Patent Description:
Rare earth phosphates possess excellent high temperature phase stability, low thermal conductivity and diffusivity, good corrosion resistance and high thermal expansion coefficient. By virtue of these characteristics, lanthanum phosphate, a member of the rare earth phosphates family, has been employed for applications as thermal barrier coatings to metallic substrates. It is considered as a potential material for thermal insulation coating on Ni-based superalloys as its properties are comparable to the properties of zirconia like high-temperature stability (melting point <NUM> ± <NUM>), high thermal expansion and low thermal conductivity. In addition, lanthanum phosphate (LaPO<NUM> ) is expected to have good corrosion resistance in environments containing sulfur and vanadium salts. However a careful control of the LaPO<NUM> stoichiometry is essential to employ such materials as TBCs primarily due to the fact that LaPO<NUM> is a line compound that melts congruently and small deviations from stoichiometry change its solidus temperature from <NUM> to <NUM> on the La-rich side or to <NUM> on the P-rich side.

Conventional solid state reactions of lanthanum phosphate necessitate high temperatures for phase formation and are also characterised by particles of irregular size and shape due to which post preparative milling techniques are required. In view of this a host of solution based preparative techniques like sol gel, co-precipitation, hydrothermal, sonochemical methods are now been developed to synthesise lanthanum phosphate particles of controlled size, morphology and composition. However, the bulk production of such particles is based on a spray calcination procedure where in the sol is atomised and fed into a heated chamber leading to the formation of particles with desirable characteristics.

In the patent <CIT> lanthanum phosphate is used to form thermal barrier coatings to protect super alloy and ceramic parts exposed to high temperature and damage by sulfur, vanadium, phosphorus and other contaminants. The coatings, between <NUM> and <NUM> micrometers in thickness, can be deposited on substrates having temperatures between <NUM>- <NUM>° C by common application methods such as EB-PVD, laser ablation and plasma spraying. The most effective coatings are crystalline and show a columnar structure with feather-like microstructure.

The patent <CIT> describes a method to coat SiC and oxide fibers with phosphate coatings to enhance the ceramic or metal matrix reinforcing properties of fibers. The phosphate coatings applied with the described method can also be employed for encapsulating radioactive waste. The patent <CIT>) reveals a means of surface treatment of metals for corrosion resistance where the coating layers are composed of rare earth elements of cerium and lanthanum in a resin. A colloidal dispersion containing rhabdophane structured rare earth phosphate and a polyphosphate is disclosed by <CIT>. The said dispersion was used to develop transparent luminescent material. A corrosion protecting coating composition, for steel and aluminium surfaces, comprising of rare earth based corrosion inhibitors with a resin and curing agent was invented by the patent <CIT>.

<CIT> reveals a process for cerium and/or lanthanum phosphate sol by introducing a first solution of salts of at least one of the rare earths into a second solution of phosphate ions with an initial pH of less than <NUM> and controlling the pH of the precipitation at a constant value of less than <NUM>. The precipitate separated from the reaction medium can be further dispersed in water with a final PO<NUM>/rare earth mole ratio of <NUM>. The sol is employed as polishing suspensions.

<CIT> discloses the deposition of adherent, fine-textured layers of particles onto a substrate and particularly to the making of mosaic-type phosphor screens for cathode ray tubes.

<NPL>) discloses a process for obtaining nanoparticles of lathanide vanadate and phosphate by aqueous synthesis, wherein the obtained nanoparticles are stabilized in colloidal suspensions and deposited in sol-gel films by spin-coating using a silicate as inorganic binder.

In metal casting operations the moulds are usually coated with layers of release agents comprising mainly of inorganic materials like alumina, zirconia, hexagonal BN, graphite etc. The layers acting as an interfacial coating prevents chemical interaction with mould material and casting metal. It is beneficial and advantageous to identify and develop suitable mould release agents that are easy to synthesize and are relatively of low costs compared to existing prior art articles.

Accordingly, present invention provides a process for obtaining a rare earth phosphate based, non-reactive and non-wettable surface for molten metals for high temperature applications, comprising the steps of:.

wherein the binders used are selected from the group consisting of polyvinyl alcohol, methyl cellulose, ethyl cellulose, and poly vinyl pyrrolidone, and wherein the plasticizers used are selected from the group consisting of polyethylene glycol and dioctyl phthalate.

In an embodiment of the present invention, the rare earth phosphate used is a single rare earth element or a mixture of rare earth elements derived from thorium depreciated monazite sand.

In another embodiment of the present invention, the surface used is selected from the group consisting of glass, metals and ceramics.

In yet another embodiment of the present invention, the solvent used is an organic solvent and is ethanol or isopropanol.

According to the present invention, the binders used are selected from the group consisting of polyvinyl alcohol, methyl cellulose, ethyl cellulose, and poly vinyl pyrrolidone.

According to the present invention, the plasticizers used are selected from the group consisting of polyethylene glycol and dioctyl pthalate.

In yet another embodiment of the present invention, the rare earth phosphate based, non-reactive and non-wettable surface is in the form of a coating, a mould or a paint.

In yet another embodiment of the present invention, said non reactive coating, mould or paint is suitable for holding the molten metal without chemically reacting at the melting temperatures of the molten metal for <NUM>-<NUM>.

In yet another embodiment of the present invention, the molten metal is either alone or in combination with alloys, metal composites, transition metals and semi-metals, wherein the molten metal is iron, aluminium, silver or zinc.

In yet another embodiment of the present invention, the formulation as prepared in step (ii) is suitable as a paint.

In an embodiment of the present disclosure, which is no embodiment of the present invention, the slurry as prepared in step (i) is useful in coating of crucibles/containers and the said process comprising the steps of:.

In yet another embodiment of the present disclosure, which is no embodiment of the present invention, the surface is made of a dispersion (sol/slurry) containing ReP particles wherein the smallest dimension of the particles are in the range between <NUM> and <NUM> microns and wherein any dimension of majority of the particles have sizes in the range between <NUM> and 1micron and wherein the particles have a rod like structure with an aspect ratio value in the range of <NUM>-<NUM>.

In yet another embodiment of the present disclosure, which is no embodiment of the present invention, dispersion/sols/slurrys are made in to the form of mould relase coating sprays/solutions.

The present disclosure 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:.

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.

The present disclosure 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.

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

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

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.

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 disclosure 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.

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 <NUM> to <NUM> are treated with orthophosphoric acid at <NUM>-<NUM>. The precipitate thus obtained is subjected to a flocculation procedure at pH <NUM> and further washed repeatedly with water to free of the chloride/nitrate ions. The washed precipitate is then peptized to a sol at pH <NUM>. The sol is then calcined to obtain powders of typically nanorod morphology with lengths up to <NUM> micron and width of <NUM>-<NUM>.

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 <NUM>. 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.

According to another aspect of disclosure, 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 <NUM> to <NUM> wt%. The powder is dispersed in water under controlled pH conditions and milled appropriately with alumina milling media for <NUM>-<NUM> 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 <NUM> is then subjected to sintering at temperatures varying in the range of <NUM>-<NUM>. Crack free sintered crucibles of lanthanum phosphate are thus obtained and can be directly employed for molten metal casting operations.

The disclosure 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 disclosure relates to the non-wettability of such monoliths and coatings with molten metals at their respective melting temperatures.

Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention. Among the following examples, Example <NUM> is no example according to the invention, but helpful to understand certain aspects thereof.

In a typical synthesis for <NUM> Lanthanum Phosphate, <NUM> Lanthanum nitrate was initially dissolved in <NUM> de-mineralized water to form lanthanum nitrate hexahydrate. Lanthanum phosphate was precipitated by adding the phosphate source <NUM>% ortho phosphoric Acid (<NUM>) drop by drop and the precipitate formed is further flocculated by addition of <NUM>% ammonia solution and pH was raised to <NUM>. 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 <NUM>) using <NUM>% nitric acid with continuous, vigorous stirring to obtain the sol. The stability of the sol is checked using zeta potential measurement.

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 <NUM> and made into a sol containing the mixture of rare earth phosphates.

The sols thus obtained in example land <NUM> was mixed with poly vinyl alcohol, sprayed or coated over surfaces and annealed to get coatings.

Claim 1:
A process for obtaining a rare earth phosphate based, non-reactive and non-wettable surface for molten metals for high temperature applications, comprising the steps of:
i. adding rare earth phosphate nanorod powders in water and/or solvent to obtain a slurry having solid loading in the range of <NUM>-<NUM> wt%;
ii. incorporating binders and plasticizers in to the slurry as obtained in step (i) to prepare a formulation;
iii. spraying or dipping the formulation as obtained in step (ii) on a surface to obtain a rare earth phosphate based, non-reactive and non-wettable surface;
wherein the binders are selected from the group consisting of polyvinyl alcohol, methyl cellulose, ethyl cellulose, and polyvinyl pyrrolidone, and
wherein the plasticizers are selected from the group consisting of polyethylene glycol and dioctyle phthalate.