RARE EARTH PHOSPHATE BASED NON REACTIVE AND NON-WETTABLE SURFACES

Abstract

The present invention provides the use of lanthanum phosphate for creating non wetting, non-reactive surfaces for molten metals like zinc and aluminium. By virtue of this property, lanthanum phosphate finds extensive applications in metallurgical industry for metal casting.

Claims

1. A metallophobic ceramic monolith made of nano rare-earth phosphates having density in the range of 96-99% (4.8-4.9 g/cc) of theoretical density for rendering a non-wetting and non-reactive surface to handle molten metals.

2. The metallophobic ceramic monolith, as claimed in claim 1, for providing a non-reacting and non-adhering surface in foundry and metallurgical applications for holding the molten metal at the melting temperature (up to 1200 C.) for 3-6 hours.

3. The metallophobic ceramic monolith as claimed in claim 1, wherein the molten metal comprises metallic elements alone or metallic elements in combination with alloys, metal composites, transition metals and semi metals.

4. The metallophobic ceramic monolith as claimed in claim 4, wherein the metallic element is Aluminium, Silver or Zinc.

5. The metallophobic ceramic monolith as claimed in claim 1, wherein the monolithic surface is formed by consolidation of nanopowders using suitable binders selected from the group of polyvinyl alcohol, methyl cellulose, ethyl cellulose, poly vinyl pyrrolidone.

6. A process for making metallophobic ceramic monolith, made of nano rare-earth phosphates, having density in the range of 96-99% (4.8-4.9 g/cc) of theoretical density for rendering a non-wetting and non-reactive surface to handle molten metals, comprising the steps of (a) Spray granulating rare earth phosphate nanorods into granules of desired sizes in the range of 1 to 100.Math. (b) Compaction processing of the above granules under uniaxial and isostatic load to obtain samples of green density of 45-55% Theoretical Density. (c) Coarsening the nanorods by high temperature calcination process and re-dispersion in aqueous medium with adequate binder to obtain a slurry having more than 70% solid loading and pouring them to porous moulds of varying shapes to obtain shaped monoliths having green density equal to 50% of theoretical density (d) Drying of samples obtained from 1(b) and 1(c) under controlled conditions (for e.g.: 45 C. and 75% RH) to obtain crack free specimens with minimum liquid content (e) High temperature treatment of above samples to obtain crack free monoliths of greater than 95% theoretical density.

7. A ceramic coating over crucibles or containers, obtained by coarsening rare earth phosphate nanorods by high temperature calcination process and re-dispersion in aqueous medium with adequate binder to obtain slurry having more than 70% solid loading and allowing it to heat treatment, so as to obtain non-wetting and non-reactive surfaces on heat treatment.

8. The metallophobic crucible or container as claimed in claim 7 for providing a non-reacting and non-adhering surface in foundry and metallurgical applications for holding the molten metal for 3-6 hours at the melting temperature up to 1200 C.

9. The metallophobic crucible or container as claimed in claim 7, wherein the molten metal comprises metallic elements alone or metallic elements in combination with alloys, metal composites, transition metals and semi metals.

10. The metallophobic crucible or container as claimed in claim 7, wherein the metallic element is Aluminium, Silver or Zinc.

11. The A process for obtaining non-wetting and nonreactive surfaces as claimed in claim 7 by coarsening rare earth phosphate nanorods by high temperature calcination process and re-dispersion in aqueous medium with adequate binder to obtain slurry having more than 70% solid loading and allowing it to heat treatment for obtaining an adherent film.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0026] FIG. 1: Flow chart of the mixed rare earth phosphate powder preparation.

[0027] FIG. 2: Viscosity vs. shear rate of slurries with varying solid loading for casting.

[0028] FIG. 3: Free standing lanthanum phosphate monolith prepared by the casting process.

[0029] FIG. 4: TG-DTA pattern of the powder mixture containing lanthanum phosphate and zinc metal.

[0030] FIG. 5: TG-DTA pattern of the powder mixture containing lanthanum phosphate and aluminium metal.

[0031] FIG. 6: EDAX spectrum of lanthanum phosphate powders after heat treatment at temperatures greater than 750 C.

[0032] FIG. 7: Free standing lanthanum phosphate article after melting operations with aluminium metal.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The present invention relates to the process of preparation of rare earth phosphate based non-reactive and non-wettable surface for molten metals for high temperature applications such as mould release coatings or crucibles for metal casting and the said process comprising the steps of: [0034] i. adding rare earth phosphate nanorod powders in water and/or organic solvents like ethanol, isoparopanol to obtain a slurry having solid loading in the range of 1-30 wt % [0035] ii. Preparation of paint formulations based on the aqueous slurry or sprayable formulations based on aqueous/organic slurry by incorporating suitable binders and plasticizers. [0036] iii. Spray/Dip coating rare earth phosphate layers on substrates of glass, metals and ceramics. [0037] iv. Melting of metals like Zn, Al, Ag in the rare earth phosphate coated monoliths and holding at the melting temperatures for 2-5 h [0038] v. adding lanthanum phosphate nanorod powders in water to obtain an aqueous slurry having solid loading in the range of 60 to 75 wt %; [0039] vi. casting the slurry as obtained in step (ii) to monoliths in the form of cylindrical crucibles of varying volumes in suitable porous moulds; [0040] vii. drying the monoliths and sintering the dried shapes in the temperature range of 1350-1550 C. to obtain crucibles/containers [0041] viii. Melting of metals like Zn, Al, Ag in the lanthanum phosphate containers and holding at the melting temperatures for 2-5 h.

[0042] The present invention also relates to the dispersion of lanthanum phosphate in the form of nanorods in aqueous medium with use of suitable dispersants to arrive at coating formulation that can be applied to substrates by the methods of dipping, spraying and casting.

[0043] The present invention also relates to the shaping of lanthanum phosphate dispersions in aqueous media to shaped monoliths of varying sizes and shape followed by the drying and sintering of the same to impart mechanical strength.

[0044] The present invention also demonstrates the suitability of such monoliths as metal melting crucibles without any reaction between each other.

[0045] The present invention also demonstrates the suitability of coatings developed from lanthanum phosphate as mould release agents in metal melting applications.

[0046] Lanthanum phosphate sample of the present invention prepared by slurry based colloidal forming technique of casting is demonstrated to be non wetting and non reacting to molten metals of zinc and aluminium. Additionally, coatings of rare earth phosphate prepared from a paint formulation in aqueous medium and coated to substrates of ceramics and metal is also shown to be non reactive of molten metals.

[0047] In metal casting operations, the mould walls are generally coated with release agents to prevent corrosion of the mould wall with molten metal and also to prevent the adherence of cast part on to the mould walls. The release agents thus facilitate easy release of cast part from the moulds. The prior art of mould release coatings is based on materials like hexagonal boron nitride, graphite etc which provide an interfacial layer between the mould and cast part. The present invention introduces rare earth phosphate based compositions as suitable materials for non-reactivity and non-wettability with molten metals like zinc and aluminium. Rare earth phosphate based powders dispersed in aqueous medium can be applied as coatings on suitable moulds for the casting purpose. Alternatively lanthanum phosphate based rare earth phosphate containers in the form of crucibles can be obtained from the aqueous dispersions of adequate solid loading and can be used directly for casting simple shapes of metal.

[0048] The process begins with the preparation of rare earth phosphates like lanthanum phosphate powders by a modified sol gel process. Solutions of lanthanum salts in water at molar concentrations in the range 0.01 to 0.1 are treated with orthophosphoric acid at 25-35 C. The precipitate thus obtained is subjected to a flocculation procedure at pH 7 and further washed repeatedly with water to free of the chloride/nitrate ions. The washed precipitate is then peptized to a sol at pH 2. The sol is then calcined to obtain powders of typically nanorod morphology with lengths up to 1 micron and width of 10-15 nm.

[0049] According to one aspect of the invention, the powder is made into a paint formulation with appropriate solid loading by employing suitable binders and plasticizers and coated to containers currently employed in foundry for molten metal casting. The coated crucibles are dried under controlled humidity conditions to ensure crack free film formation and further heat treated at temperatures greater than 800 C. The coated foundry crucibles thus obtained can be employed for molten metal casting operations wherein the coating layer is non-wetting and non-reactive to molten metals.

[0050] According to another aspect of invention, monoliths of lanthanum phosphate in the form of crucibles for example, can be cast from aqueous slurry of the powder with solid loading in the range of 60 to 75 wt %. The powder is dispersed in water under controlled pH conditions and milled appropriately with alumina milling media for 5-15 hrs. Slurry is further deaired and poured onto plaster of paris moulds in a slip casting procedure. After sufficient buildup of thickness the excess slurry is drained and the cast shape is subjected to ambient air drying until they are released from the moulds. The dried monoliths after an oven drying at 80 C. is then subjected to sintering at temperatures varying in the range of 1300-1600 C. Crack free sintered crucibles of lanthanum phosphate are thus obtained and can be directly employed for molten metal casting operations

[0051] The invention more specifically relates to the synthesis and dispersion of lanthanum phosphate powders in an aqueous medium followed by its shaping to monoliths by slip casting and sintering to perform as non-reactive containers for molten metal casting. In another embodiment the aqueous/organic dispersions of rare earth phosphates with suitable solid loading and binders can perform as paint/spray formulations for application as coatings on other glass, ceramic and metallic substrates. Another embodiment of the invention relates to the non-wettability of such monoliths and coatings with molten metals at their respective melting temperatures.

EXAMPLES

[0052] Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example I

Preparation of Lanthanum Phosphate Nanorods Powder

[0053] In a typical synthesis for 20 g Lanthanum Phosphate, 37.0198 g Lanthanum nitrate was initially dissolved in 1710 ml de-mineralized water to form lanthanum nitrate hexahydrate. Lanthanum phosphate was precipitated by adding the phosphate source 88% ortho phosphoric Acid (5.44 ml) drop by drop and the precipitate formed is further flocculated by addition of 25% ammonia solution and pH was raised to 7. The precipitate was then washed with luke warm water. The washed precipitate was then filtered and redispersed in de-mineralized water and further peptized to form lanthanum phosphate sol (pH 2) using 20% nitric acid with continuous, vigorous stirring to obtain the sol. The stability of the sol is checked using zeta potential measurement.

Example 2

[0054] In another example, mixture of rare earth nitrates comprising the elements of La, Pr, Sm and Ce was dissolved in demineralized water and treated with stoichiometric amounts to precipitate the rare earth phosphate mixture. The precipitate was treated as mentioned in example 1 and made into a sol containing the mixture of rare earth phosphates.

Example

Example 3

Coating of Lanthanum Phosphate Nanoroads Powder

[0055] The sols thus obtained in example land 2 was mixed with poly vinyl alcohol, sprayed or coated over surfaces and annealed to get coatings.

Example 4

Preparation of Containers/Monoliths

[0056] Lanthanum Phosphate precipitated from Lanthanum salt by adding ortho phosphoric acid is further flocculated by addition of 25% ammonia solution till the pH was maintained between 6.8-7.8. The precipitate thus obtained was washed, dried and calcined at 1000-1200 C. to obtain Lanthanum phosphate powders. The powders were then dispersed in water at pH 2 and ball milled for 6-12 h using alumina as milling medium. 1-2 drops of octanol was added to the suspension to prevent foaming. The slurry after deairing was slip cast to shapes of crucibles and containers using Plaster of Paris moulds. After the build-up of sufficient thickness the slip cast part was de-moulded and dried at 80 C. under controlled conditions of humidity and temperature (50 C. and 65-75% RH) and further under normal oven conditions for 15 h at 80 C. The dried monolith is then sintered in the temperature range 1300-1500 C. and the crack free monoliths thus obtained are used for aluminum metal casting.

Advantages of the Invention

[0057] Rare earth phosphate based non-reactive surfaces obtained by the process described in this invention provides non-reactive, non-wetting characteristics towards molten metals. This property enables the development of mould release coatings in metallurgical industry for casting operations [0058] The formulations thus developed substitute the highly expensive material of boron nitride used commonly for the said purposes [0059] The dispersibility of powders in aqueous/organic media enables the formation of paintable/sprayable formulations conducive for industrial environments compared to physical/chemical vapour deposition processes that require sophisticated instrumentation [0060] The developed powders are also amenable to environmentally benign colloidal forming techniques that promote monolith fabrication [0061] The non-wettability and non-reactive nature of the surfaces enable repeated use of containers and helps in minimum material losses when used for precious metal castings of gold and silver.