CN1127734A

CN1127734A - Method for preparing transparent polycrystalline spinel

Info

Publication number: CN1127734A
Application number: CN 95117836
Authority: CN
Inventors: 董存新; 彭载学; 刘德铭; 王雁鹏; 王向阳; 李薇; 倪代秦; 李瑞龙
Original assignee: Artificial Crystal Inst State Bareau Of Blag Materials Industry
Current assignee: Artificial Crystal Inst State Bareau Of Blag Materials Industry
Priority date: 1995-12-15
Filing date: 1995-12-15
Publication date: 1996-07-31
Anticipated expiration: 2015-12-15
Also published as: CN1049881C

Abstract

The process for preparing transparent polymorphic spinel includes bimetal alkoxide method to obtain magnesium aluminate, vacuum drying, high-temp calcining to directly obtain spinel powder, and hot pressing, and features less investment for apparatus, simplified technology and low cost.

Description

Preparation method of transparent polycrystalline spinel

The invention belongs to the field of transparent polycrystalline optical materials.

The transparent spinel material is a high-speed guided missile fairing or window material used in the present and future, has high mechanical strength, good permeability and thermal conductivity, low elastic modulus and thermal expansion coefficient, can withstand the pneumatic thermal shock caused by high-speed flight and resist the erosion of environmental rainwater, and the successful development of the material can accelerate the update of missile weapons in China and shorten the difference between the missile weapons and foreign countries.

Foreign polycrystalline spinel (MgAl)₂O₄) The development of materials was started in the sixties and in 1976 US patent 3974249 discloses a process for the preparation of transparent spinels. At that time, the prepared spinel material has only low transmittance in ultraviolet and visible wave bands, the transmittance is reduced under the exposure of ultraviolet rays, and the material is easy to be worn and corroded. In the nineties, US patent 4930731, 4983555, 5244849, in turn, disclosed the preparation of transparent polycrystalline spinel and its application technology in succession, in which the optical and mechanical properties of the spinel material were greatly improved. EU patent0334760 and US patent 5152940 also disclose a method and a process for the preparation of spinel sintered bodies, respectively.

However, the preparation methods disclosed in the above patents all adopt a two-step method of hot pressing in vacuum or sintering in atmosphere and then hot isostatic pressing treatment, and the hot isostatic pressing equipment has large investment and complex process, which results in high material preparation cost. In addition, the prior art uses magnesium oxide (MgO and aluminum oxide (Al)₂O₃) The powder mixture of (1) is used as a starting material for pressing spinel polycrystal, and it is difficult to achieve the homogenization of the starting material, i.e., MgO, Al in the micro-domain range, no matter how well the two powders are mixed₂O₃The molar ratio can not reach 1: 1, which causes the local heterogeneous phenomenon of the pressed polycrystalline spinel, influences the optical performance of the material, and the transmittance is still low at the wave band of more than 4.5 mu m.

The invention aims to overcome the defect of uneven raw materials in the prior art, and seeks a simpler pressing method to reduce the preparation cost of materials.

In order to achieve the purpose, the invention firstly adopts a double metal alkoxide method to directly prepare spinel powder, namely a compound magnesium aluminate (MgAl) is prepared by hydrolyzing an aluminum magnesium isopropoxide double metal alkoxide₂O₄). The spinel powder thus prepared has good homogeneity and no chemical component in micro areaThere is a difference and the impurity content is low. Because the raw materials of the invention reach the homogenization, the invention adopts the hot pressing process to complete the pressing of the polycrystalline spinel material in one step, and obtains the transparent polycrystalline spinel product with the material performance equivalent to or even better than that in the prior art.

The technical scheme of the invention is explained in detail in two parts as follows:

preparation of spinel powder

High-purity metallic aluminum (99.99 percent), magnesium (99.99 percent), isopropanol and trace HgCl are added into a reactor₂Heating and refluxing for 48 hours, standing the reaction solution until the reaction solution is clear after the reaction is finished, transferring the clear solution into a reduced pressure distillation device for reduced pressure distillation to obtain colorless viscous aluminum magnesium isopropoxide, wherein the reaction is carried out under strict anhydrous conditions.

Adding water and organic solvents such as isopropanol or ethanol into the isopropanol aluminum magnesium for hydrolysis, and carrying out vacuum drying and high-temperature calcination at the temperature of 1000-1100 ℃ on the hydrolysis product to prepare the spinel powder.

The purity of the synthesized spinel powder can reach more than 99.9 percent, and the content of non-ferrous metal impurities is less than 30ppm (see table 1).

TABLE 1 spinel powder impurity content

Element(s)	Fe	Cr	Zn	Na	Mn	Si	Ca	Ga	Zr	C	S	V
Element(s)	Fe	Cr	Zn	Na	Mn	Si	Ca	Ga	Zr	C	S	V	Content (wt.) (ppm)	11.0	0.1	5.0	129	3.0	200		5.0	1.4	300	10	3.0

The analysis means is as follows: neutron activation, plasma emission spectroscopy

The synthesized spinel powder has narrow particle size distribution range, more than 90% of the powder has particle size less than 1.0 μm, and the average particle size is 0.3-0.4 μm (see Table 2).

TABLE 2 spinel powder particle size distribution

Sample (I) Distribution of	＞2μ	2-1.5	1.5-1.0	1.0-0.8	0.8-0.6	0.6-0.4	0.4-0.3	0.3-0.2	0.2-0.1
Sample (I) Distribution of	＞2μ	2-1.5	1.5-1.0	1.0-0.8	0.8-0.6	0.6-0.4	0.4-0.3	0.3-0.2	0.2-0.1	1	1.3	1.1	1.3	1.9	4.3	13.2	15.0	29.4	32.5
2	1.8	1.5	1.8	2.7	5.6	16.1	17.3	27.1	26.0	1	1.3	1.1	1.3	1.9	4.3	13.2	15.0	29.4	32.5

The measurement means is as follows: british MK-3 type particle size analyzer

Preparation of a transparent polycrystalline spinel

High-strength, high-purity and high-density graphite is selected as a hot-pressing mold, and the pressure resistance requirement of the mold is 500-700Kg/cm₂Plating a layer of pyrolytic graphite on the surface of graphite or padding a layer of carbon-carbon fiber on the surface of graphite contacting with spinel powder to prevent the graphite mold from aligning the spinelThe pollution of raw materials is realized by pressurizing an upper movable pressure head and a lower movable pressure head of the vacuum hot-pressing furnace by a hydraulic or pressure testing machine and adopting a medium-frequency power supply for induction heating.

Before the hot pressing process is started, levigating LiF with the weight of 1-2 percent₃Adding spinel powder as sintering aid, mixing, cold pressing in hot pressing mold at 5-10MPa, vacuumizing to about 5Pa, low temperature heating with power supply, and baking material and furnace moisture. When the temperature is raised to the range of 1000-1100 ℃, the temperature is maintained for 30 minutes, raw material impurities are removed, and the pressure tester is started to pressurize to prevent the powder from shrinking in the middle of the die and causingThe spinel edge devitrifies. Along with the rise of the temperature, the pressure rises simultaneously, when the temperature reaches 1500-1700 ℃, the pressure also rises to 40-55 tons (phi 100mm mould) correspondingly, and the temperature is preserved for 2-5 hours, then the temperature reduction procedure is executed,and the temperature reduction rate is controlled within 5 ℃/min. The main physical properties of the transparent spinel polycrystal obtained by the hot pressing process are shown in Table 3.

TABLE 3 Main physical Properties of Hot-pressed spinel polycrystalline body Unit data Density of 3.58 melting Point ℃ 2135 Knoop hardness Kg/mm²1200 bending strength MPa 1220 Young modulus GPa 190 Poisson's ratio 0.2608 linear thermal expansion coefficient of 10^-6Per DEG C40-200 ℃ 5.740-800 ℃ 7.0 thermal conductivity (30 ℃) W/M.K 15Dielectric constant

10³Hz 8.2

10⁶Hz 8.2

9.3×10⁹Hz 8.3

Because the raw materials are optimized, the pressing of the transparent polycrystalline spinel material is finished by one step only by adopting a hot pressing process, hot isostatic pressing equipment with huge investment is saved, the process is simplified, the material preparation cost is reduced, and a transparent polycrystalline spinel product with the material performance equivalent to or even better than that in the prior art is also obtained.

Example 1 (preparation of starting Material)

Weighing 70 g of pure magnesium and 1160 g of aluminum isopropoxide by a tray balance, filling into a 5000ml three-necked bottle, adding 1000ml of isopropanol, slowly heating until the mixture is slightly boiled, and adding a trace amount of HgCl₂And refluxing for 48 hours, controlling the temperature at 80-90 ℃, standing and clarifying the product after the reaction is finished, filtering out precipitates, transferring clear liquid into a reduced pressure distillation device, distilling out excessive isopropanol, and distilling out the isopropanol aluminum magnesium. Hydrolysis was carried out by adding 1 mole of product to 8-24 moles of water and 1000-1500ml of isopropanol. The hydrolysate is dried in a vacuum drying oven, the drying temperature is controlled at 90-120 ℃, the dried powder is calcined in a high temperature furnace, the dried powder is firstly dried at 600 ℃ for 2 hours under 500 ℃ and then calcined for 1.5-2 hours under 1100 ℃ under 1000 ℃ and the like.

Example 2 (preparation of starting Material)

Weighing 70 g of pure magnesium and 154 g of aluminum, adding 3000ml of isopropanol, slowly heating to slight boiling, and adding a trace amount of HgCl₂Initiating to generate a large amount of bubbles, heating and refluxing for 48 hours after the reaction is stable, standing the product after the reaction is finished, and distilling under reduced pressure. The subsequent process is the same as above.

Example 3 (preparation of spinel polycrystal)

Weighing 90 g of spinel powder, adding 1.4-1.8 g of LiF, mixing, finely grinding, filling into a graphite mold with the inner diameter of phi 100mm, carrying out pyrolytic graphite treatment on the graphite mold, and then filling a layer of carbon-carbon fibers on the upper surface and the lower surface of the graphite mold. After the raw materials are put into a die, cold pressing and molding are carried out, low-temperature heating is carried out, when the temperature is raised to 1600 ℃, the pressure is also raised to 50 tons, heat preservation and pressure maintaining are carried out for 3-4 hours at the temperature, after the heat preservation time, a cooling program is executed, and the cooling rate is controlled within 5 ℃/min. After cooling, the crystals are taken out and subjected to coarse grinding, fine grinding and polishing. The spinel (2mm thick) has a transmittance of 80% or more (FIG. 1) at 0.3-0.5 μm and a transmittance of 87% at 3-5 μm (FIG. 2)

FIG. 1 is a graph showing a UV and visible light transmittance curve

FIG. 2 is a schematic view of an infrared transmittance curve

Claims

1. A method for preparing a transparent polycrystalline spinel, characterized by: adopts a double metal alkoxide method, namely magnesium aluminate (MgAl) is obtained by hydrolyzing aluminum magnesium isopropoxide double metal alkoxide₂O₄) The compound is directly prepared into spinel powder through vacuum drying and high-temperature calcination at 1100 ℃ under 1000 ℃ and then the pressing of the transparent polycrystalline spinel material is completed in one step by adopting a hot pressing process.