-2008-form-5.pdf

368-del-2008-GPA-(28-04-2008).pdf

Abstract_Clean.pdf

Clean copy.pdf

complete spec.pdf

FER Response.pdf

Others.pdf


Patent Number 265659
Indian Patent Application Number 368/DEL/2008
PG Journal Number 11/2015
Publication Date 13-Mar-2015
Grant Date 03-Mar-2015
Date of Filing 12-Feb-2008
Name of Patentee DIRECTOR GENERAL, DEFENCE RESEARCH & DEVELOPMENT ORGANIZATION
Applicant Address MINISTRY OF DEFENCE, GOVERNMENT, OF INDIA, ROOM NO.348, B-WING DRDO BHAVAN, RAJAJI MARG, NEW DELHI-110011
Inventors:
# Inventor's Name Inventor's Address
1 PRABHAKARAN KUTTAN NAVAL MATERIALS RESEARCH LABORATORY, SHIL-BADLAPUR ROAD, ANAND NAGAR P.O.ADDL. AMBERNATH, THANE-421506
2 PATIL SHIVAJIRAO DINESH NAVAL MATERIALS RESEARCH LABORATORY, SHIL-BADLAPUR ROAD, ANAND NAGAR P.O.ADDL. AMBERNATH, THANE-421506
3 DAYAL RAJIV NAVAL MATERIALS RESEARCH LABORATORY, SHIL-BADLAPUR ROAD, ANAND NAGAR P.O.ADDL. AMBERNATH, THANE-421506
4 SHARMA CHANDRA SURESH NAVAL MATERIALS RESEARCH LABORATORY, SHIL-BADLAPUR ROAD, ANAND NAGAR P.O.ADDL. AMBERNATH, THANE-421506
5 GOKHALE MADHUSUDAN NITIN NAVAL MATERIALS RESEARCH LABORATORY, SHIL-BADLAPUR ROAD, ANAND NAGAR P.O.ADDL. AMBERNATH, THANE-421506
PCT International Classification Number C01G 49/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA