Sintered boron nitride body and method for producing a sintered boron nitride body

Abstract

In order to provide a sintered, hexagonal boron nitride body (2a, 2b), same is produced by at least one pressing process and subsequent sintering process from a powder (P) made of a hexagonal boron nitride, its density being deliberately set to a value of <1.6 g/cm.sup.3. Studies have shown that, due to the selection of this lower density, the boron nitride body (2a, 2b) exhibits very high isotropy, when compared with conventional hexagonal boron nitride bodies. This relates in particular to thermal conductivity and the coefficient of thermal expansion, which are also largely temperature-independent.

Claims

1. A sintered boron nitride body, produced by at least one pressing process and a subsequent sintering process from a powder comprising a hexagonal boron nitride powder, and wherein said sintered boron nitride body comprising a density of less than 1.6 g/cm.sup.3 and a thermal conductivity in a first direction in space that differs by less than 15 W/mK from the thermal conductivity in a second direction in space.

2. The sintered boron nitride body according to claim 1 wherein the sintered boron nitride body having a crystal structure comprising layers of a planar, honeycomb structure hexagonal boron nitride, and the sintered boron nitride body being isotropic with regard to the first direction in space and the second direction in space wherein the first direction in space is oriented perpendicular to the layers of hexagonal boron nitride and the second direction in space is oriented parallel to the layers of hexagonal boron nitride.

3. The sintered boron nitride body according to claim 2 wherein the sintered boron nitride body has a coefficient of thermal expansion in the first direction that differs by less than 0.25*10.sup.6 K from the coefficient of thermal expansion in the second direction.

4. The sintered boron nitride body according to claim 2 wherein the sintered boron nitride body has a thermal conductivity having a maximum deviation of +/10 W/mK over a temperature range from room temperature to about 1000 C.

5. The sintered boron nitride body according to claim 1 wherein the sintered boron nitride body has a thickness of greater than 30 mm and an area equal to at least least 30 cm.sup.2.

6. The sintered boron nitride body according to claim 5 wherein the sintered boron nitride body has a thickness of greater than 40 mm.

7. The sintered boron nitride body of claim 1, wherein the sintered boron nitride body has a thermal conductivity of 20 W/mK to 35 W/mK in the a-direction and c-direction.

8. A sintered boron nitride body, produced by at least one pressing process and a subsequent sintering process from a powder comprising a hexagonal boron nitride powder, and wherein said sintered boron nitride body comprising a density of 0.9 g/cm.sup.3 to 1.2 g/cm.sup.3 and has a coefficient of thermal expansion in a first direction that differs by less than 0.25*10.sup.6 K from the coefficient of thermal expansion in a second direction.

9. A sintered boron nitride body, produced by at least one pressing process and a subsequent sintering process from a powder comprising a hexagonal boron nitride powder, and wherein said sintered boron nitride body comprising a density of 0.9 g/cm.sup.3 to 1.2 g/cm.sup.3 and has a thermal conductivity having a maximum deviation of +/10 W/mK over a temperature range from room temperature to about 1000 C.

10. A sintered boron nitride body, produced by at least one pressing process and a subsequent sintering process from a powder comprising a hexagonal boron nitride powder, and wherein said sintered boron nitride body comprising a density of 0.9 g/cm.sup.3 to 1.2 g/cm.sup.3 and has a thermal conductivity of 20 W/mK to 35 W/mK in the a-direction and c-direction.

11. A sintered boron nitride body, produced by at least one pressing process and a subsequent sintering process from a powder comprising a hexagonal boron nitride powder, and wherein said sintered boron nitride body comprising a density of 0.9 g/cm.sup.3 to 1.2 g/cm.sup.3 and has a thermal conductivity in the first direction that differs by less than 15 W/mK from the thermal conductivity in the second direction.

Description

DESCRIPTION OF THE FIGURES

(1) Embodiments of the invention are explained in more detail below with reference to the figures.

(2) FIG. 1 shows schematically the course of the production process in two different alternatives and

(3) FIG. 2 shows a measurement diagram of the temperature dependence of the thermal conductivity of a boron nitride body according to the invention, in comparison with a conventional boron nitride body.

DESCRIPTION OF THE EMBODIMENT

(4) For producing a sintered boron nitride body 2a, b made of nearly 100% hexagonal boron nitride, a powder P, the individual powder particles of which are composed of hexagonal boron nitride, is first provided in a method step A. In addition to the crystalline, hexagonal boron nitride particles, the powder P also contains a small proportion of boron oxide. Typically, this proportion lies in the range of 1 to 5, in particular to 2% by weight. Aside from the boron nitride, the powder P does not contain any further constituents.

(5) In method step B, this powder is introduced into a press mold and subjected to an isostatic cold-pressing, so that a cold-pressed molded body 4 is subsequently obtained. In this isostatic pressing, a compaction pressure is exerted from all sides on the molded body 4 to be formed, as shown by the arrows 6. In this cold-pressing process, the cold-pressed molded body 4 is compacted to a density in the range of about 1 to 1.3 g/cm.sup.3.

(6) In the first variation of the method, a hot-pressing is subsequently still carried out in method step C, in which the cold-pressed molded body 4 is uniaxially subjected to a further pressing process at a temperature of about 1200 C. to 1500 C. The exerted pressing force is again shown by the arrows 6. In this pressing process, the result is a hot-pressed molded body 8. In the hot-pressing process, said body is set to a density that is typically about 0.2 g/cm.sup.3 greater than the desired final density of the sintered boron nitride body 2a. A density in the range of 1.4 to 1.7 g/cm.sup.3 is therefore typically set for the hot-pressed molded body 8. Finally, in the subsequent method step D, the actual sintering or tempering process takes place. The sintering process is typically carried out at about 1700 C. to 2000 C., in particular at about 1800 C. in an inert atmosphere, particularly in a nitrogen atmosphere. The dwell time is several hours, preferably about 3 to 5, and in particular 4 hours.

(7) At the temperatures of the hot-pressing process, the boron oxide present is only fused, and thus active as a binder, in order to form a hot-pressed molded body 8 of high strength. At the higher sintering temperatures, the still remaining boron oxide evaporates, and the individual boron nitride particles sinter together. Due to the evaporation of the boron oxide, the density of the final manufactured boron nitride body 2a is reduced to a density in the range of 1.2 to 1.5 g/cm.sup.3, depending on the setting in the hot-pressing process.

(8) In the second method alternative, hot-pressing according to method step C is dispensed with, and the cold-pressed molded body 4 is directly subjected to a sintering process D in order to create the sintered boron nitride body 2b. Same accordingly again has a significantly lower density in the range of about 1 g/cm.sup.3 to 1.2 g/cm.sup.3, in particular 1 g/cm.sup.3, compared with the sintered hot-pressed boron nitride body 2a.

(9) The final manufactured boron nitride bodies 2a, 2b each have a thickness d, which is preferably >30 mm and in particular >40 mm.

(10) The boron nitride bodies 2a, 2b produced in this way have very high isotropy, particularly with regard to thermal conductivity and the coefficient of thermal expansion. A distinction is made between a first, parallel direction in space, the c-direction, and a second direction, perpendicular thereto, the a-direction. The parallel c-direction in space is oriented parallel to the direction of pressing in the uniaxial hot pressing according to method step C.

(11) In general, these two directions in space a, c are two directions in space that are perpendicular to one another along main axes of the sintered boron nitride body 2a, 2b.

(12) The diagram according to FIG. 2 shows the temperature dependence of the thermal conductivity of a hot-pressed boron nitride body 2a according to the invention having a density of 1.5 g/cm.sup.3 in comparison with a conventionally manufactured boron nitride body of high density. The thermal conductivity is plotted in W/mK against the temperature. The measuring points marked as triangles are the measuring points of a conventional boron nitride body, in fact, in the c-direction in space. The measuring points marked with a diamond are the measuring points of a boron nitride body 2a of low density according to the invention, likewise in the c-direction in space. The measuring points marked with a square are the measuring points for a boron nitride body 2a according to the invention in the a-direction.

(13) Based on the values of a conventional boron nitride body, it is apparent first that they are heavily temperature-dependent and drop from an initial value of about 70 W/mK at room temperature to a value of about 25 W/mK at a temperature of about 1100 C. In contrast, the values for both directions in space a, c in the boron nitride body 2a according to the invention are largely constant. They fluctuate only slightly around a mean value of about 25 W/mK over the entire temperature range from about room temperature to about 1100 C.

(14) It is furthermore readily apparent that the values for the two directions in space a, c are virtually indistinguishable or differ only slightly; hence, that the boron nitride body 2a has very high isotropy with regard to its thermal conductivity. The thermal conductivity is therefore largely independent of the orientation of the boron nitride body 2a.

(15) It further apparent that, although the values of the thermal conductivity of the body 2a according to the invention at room temperature lie significantly below those of a conventional boron nitride body, the values nearly approximate them with increasing temperature due to a sharp drop in the temperatures of the conventional sintered bodies and already lie very close together in the range of a later area of application, for example in a temperature range greater than 800 C. Surprisingly, the lower density of the boron nitride body 2a according to the invention therefore simply does not lead to lower thermal conductivities at the later application temperatures.

(16) Similarly, the boron nitride body 2a also exhibits very high isotropy with regard to the coefficient of thermal expansion which is likewise, similar to the thermal conductivity, substantially independent of the respective temperature.

(17) The following table additionally lists several further properties of a conventional sintered boron nitride body as a comparative sample and those of a boron nitride body 2a according to the invention.

(18) TABLE-US-00001 Properties Comparative sample BN body 2a Direction in space c (par- a (perpen- c (par- a (perpen- allel) dicular) allel) dicular) Density (g/cm.sup.3) 1.90 1.50 Thermal conductivity 78.00 130.00 25.00 32.00 at 25 C. (W/mK) Coefficient of thermal 1.60 0.40 0.20 0.35 expansion at 1200 C. (10.sup.6/K) Specific heat at 25 C. 0.81 0.90 (J/(g*K)) Max. temp. inert 2000 2000

(19) The two boron nitride bodies are self-binding systems, to which no additional binder had thus been added. The binder functionality is assumed by the boron oxide that is usually still contained in the starting powder as a contamination, so to speak, in an amount of 1 to 2% by weight. The characteristic color of such hexagonal boron nitride bodies is white. The density of the comparative sample according to the prior art was 1.9 g/cm.sup.3, whereas the density of the boron nitride body 2a according to the invention was 1.5 g/cm.sup.3.

(20) The thermal conductivity at 25 C. shows strong anisotropy in the comparative sample and is 78 W/mK for the parallel c-direction in space and 130 W/mK for the perpendicular a-direction in space. In contrast, the values for the boron nitride body 2a according to the invention differ by only 7 W/mK. As previously explained in connection with FIG. 2, while the values at 25 C. lie clearly below those of the conventional boron nitride body, they become increasingly similar at higher temperatures.

(21) With respect to the coefficient of thermal expansion, measured at 1200 C., the anisotropy is even more pronounced in the comparative sample and differs by factor of 4. For the parallel c-direction in space it is 1.6*10.sup.6/K, and for the perpendicular a-direction in space it is 0.4*10.sup.6/K. In contrast, the boron nitride body 2a according to the invention shows an absolute difference of only 0.15.Math.10.sup.6/K. Here as well, the coefficient of thermal expansion thus shows very high isotropy when compared with the conventional comparative sample. Moreover, the boron nitride body 2a according to the invention is also characterized by a significantly lower coefficient of thermal expansionat least with respect to the parallel c-direction in space, which is lower than that of the comparative sample by about a factor of 8. The specific heat for the two boron nitride bodies is roughly comparable. The same is also true for the maximum service temperature in inert ambient conditions, hence, under a protective gas atmosphere/nitrogen atmosphere, for example.

(22) Studies have furthermore shown that the boron nitride body according to the invention also exhibits adequate electrical breakdown strength.

(23) In summary, it should therefore be noted that it is possible to achieve particularly advantageous physical properties by specifically setting only a low density for the sintered boron nitride body 2a, 2b. In particular, this results in high isotropy. This relates to thermal conductivity and also to the coefficient of thermal expansion. In addition to high isotropy, the boron nitride body 2a, 2b is moreover characterized by being substantially temperature-independent, in particular with regard to thermal conductivity, over a temperature range from room temperature to temperatures greater than 1000 C. Overall, compared with a conventional hexagonal boron nitride body, a significantly improved, evenly built up, isotropic hexagonal boron nitride body 2a, 2b is achieved, whose possible uses are considerably more flexible for the most varied areas of application, due to this improved isotropy.