KR20170058048A

KR20170058048A - Forming Method of MgAl2O4 Spinel

Info

Publication number: KR20170058048A
Application number: KR1020150161768A
Authority: KR
Inventors: 장철우; 함성원
Original assignee: 경일대학교산학협력단
Priority date: 2015-11-18
Filing date: 2015-11-18
Publication date: 2017-05-26
Also published as: KR101746128B1

Abstract

The invention MgAl2O4 nm in dispersed phase of the powder was added in addition to organic materials monomers in addition to the dispersing agent to a stable seulreorieul with little agglomeration and settling of the particles according to the sign of the MgAl2O4 particulate create unique molding subsidiary filter pressing of the present invention the solution + Gel casting technique to provide a highly uniform molded article having no localized uneven distribution of pores which is essential for obtaining transparent MgAl 2 O 4 ceramics. The monomer added in the present invention adsorbs on the surface of the nanoparticles to geometrically inhibit the precipitation of the particles and doubles the stability of the MgAl 2 O 4 slurry in addition to the dispersant that gives electrostatic repulsion. The combination of this doubled stability of the slurry and the combination of filter pressing and gel casting makes it possible to produce highly uniform MgAl2O4 compacts which are essential for the production of transparent ceramics.

Description

Manufacturing Method of MgAl2O4 Spinel Forming Method {Forming Method of MgAl2O4 Spinel}

The present invention relates to a method of manufacturing a transparent MgAl 2 O 4 spinel ceramic used for a transparent bulletproof window, a transparent IR searcher protective dome, a window for a high energy RF device, and the like, and more particularly, to a method for manufacturing a transparent MgAl 2 O 4 spinel The present invention relates to a method for producing a uniform shaped body by a wet process such as gel casting and a filter press using a raw material as a starting material. MgAl 2 O 4 Spinel raw material powder is agglomerated by hydration reaction with water as a solvent in the dispersing step and it is difficult to obtain a transparent MgAl 2 O 4 ceramic because it is difficult to manufacture a uniform molded body and a sintered body which are indispensable conditions for manufacturing transparent ceramics . The present invention overcomes the technical difficulties of MgAl2O4 spinel wet molding, and relates to a method for producing a uniform MgAl2O4 spinel molding required for the production of transparent ceramics.

(Prior art)

Conventional transparent MgAl 2 O 4 Spinel ceramics have been produced by using a micron raw material of 0.5 to 5 μm and sintering at a temperature of 1,650 ° C. or higher by adding LiF to facilitate sintering sintering (US2006021726A1). When MgAl 2 O 4 Spinel ceramics are produced by this method, LiF added as a sintering accelerator remains in the grain boundaries, and mechanical properties such as strength and hardness are lowered. Therefore, transparent bullet-proof windows requiring durability in a more extreme environment, IR Explorer protection dome and so on.

In recent years, there have been introduced techniques for fabricating transparent MgAl 2 O 4 spinel ceramics having excellent mechanical properties without adding a sintering accelerator such as LiF by using a nano raw material having a very good sintering property as a starting material. In order to manufacture transparent MgAl 2 O 4 ceramics, it is most important to completely remove the pores in the final product. For this purpose, it is most important to produce a uniform molded article having no large pores generated by the particle agglomerates. In order to produce a uniform molded article using the nano powder, the powders must be dispersed in the dispersion medium in a monodispersed state, and the uniform monodispersed state must be maintained in the molded article. In general, the nano-sized MgAl2O4 raw material powder is very strong in cohesion so that a special dispersion process is required in the raw material dispersion process. In addition, the MgAl2O4 raw material reacts with water as a dispersion medium during the dispersion process (hydration reaction) It is very difficult to obtain a uniform molded article. Due to these problems, it is known that it is very difficult to produce a uniform MgAl2O4 molded body by the wet molding method, which is known to be easiest to ensure the uniformity of the molded body. Therefore, the transparent MgAl2O4 ceramics are mostly produced by dry forming. It is possible to disperse nano-sized ultrafine particles by using a special dispersing device → manufacture granular powder by spray drying → dry pressing (dry molding) → first air sintering → second hot hydrostatic pressure And sintering (HIP).

The following methods have been proposed for the preparation of uniform MgAl2O4 compacts using nano powders, and they are explained by the dry molding method and the wet molding method.

First, the technologies related to dry molding are as follows.

An organic solvent such as isopropanol or ethylene glycol (Journal of European Ceramic Society, Vol. 32, pp. 2869 ~ 2886, 2012) was used as a dispersion medium instead of water to solve the problem of agglomeration and sedimentation of particles due to hydration of MgAl 2 O 4 nano- A method of applying a dry process is proposed. In this method, a hard cake, which is a non-uniformity inducing factor in a molded article, is displayed during silver spray drying, or hard cake is hardly produced through strict process control during spray drying There is a problem that a large pore distribution is left in the inside of the molded body due to uneven delivery of the pressure applied to the granular powder in the process of putting the spray-dried granular powder into a mold and molding it. Such a hard cake and a wide pore distribution cause a large pore to remain in the material in the subsequent sintering process. Recently, in order to overcome the disadvantages of such a dry process, the uniaxial press-molded primary molded product is subjected to cold isostatic pressing at a very high pressure of 350 to 700 MPa (generally, a pressure of 200 MPa or less is applied) However, since the cost of a cold isostatic press is several hundred times higher than that of a wet type apparatus, Considering mass production in the future, it is economically disadvantageous.

Second, the technologies related to wet molding include the following.

In order to ensure the uniformity of the MgAl2O4 compact, wet casting slip casting techniques have been introduced. In order to suppress agglomeration of particles by hydration of MgAl2O4 powder, a method of slip casting MgAl2O4 surface modified with a solution of H3PO4 and Al (H2PO4) 3 in ethyl alcohol (Bulletine of Material Science, Vol. 34, No 2, pp. 327-335, 2011). In this method, MgAl 2 O 4 powder having a size of the starting material of micron unit (0.5-1.0 μm) is used, and there is no example applied to nano powder having strong cohesion. In addition, an expensive organic solvent such as ethyl alcohol is excessively required in the course of surface modification of raw materials, and additional processing apparatuses and processes are added, which is economically disadvantageous. A slip casting method using nano powder (Journal of american ceramic society, Vol. 94, No. 5, pp.1388 ~ 1396, 2011) has also been proposed. A polymer dispersant such as NH4PAA (Ammonium Poly Acrylic Acid) The solution is put into a dispersion process to obtain a well-dispersed solution by a ball mill method, and this solution is poured into a gypsum mold to perform slip casting. In this method, a slurry is prepared by adding a dispersant NH4PAA. Since the slurry has a characteristic that NH4PAA molecules agglomerate with each other over time, slurry casting, which requires the slurry to remain on the gypsum mold for a long time, It is difficult to produce a molded body. In this paper, it is not suggested how to make a slipcasted molded body transparent. In addition, slip casting is a process that tends to cause contamination of a molded body from a gypsum mold used as a molding mold, thereby reducing the reproducibility or yield of the material after the heat treatment. Another wet forming method, gel casting method, is a method of solidifying a mixture of ceramic particles and a monomer by a polymerization reaction within 1 hour, minimizing the problem of coagulation and sedimentation of ceramic particles in the dispersion over time Therefore, it is a very economical method in terms of mass production in which the green body is very uniform and has a very high molding strength, so that green processing is easy and there is no problem such as local breakage during handling. An example of applying this gel casting method to the production of MgAl 2 O 4 molded body is proposed in a recent publication (Journal of american ceramic society, Vol. 93, No. 9, pp. 2665 ~ 2666, 2010). In this paper, MgAl2O4 powders of submicron size with an average particle size of 0.1 ㎛ or more were used, and no gel casting of nano powder was proposed. In the gel casting method, when the slurry is prepared, the content of water is usually 30 wt% or less of the total weight of the slurry. When the content exceeds this amount, the density of the formed body is lowered and excessive drying and sintering shrinkage occurs. This happens. Since the nano powder has a large specific surface area and requires an excess amount of water of 50 to 60 wt% in the dispersion, if the molding method such as gel casting is applied, the warping or distortion of the product tends to occur seriously, so that the gel casting of MgAl 2 O 4 nano powder Has not yet been proposed. In addition, there is no proposal for manufacturing a MgAl 2 O 4 molded body by a filter press method which is a wet process which is widely used in the production of a ceramic molded body. This is because there is a problem of agglomeration and sedimentation of particles with time as in slip casting.

Accordingly, it is an object of the present invention to provide a method of manufacturing a uniform shaped body, which is essential for manufacturing a transparent MgAl 2 O 4 ceramic by overcoming the limitations of existing process technology and applying economically favorable wet process. In the dispersion step of the MgAl 2 O 4 nanopowder, in addition to the dispersant, an organic matter of monomers was further added to prepare a stable slurry having little aggregation and sedimentation of particles due to hydration of the MgAl 2 O 4 grains, and this solution was used as a filter- It is an object of the present invention to provide a highly uniform molded article which is formed by a composite technique of casting and which does not have a nonuniform distribution of local pores which is essential for obtaining a transparent MgAl 2 O 4 ceramic. The monomer added in the present invention adsorbs on the surface of the nanoparticles to geometrically inhibit the precipitation of the particles and doubles the stability of the MgAl 2 O 4 slurry in addition to the dispersant that gives electrostatic repulsion. It is an object of the present invention to provide a uniform MgAl 2 O 4 molded body which is essential for the production of transparent ceramics by using the doubled stability of the slurry and the filter pressing + gel casting molding method.

(Construction and function of the invention)

Hereinafter, the present invention will be described in detail.

The present invention comprises a process of the following steps.

1) A step of preparing a monomer solution containing 5 ~ 20 wt% of acrylamide and methylenebisacrylamide, which are the monomers for gelcasting, in distilled water.

2) A high purity MgAl 2 O 4 nanopowder having an average particle diameter of 57 nm and a specific surface area of 30 m 2 / g and a purity of 99.99% or more was added to the monomer solution so as to be 50 to 60 wt% of the total amount of the slurry (monomer solution + MgAl 2 O 4 powder) Mixing Citric Acid as a dispersant in 0.3 ~ 2.0wt% to prepare a well-dispersed slurry. Since the monomer is very small in molecular size and adsorbs on the surface of the nano powder to geometrically interfere with the agglomeration of the nano powder, it is effective to obtain a very stable slurry without aggregation and sedimentation of MgAl 2 O 4 particles. These monomers are effective not only in dispersing but also in imparting the strength of the molded body in the post-process gelling (polymerizing) process.

3) adding 1 μl of Ammonium persulfate solution as a gelation initiator per 1 g of the slurry unit and gently mixing the dispersed slurry in order to gel the monomers;

4) The above-mentioned MgAl 2 O 4 slurry was poured into a mold equipped with a filter having a pore size of 0.2 μm or less and pressurized to filtrate the aqueous solution containing the slurry. The formed body was separated from the mold, placed in an oven heated to 60 ° C, Gt; gelation < / RTI > The filter pressing and gel casting composite technology of the present invention is a molding method for removing 50 to 70 wt% of the amount of water added during dispersion in the filter pressing process and gelling the product. It is possible to produce a uniform molded product having a molding density of 40% And it is possible to fully utilize the advantages of the gel casting technique which was impossible due to the high specific surface area of the nano powder, so that a green body capable of being machined can be manufactured and a transparent MgAl 2 O 4 ceramic can be easily manufactured due to the uniformity of the green body . The molded body subjected to filter pressing for gelation does not necessarily have to be separated from the mold, and the mold itself may be heated to gel on the spot.

5) Firstly, the formed body is sintered at a temperature of 1,550 ° C. to 1,600 ° C. in an atmospheric environment, and then subjected to hot isostatic sintering (HIP) at 1,380 ° C. to 1,550 ° C. to form a transparent MgAl 2 O 4 ceramic .

Hereinafter, the reason for limiting the number will be described.

If the monomer content of the monomer solution for gel casting is less than 5 wt%, the dispersing effect of the MgAl 2 O 4 nano powder can not be sufficiently exhibited, and the amount of the polymer in the molded body obtained through gelation is insufficient. If the addition amount of the monomer exceeds 20 wt%, the dispersion effect of the nano powder and the green strength of the molded article are not greatly improved and economically disadvantageous. Therefore, the monomer content in the monomer solution is preferably limited to 5 to 20 wt%.

When the average particle size of the MgAl 2 O 4 nano powder put into the monomer solution exceeds 0.1 μm, the sintering temperature is higher by 100 to 150 ° C. than the nano powder, so that the equipment cost is increased and the constituent particles of the MgAl 2 O 4 sintered body obtained after sintering It is preferable to use a nano raw material having an average particle diameter of less than 0.1 占 퐉 in view of the mechanical properties of the material and economical efficiency of the material.

The amount of the nano powder to be charged into the monomer solution is preferably in the range of 50 to 60 wt%. If the amount is less than 50 wt%, it takes a long time to prepare the desired shaped volume in the filter pressing process, which is economically disadvantageous. The viscosity of the slurry for forming a molded article excessively increases and the fluidity becomes insufficient, which makes it difficult to inject the slurry into the mold, and a large amount of bubbles remaining in the slurry is contained, resulting in defects after sintering.

The addition amount of citric acid as a dispersant to be added during slurry preparation is preferably 0.3 to 2.0 wt%. If the addition amount is less than 0.3 wt%, the dispersing agent becomes insufficient and dispersion of the particles becomes difficult. When the addition amount exceeds 2.0 wt%, the slurry viscosity increases and sedimentation occurs.

The mold used for pressing the filter is made of metal such as steel or aluminum. It is preferable to use a membrane filter having a pore size of 0.2 μm or less or a ceramic filter plate such as alumina or silica when the slurry is pressurized and drained. If a filter having a pore size exceeding 0.2 μm is used, The nano powders are discharged together with the monomer solution. Since the temperature and time of the subsequent gel casting process are generally known conditions, the values are not particularly limited.

The compacts are firstly sintered in the atmosphere by heating at a temperature of 1,550 ° C to 1,600 ° C. If the sintering temperature is less than 1,550 ℃, the sintered density becomes less than 95% of the theoretical density, and it is difficult to obtain a transparent MgAl2O4 ceramic without pores in the second hot hydrostatic sintering (HIP). When the sintering temperature exceeds 1,600 ℃, However, since the size of the sintered body constituting particles increases, mechanical properties such as strength and hardness are lowered. Therefore, the first sintering temperature is preferably limited to a range of 1,550 ° C to 1,600 ° C. These primary sintered bodies are subjected to hot hydrostatic pressure (HIP) sintering at 1,380 ° C. to 1,550 ° C., preferably 1,450 ° C. to 1,500 ° C., to produce transparent MgAl 2 O 4 ceramics.

As described above, according to the present invention, a slurry in which a dispersant, Citric Acid, is dispersed in an aqueous monomer solution composed of acrylamide and methylenebisacrylamide and MgAl 2 O 4 spinel nano materials having an average particle size of d ₅₀ = 57 nm is dispersed is formed by a filter press and a gel casting method Thereby making it possible to produce a uniform molded article essential for the production of transparent MgAl 2 O 4 spinel ceramics.

[Fig. 1] is a transparent MgAl2O4 ceramic photograph obtained from a molded body formed by the filter press + gel casting method of the present invention.

Hereinafter, the present invention will be described in detail by way of examples.

(Example)

100 g of MgAl 2 O 4 raw material having an average particle diameter d ₅₀ = 57 nm, a specific surface area of 30 m 2 / g and a purity of 99.99%, and a dispersant, Citric Acid, in distilled water were added to 90 ml of a solution containing 10% by weight of a mixture of acrylamide and methylenebisacrylamide in a ratio of 24: g and YTZ balls were ball milled for 2 hours in a mixed medium to prepare a well dispersed MgAl 2 O 4 slurry. At this time, the pH of the slurry was adjusted to be in the range of 8 to 10. Ammonium persulfate solution, which is a gelation initiator, was added and mixed in an amount of 1 1 per 1 g of the slurry, and then the bubbles in the slurry were removed by vacuum defoaming. The slurry was poured into a mold equipped with a membrane filter having a pore size of 0.2 탆 and pressed to produce a molded body. The relative density of the molded body after filter pressing was 40%. The molded article was placed in an oven, heated to 60 DEG C and held for 1 hour to gel the monomers in the molded article and dried. And sintered at 1,570 ° C for 2 hours in an air atmosphere. The sintered bodies after the first sintering had a relative density of 98% or more. The primary sintered bodies were subjected to hot isostatic pressing at 1,500 ° C and 180 MPa for 3 hours in order to remove the internal pores. The densities of the sintered bodies after hot isostatic pressing were more than 99.9%, and it was confirmed that the sintered bodies were transparent as shown in Fig.

(Comparative Example)

100 g of a MgAl 2 O 4 nano raw material having an average particle size d ₅₀ = 57 nm, a specific surface area of 30 m 2 / g, a purity of 99.99%, and 5.0 g of NH 4 PAA as a dispersant were added to 90 ml of distilled water, and the YTZ balls were ball- MgAl2O4 slurry was prepared. The slurry thus prepared was vacuum degassed and poured into a gypsum mold for slip casting. After the casting was completed, the molded body having been demolded from the gypsum mold was dried in air and at room temperature for 3 days, and the relative density of the molded body was 38 to 40%. The compact was sintered at 1,570 ° C for 2 hours in the atmosphere. The sintered bodies after the first sintering had a relative density of 98% or more. The primary sintered bodies were subjected to hot isostatic pressing (HIP) under the conditions of 1,500 ° C and 180 MPa for 3 hours in order to remove the pores inside. The densities of the sintered bodies after hot hydrostatic sintering were more than 99.9%, and it was confirmed that the sintered bodies were translucent.

Claims

Adding a dispersing agent to the monomer solution consisting of Citric Acid Acrylamide and Methylenebisacrylamide and a mean particle size of d ₅₀ = 57㎚ of MgAl2O4 Spinel MgAl2O4 Spinel manufacturing a transparent ceramic by molding and sintering a slurry obtained by dispersing the nano-material to a filter press + gel casting method A method for producing a uniform MgAl 2 O 4 spinel molded article

The method according to claim 1, wherein the content of the acrylamide and the methylenebisacrylamide monomer is 5 to 20 wt% of the aqueous solution, and the amount of the citric acid is 0.3 to 2.0 wt% of the MgAl 2 O 4 spinel nano powder.

A method of producing a uniform MgAl 2 O 4 spinel molding capable of producing a transparent MgAl 2 O 4 Spinel ceramic by first sintering a formed body manufactured by the method of claim 1 and then secondarily hot-pressing the mixture by hot hydrostatic pressure (HIP)