DE1278318B - Process for the production of polycrystalline molded bodies from sintered corundum - Google Patents

Description

Process for the production of polycrystalline moldings from sintered corundum The invention relates to a process for the production of polycrystalline shaped bodies made of sintered corundum with improved radiation permeability and polycrystalline Sintered corundum bodies, which are particularly suitable for optical purposes.

A method for producing tools and devices from aluminum oxide is already known, in which aluminum oxide of great purity is formed without special binders and at a temperature of over 1600 ° C in a carbon-free environment in a single operation to form a solid and dense body is sintered together. The sintering is expediently carried out in a vacuum or in an inert atmosphere. The sintered body obtained is of a translucent, Mannor-like nature and, because of its hardness, is suitable for grinding and polishing purposes as well as for the production of mortars or similar devices for the chemical industry.

It has also been known for a long time that completely dense sintered aluminum oxide objects can be produced by firing at 1850 to 1900 ° C , which have a pale sky-blue color and are translucent. It is also already known that the addition of small amounts of magnesium oxide and zirconium oxide promotes sintering and improves light transmission. Furthermore, it is already known to carry out the sintering of aluminum oxide in a hydrogen atmosphere.

The sintered bodies produced by the previously known method In the most favorable case, aluminum oxide is made of translucent bodies which are light let through, but at the same time scatter the light in all directions, so that through them one cannot clearly see the shape of other bodies.

The known sapphire single crystals consisting of aluminum oxide have extremely favorable optical and physical properties even at high temperatures, for example great hardness, high density and good radiation permeability, but only bodies with limited size and shape can be produced from single crystals.

The invention is now based on the object of creating a polycrystalline shaped body made of sintered corundum which, compared to the known sintered corundum, has additional application possibilities and has similar properties to a sapphire single crystal, i.e. H. high mechanical and thermal strength as well as good radiation permeability, and which retains these properties even at elevated temperatures. This object is now achieved by a process for the production of polycrystalline molded bodies from sintered corundum with improved radiation permeability, which is characterized according to the invention in that formed, finely divided aluminum oxide of high purity together with at most 0.5 percent by weight of magnesium oxide in a vacuum or in a hydrogen atmosphere at a temperature of 1700 until 19501 C is sintered until the spinel content is largely reduced, thereby increasing the density and the radiation permeability in the visible and infrared range. In this way, polycrystalline sintered corundum bodies which are particularly suitable for optical purposes are obtained, which are characterized by a straight-through radiation transmission with a thickness of 0.94 mm in the wavelength range from 0.003 to 6.6 #L of more than 10%. Polycrystalline sintered corundum bodies according to the invention can be used, for example, for the production of windows in blast furnaces and high-speed projectiles and as lamp envelopes. They are also suitable for most applications for which sapphire has been used, and have the advantage that they can be in any shape, e.g. B. as preformed casings can be produced.

In the method according to the invention, a mixture of finely divided aluminum oxide and 1/10 to 1/2 percent by weight of finely divided magnesium oxide is preferably used. The grain size of the total alumina can average between 0.05 and 0.2 t, or up to 70 1 / o of the alumina can have a grain size of 5 to 10 t and not less than 30 1/9 of the alumina have a grain size of less than 1 tt, the smaller particles serving as binders for the larger particles. If desired, mixtures can also be used which contain a smaller percentage of particles having a size of 5 to 10 t and relatively more particles having a size of less than 1 li.

The purity of the aluminum oxide can vary within certain limits. 99.60 / 9 pure aluminum oxide gives only slightly worse results than 99.9911 / o pure aluminum oxide. In general, the purity should not be less than 99% and should be as high as possible, so that the proportion of the spinel phase, which affects the transparency of the body due to the diffraction of the incident radiation, is as small as possible.

It has been found that excessive grain growth during the Sintems must be prevented so that the necessary degree of transparency is achieved will. Rapid growth is undesirable as it causes cracks at the grain boundaries and many of the existing pores in the grain become trapped and not easily can diffuse out of the compact. It is believed that the addition of magnesium oxide restricts grain growth in that the grain boundaries are defined and prevented from doing so will capture the existing pores. It goes without saying that this too Stress cracks occur less often. Dense cracks at the grain boundaries make themselves through Noticeable reduction in transparency.

While the addition of finely divided magnesium oxide to finely divided Aluminum oxide enables the aluminum oxide to form a sintered corundum with considerable Transparency is sintered, a deterioration of the transparency occurs at the same time due to the increased formation of a spinel (MgA1,0.) as the second phase is conditional. The spinel reduces the "straight-ahead permeability" by increasing it Diffraction of light in the body. There is therefore an optimal range for the addition of magnesium oxide, and the best optical properties will be obtained when the on Mag, nesium oxide added weight fraction is within this range. The straight-ahead permeability gives the ratio between the amount entering a material at a certain angle and the radiation entering at the same angle.

The invention will now be explained in more detail with reference to drawings. In which F i g. 1 Curves showing the permeability of sintered corundum as a function of the magnesium oxide content, FIG . FIG. 2 is a curve showing the dependence of the shrinkage on the sintering temperature, and FIG. 3 the straight-ahead permeability of sintered 1 corundum bodies and of an optical sapphire crystal as a function of the wavelength.

F i g. 1 shows the relationship between the added magnesium oxide (in percent by weight), the spinel formation and the dependent transparency. The abscissa indicates the magnesium oxide content and the ordinate the dependent "straight-ahead permeability" of a sintered corundum of about 1.7 mm thickness. Curve 5 shows that the transparency of a sintered corundum, starting from the addition of O # 1 / q magnesium oxide, rises steeply and assumes a maximum value (with infrared radiation) when about 10 percent by weight of magnesium oxide is added. For visible wavelengths, slightly smaller additives are usually preferred, e.g. B. at most about 1/16%. If more than 1ho% magnesium oxide is added, the transparency decreases, but not as quickly as between 0 and 1 / lo%. So it can be added up to 1/2 percent by weight. In most cases, an addition between Iho and 1/4 percent by weight is recommended. Such a curve is believed to result from the fact that small amounts of MgO effectively prevent abnormal growth of grains and inclusion of pores without increasing the formation of spinel as the second phase so much that the transparency is impaired would. However, if the magnesium oxide content is more than about 1/10/0, the spinel formation increases to such an extent that the transparency decreases considerably.

The curves 6, 7 and 8 in FIG. 1 show how the percentage of spinel formed increases uniformly with increased magnesium content, and also show the effect of sintering temperature on spinel formation. The temperature influences will be discussed further below.

After the aluminum and magnesia are mixed, the mixture is compacted under a pressure sufficient to produce compacts having at least 351070 the theoretical density of an alumina single crystal. Usually, the pressure which must be applied to form a compact of appropriate bulk density is about 5.5 to 55 kg / cm and depends on the size of the body to be produced. In most cases, a pressure greater than 22 kg / CM2 is recommended. If the bulk density is less than 3511 / o. the theoretical density is used, the optical properties are usually insufficient.

If the raw, unfired compact is more than about 0.95 mm thick, it should be placed in an oxygen-containing atmosphere, e.g. B. in air, must be pre-fired for at least 1 hour, so that the best possible permeability is achieved. Bodies that are less than 0.95 mm thick can be made transparent without pre-baking, although the pre-baking does not in any case adversely affect the permeability of the resulting body. The duration of the pre-burning is not critical; but it must be more than 1 hour. Longer pre-burning times are permissible, but only insignificantly improve the transparency of the finished body.

The temperatures during the prebake should be more than 10001 C and less than about 1700 ° C. The preferred range is from 1000 to 1200 'C. samples at 11501 C in air for 1 hour at 1240' two days were pre-baked for 1 hour and at 1400 'C C 1 week at 13001 C, showed no significant Difference in transparency when finalized using the following procedural steps.

The graph of FIG. 2 shows the degree of shrinkage as a function of temperature. Each sample was pre-fired in air at a specified temperature for 1 hour. The highest degree of shrinkage occurs between approximately 1200 and 15001C . Later, the shrinkage increases only slightly, while the density approaches the theoretical maximum. If the compact is pre-fired at temperatures between 1000 and 1200.degree. C. , it is given sufficient hardness so that it can be brought into the desired shape. The required degree of density is achieved by the final sintering.

The last process step in the production of a transparent sintered corundum is the sintering of the compact at temperatures of at least 1700 ° C., preferably at temperatures between 1800 and 1950 ° C. It is assumed that higher temperatures are more advantageous since the enclosed pores diffuse more out of the body can, resulting in better optical properties. The compact can be fired for different times depending on the sintering temperature. A lower temperature, e.g. B. 1700 'C, requires about 24 hours, while at a temperature of 1900' C the duration can be reduced to a maximum of 6 to 10 hours. Even at higher temperatures, a longer time can be spent so that the body receives a slightly better transparency than could be achieved with a shorter duration. For example, bodies that had been fired at 1900 ° C for 1000 minutes had very good permeability. For the greatest possible permeability, this final step is best carried out in a hydrogen atmosphere. However, heating in a vacuum also provides an optical transmittance that approximates that achieved in hydrogen.

Curves 6, 7 and 8 in FIG. 1 show that a lower sintering temperature results in less pronounced spinel formation. The strength of the spinel formation within the preferred range for the addition of magnesium oxide does not deviate so much, even when higher temperatures are used, that one therefore refrains from using such temperatures which impart a degree of transparency which cannot be achieved at lower temperatures.

Curve 10 in FIG. 3 gives the optical transmittance of a 0.94 mm thick, disk-shaped sintered corundum according to the invention. This article was made from a mixture of finely divided magnesia and alumina with a grain size of less than 1 micron (0.05 to 0.2 #t) so that about 1 / s percent by weight magnesia was contained in the final mixture. The magnesium oxide was thoroughly mixed with the aluminum oxide in a mixer with the aid of rotating aluminum paddles. Aluminum blades are used to prevent any discolouring impurities from getting into the powder mixture.

The mixed magnesia-alumina powder was then pressed into a disk-like mold under a pressure of 22 kg / cm 2 and prebaked in air at 1200 ° C. for 1 hour. After prebaking, the disk was placed in a molybdenum furnace, brought to a temperature of 1700 ° C., which was maintained for 100 minutes, and cooled to room temperature. The body was then reheated in a hydrogen atmosphere to a temperature of 1900 ° C , to which it was exposed for 1000 minutes before being cooled to room temperature. The body treated in this way was prepared for the optical measurements by polishing it on the opposite sides. The final thickness, as indicated above, was 0.94 mm.

The "straight transmission" of the finished body exceeded 1011 / o for wavelengths between about 0.003 and 6.6 #t and reached a peak value of approximately 33 "1 / o for 5.5 [t about 3 and 6.3 #i more than 20%, which makes the body particularly valuable when the permeability of the mean wavelengths of infrared radiation is decisive The measurements were carried out with the aid of a spectrometer developed by D. TF M a rp 1 is described 490, 1956, e in the "Journal of the Optical Society of America," Vol. 46, and a beam of 7 'was used. table 1 below lists the characteristic and the above-mentioned permeability values. Table 1 Wavelength in #t »straight-ahead permeability« in 0 / 'o 0.0027 10.0 0.0030 11.3 0.50 13.0 1.00 14.0 2.00 17.8 3.00 19.0 4.00 25.2 5.00 27.8 5.50 34.0 6.00 28.5 7.00 0.5 The permeability values of a second body, which had been made by largely the same process as the first body, are shown by curves 11 to 15 in FIG. 3 shown. In this embodiment, however, the body was made from a mixture of finely divided alumina which contained 1/10 percent by weight of finely divided magnesium oxide. The compaction pressure was 340 kg / cm2 and the finished body was again disk-shaped and about 4.2 mm thick. The raw, unfired compact was prebaked at a temperature of 12001 ° C. for 1 hour, and the prebaked compact was then fired in hydrogen at 1900 ° C. for 1,000 minutes. After cooling , the opposite sides of the body were polished and its permeability was measured with the aid of a Perkins-Ehner spectrophotometer. The structure and operation of this device are described in the Journal of the Optical Society of America, Vol. 40, No. 1, Pp. 29 to 41 of January and No. 2, pp. 93 to 101, of February 1950 . The incoming and outgoing beams had a width of 60, and the distance of the tested sintered corundum from the focal point was increased to 3.8 cm compared to the usual 2.5 cm.

As curve 11 indicates, the 4.2 mm thick body peaked at about 29.d / o for wavelengths of about 5 it and had a transmittance of more than 101 / o for all wavelengths between 3 and 5.9 #t .

Curves 12-14 indicate the permeability values of the thinner bodies made from the 4.2 mm thick that produced curve 11 . The values of curve 12 were measured on a 3.3 mm thick body which had been made from the 4.2 mm thick and polished again. This procedure was repeated and the body was gradually reduced to a thickness of 2.2, 1.2 and 0.5 mm so that the effect of the thickness on the permeability could be determined.

The 3.3 mm thick body peaked at about 3811 / o for wavelengths of about 5 #t. The 2.2 mm thick body peaked at about 50111 for a wavelength of about 5 #t and the 1.2 mm thick body had a peak transmittance of about 66% for a wavelength of 5 #t. The 0.5 mm thick body peaked at about 79% at 4.9 #L. If one compares these peak values in their dependence on body strength, it becomes evident that the permeability increases linearly with decreasing strength. It can also be determined that the "straight-ahead permeability" of the 3.3 and 2.2 mm thick bodies for all wavelengths between 2.4 and 6 g or between 1.5 and 6.2 #t is more than 1011 / o. The 1.2 mm body had a transmittance of about 19 11 / o at a wavelength of 1 [t at which measurements were started; up to about 6.6 R it was more than 10 %. The 0.5 mm thick body had a permeability of 42 % at 1 left, which was up to about 6.9 [t more than 1011 / o. Table U below gives further values for permeability. Table U Wavelength »straight-ahead permeability« in II / o Body thickness in mm M ft 0.5 1.2 2.2 3.3 i 4.2 1.0 42.0 19.0 70 20 2.0 2.0 55.0 32.0 1.5: 0 7: 5 4.0 3.0 66.0 46.0 27.0 17.0 10.0 4.0 74.5 60.0 42.0 30.0 22.0 5.0 79.0 66.0 50 '0 380 29.0 6.0 66.5 45.0 25.0 12: 0 7.0 7.0 7.0 1.0 0.0 0.0 0, o The curve 16 of FIG. 3 indicates the "straight-ahead permeability" of a 0.49 nm thick optical sapphire. If this curve is compared with that of the 0.5 mm thick sintered corundum (curve 15), it is found that the permeability of this sintered corundum comes very close to that of sapphire. It is therefore possible to use a sintered corundum instead of a sapphire, for purposes for which only a sapphire seemed suitable up to now. In addition, sintered corundum can be used if a slightly lower optical transmission is acceptable, but the physical properties must be the same as those of sapphire.

In the above-described embodiments, the raw materials, i.e. H. the finely divided aluminum oxide and magnesium oxide powder, mixed with the aid of a mechanically agitated mixer. It is not necessary that such apparatus be used as other methods that mix the two powders can be equally effective.

For example, a body was made by dissolving magnesium carbonate in hydrochloric acid to form a magnesium chloride solution. The aluminum oxide was calcined ammonium aluminum oxide and the magnesium chloride was added in an aqueous suspension of the aluminum compound so that 0.2 to 0.5 percent by weight of magnesium oxide was contained in the clay. In the embodiment mentioned, 0.5 mol of magnesium oxide was added, and this corresponds to about 0.2 percent by weight of magnesium oxide.

The suspension of the clay was ground with the dissolved magnesium chloride in a polyethylene bottle for about 3 hours with the help of clay balls. Grinding can take 2 to 4 hours. Magnesium hydroxide then precipitated on the clay while ammonia was added until the pH rose to 7. The suspension was vigorously agitated during the precipitation. This suspension was placed in a dryer to remove the water. A gel formed and the gel was homogenized by stirring. The drying was ended at a temperature of 1501 ° C. and the resulting dried cake was pulverized to a grain size of 16 meshes per square centimeter and then calcined at 13001 ° C. for 6 hours. The calcination time can be between 24 hours at 12001 C and about. 4 hours at 1350 ° C. The respective duration is more or less determined by your own discretion.

After the calcination, the cake was finely divided mechanically and the obtained # II1ver was molded into a green compact under a pressure of about 340 kg / cm 2. The green compact was placed in a molybdenum furnace and fired at 17751 C in a hydrogen atmosphere for 24 hours. The final firing took place in the air at a temperature of 15001 C. The duration of the second firing in the air is not decisive. It can be omitted if desired. After the body was fired, it was polished on its opposite sides so that it was finally about 1/2 mm thick, and then its transmittance was measured using the aforementioned Perldns-Elmer spectrophotometer. The values achieved are listed in the table below for the corresponding wavelengths. Table IH Wavelength in #t »straight-ahead transmission« in% 2 16.5 3 21.0 4 31.0 5 42.0 5.5 46.0 5.9 48.0 6 47.5 6.5 38.0 7 15.0 7.5 0 If it is desired to increase the density of the sintered corundum even more, the formation of spinel during sintering (MgA1.0.) Can be prevented by keeping the body at high temperatures for a very long time, e.g. B. at temperatures corresponding to those of the final firing, burns in a hydrogen atmosphere or under vacuum. For example, a sintered corundum with a content of 0.5 mol percent magnesium oxide in the form of a spinel was burned at 18501 C in hydrogen for 24 hours, and after this time it was free of the spinel at a depth of 1 mm. A dense, pure sintered corundum can therefore be produced which has been cleaned by firing, the duration of the firing process depending on the thickness of the body. That is to say, the stronger the body, the longer it has to be burned, and the thinner the body, the less time it takes to rid it of the spinel.

It is clear that the invention allows sintered corundum of large To produce purity, high density and extraordinary permeability, which is particularly noticeable in the area of visible and infrared light and enables corresponding applications.

Claims (2)

Patent claims: 1. A process for the production of polycrystalline shaped bodies from sintered corundum with improved radiation permeability, characterized in that shaped, finely divided aluminum oxide of high purity is sintered together with a maximum of 0.5 percent by weight of magnesium oxide in a vacuum or in a hydrogen atmosphere at a temperature of 1700 to 19501 C. is until the spinel content is largely reduced, thereby increasing the density and the radiation permeability in the visible and infrared range. 2. The method according to claim 1, characterized in that 1 / "to 1/41, preferably 1/1 , percent by weight magnesium is used. 3. The method according to claim 1 or 2, characterized in that the formed, magnesium oxide-containing aluminum oxide in an oxygen-containing Atmosphere is prebaked for at least 1 hour at a temperature of about 1000 to 1200 ° C. 4. A method according to any one of the preceding claims, characterized in that finely divided aluminum oxide is used, of which up to 70 % by weight consists of particles with a size of 5 to 10 #t and at least 30 % by weight of particles with a size of less than 1 [t. 5. The method according to any one of the preceding claims, characterized in that the magnesium oxide-containing aluminum oxide is formed into a compact, the density of which is at least 3511 / o of the theoretical Density of an aluminum oxide single crystal is 6. Polycrystalline sintered corundum body, in particular f For optical purposes, characterized by a straight-ahead radiation transmittance with a thickness of 0.94 mm in the wavelength range from 0.003 to 6.6 #t of more than 100/0. Considered publications: German Patent Specifications No. 560 575, 670 23 1; Tonindustrie-Zeitung, 58, pp. 449 to 451, 463 to 465, 477 to 478 and 485 to 487 (1934); J. of Am. Ceram. Soc., 41, pp. 55-62 (1958); 39, pp. 141-145 (1956); 38, pp. 33 to 37 (1955); Z. angewandte Phys., 6, pp. 157 to 161 (1954).