KR940006424B1 - Alumina ceramics - Google Patents

Abstract

The high transparent polycrystalline ceramics for high pressure sodium lamp is composed of alumina which has more than 99.9% of the purity and to which 0.03-0.1 wt.% magnesia and 0.002-0.07 wt.% zirconia are added with the types of carbonate, acetate, and chloride. The mixture is molded by pressing and calcinated at 800-1,300 deg.C for 1-5 hrs., the surface of the calcinated body is polished smoothly by nylon or rug-like buff, and sintered at 1,750-1,900 deg.C for 1-15 hrs. in hydrogen or vacuum atmosphere.

Description

Translucent Polycrystalline Alumina Ceramics

The present invention relates to a light-transmitting polycrystalline alumina ceramics mainly used in a light-transmissive alumina light tube of a high pressure sodium lamp, and particularly to calcined alumina ceramics formed by adding magnesium oxide and zirconium oxide to high purity aluminum oxide. The present invention relates to a composition of a translucent alumina ceramics having improved light transmittance and workability of polycrystalline alumina ceramics by sintering after calcining the surface of the calcined body, and a method of manufacturing the same.

Translucent alumina ceramics are generally used as window materials for light tubes and infrared sensors of high pressure sodium lamps because they exhibit good light transmittance to visible light and infrared light, and also have excellent thermal, mechanical, chemical properties, and electrical stability.

Transmittance of polycrystalline alumina ceramics is determined by surface roughness of ceramics, size and uniformity of crystal grains, porosity and impurity content.

That is, the light passing through the polycrystalline alumina ceramics is reflected and scattered at the surface, grain boundaries, pores and other impurities of the alumina ceramics, and as the light path is prolonged by such reflection and scattering, consequently, the light transmittance is decreased. Caused.

In other words, the light transmittance of polycrystalline alumina ceramics minimizes the reflection and scattering of light due to grain boundaries, pores and impurities through the use of high purity raw materials and control of additives and firing conditions, and smoothly polishes the surface of ceramics to reflect them from the surface. Can be increased by preventing scattering.

As a general surface polishing method of the light-transmissive alumina ceramics, two types of methods are used, a chemical polishing method and a mechanical polishing method.

The chemical polishing method was developed by GEScott, Jr. et al., Wherein translucent alumina ceramics were deposited in a sodium borate solution at a temperature in the range of 762-857 ° C. and then hydrofluoric acid (HF) was applied to the surface of the alumina ceramics. It is a method to obtain a smooth surface by removing the attached deposits and drying them in the air.

However, such a chemical polishing method is complicated and time-consuming, and therefore, there is a difficulty in applying it to mass production of a product having a complicated shape such as a translucent alumina light tube for a high pressure sodium lamp.

On the other hand, the mechanical polishing method is a method of mechanical polishing again in order to smooth the surface after grinding and cutting the surface of the sintered translucent alumina ceramics.

However, since the alumina ceramics sintered body is very hard, a considerable load is required when mechanical polishing, and such loads may cause defects such as damage to the alumina ceramics or microcracks, thereby lowering the reliability of the product. There is a problem.

Therefore, the present invention calcined the alumina ceramic molded body as a way to solve the problems of the conventional mechanical polishing method to obtain a calcined body having a suitable strength for surface polishing, and then smoothly polished the surface of the calcined body. It is an object of the present invention to provide a light-transmissive polycrystalline alumina ceramic composition and a light-transmitting polycrystalline alumina ceramics composition which can improve the light transmittance of polycrystalline alumina ceramics and increase the workability and yield of the product by sintering at.

The composition of the light-transmitting polycrystalline alumina ceramics of the present invention is made by adding 0.03 to 0.1 w / o magnesium oxide and 0.002 to 0.07 w / o zirconium oxide to 99.99% or more of high purity aluminum oxide.

At this time, when the amount of magnesium oxide added is 0.03 w / o or less or the amount of zirconium oxide is 0.002 w / o or less, abnormal growth of crystal grains occurs, and as a result, pores are formed in the crystal grains. .

On the contrary, when magnesium oxide is added at 0.1 w / o or more or zirconium oxide is added at 0.07 w / o or more, the amount of magnesium oxide added is 0.03 to 0.1 w because secondary phases are precipitated at grain boundaries, which also acts as a factor of decreasing transmittance. It is preferable to keep it at / o and to add zirconium oxide in the range of 0.002 to 0.07w / o.

Meanwhile, the magnesium oxide and zirconium oxide added in aluminum oxide in the light transmissive polycrystalline alumina ceramic composition of the present invention are preferably in the form of oxides, even when added in the form of chloride, carbonate, or acetate, which is a preliminary step of the oxide. Synthesis of is possible.

The light-transmitting polycrystalline alumina ceramic manufacturing process of this invention is as follows.

First, aluminum oxide, magnesium oxide, and zirconium oxide having the composition range of the present invention are mixed and pulverized by wet, followed by spray drying to synthesize granulated powder of about 80 μm.

Next, the synthesized powder was press-molded to form a tube-shaped molded body at a temperature of 800 to 1300 ° C. for 1 to 5 hours, and then the surface of the calcined body was cleaned using a nylon or molten buff. Polish

The finished calcined body is sintered in a hydrogen atmosphere or vacuum at a temperature of 1750-1900 ° C. at a heating rate of 300 ° C./h or higher to form translucent polycrystalline alumina ceramics.

If the calcination temperature is lower than 800 ° C. during calcination of the light-transmitting polycrystalline alumina ceramics manufacturing process of the present invention, the strength of the calcined body is because only the binder is burned out and the reaction between the ceramic particles rarely occurs. Rather, compared with the strength of the molded body, the risk of breakage during surface polishing of the calcined body is high. On the contrary, when the calcining temperature exceeds 1300 ° C, the reaction between ceramic particles proceeds rapidly, and the surface hardness of the calcined body is rapidly increased. Since it is difficult to polish the surface, the calcination temperature should be maintained in the range of 800 ~ 1300 ℃.

Embodiments of the present invention are as follows.

EXAMPLE

Magnesium oxide and zirconium oxide as a sintering aid were added to the aluminum oxide powder with a purity of 99.99% or less and an average particle diameter of less than 1 μm for the production of a transmissive alumina light emitting tube for a 400 watt high pressure sodium lamp. After mixing, spray drying was performed to obtain granulated powder having an average particle diameter of 80 μm.

Subsequently, the granulated powder was molded at a pressure of 2 ton / cm 2 using a dry hydrostatic pressure forming device composed of tungsten carbide mandrel and polyurethane, to prepare a cylindrical molded body.

By the way, the inner surface of the cylindrical molded body produced in this way was as smooth as a mirror surface because it was in contact with the tungsten carbide mandrel, but the outer surface of the molded body showed an uneven rough surface.

After calcining such a molded body at a temperature of 1000 캜 in the air for 2 hours, the surface of the calcined body was smoothly polished using a nylon buff.

Next, the calcined calcined body was calcined for 3 hours at a temperature of 1850 ° C. in a vacuum of 10 ⁻⁴ Torr using a high temperature vacuum furnace, and then average particle size, maximum surface roughness and straight line were obtained for the cylindrical sintered body. The transmittance was measured and the results are shown in Table 1.

TABLE 1

Characteristics of Translucent Alumina by Additive Composition and Calcined Surface Polishing

As can be seen in Table 1 above, in the case of the embodiment specimens made of the material of the alumina ceramics composition range of the present invention by the alumina ceramics manufacturing process of the present invention, a smooth surface having a maximum surface roughness of 4 μm or less in the alumina light emitting tube and thus Good linear transmittance of 7% or more was shown.

Claims (4)

A translucent polycrystalline alumina ceramics composition, characterized by adding 0.03 to 0.1 w / o of magnesium oxide and 0.002 to 0.07 w / o of zirconium oxide to aluminum oxide having a purity of 99.99% or more. The translucent polycrystalline alumina ceramics composition according to claim 1, wherein magnesium oxide and zirconium oxide are added in the form of chloride, carbonate or acetate which is a preliminary step of the oxide. A mixed powder obtained by adding and mixing 0.03 to 0.1 w / o magnesium oxide and 0.002 to 0.07 w / o zirconium oxide to an aluminum oxide powder having a purity of 99.99% or more is obtained to obtain a molded product, which is calcined and processed in the air. A calcined polycrystalline alumina ceramic composition, comprising: preparing a calcined body having a suitable strength to and smoothly polishing the surface of the calcined body and then sintering in hydrogen or a vacuum atmosphere. The translucent polycrystalline alumina ceramic composition according to claim 3, wherein the calcination step is performed at a temperature of 800 to 1300 ° C for 1 to 5 hours.