AQUEOUS SUSPENSION CONTAINING METAL CARBIDE PARTICLES

Abstract

The present invention relates to aqueous suspensions containing 30 to 95 wt.-% metal carbide particles and a dispersant, and to a process for coating substrates using said aqueous suspensions. The invention also relates to the coated substrates that can be produced by the process according to the invention and to the uses thereof.

Claims

1-15. (canceled)

16. An aqueous suspension comprising at least one metal carbide particle and at least one dispersant, wherein the proportion of the at least one metal carbide particle is in the range from 30% to 95% by weight based on the total weight of the suspension.

17. The aqueous suspension of claim 16, wherein: the at least one metal carbide particle is selected from the group consisting of carbides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, silicon, and mixtures thereof; the at least one metal carbide particle has an average particle size in the range from 0.05 to 25 m; the at least one metal carbide particle has a content of individual elemental impurity of <300 ppm; and/or the dispersant is selected from the group consisting of polyacrylic acid, tetrabutylammonium hydroxide, and mixtures thereof.

18. The aqueous suspension of claim 16, which comprises at least one additive selected from the group consisting of a base, a defoamer, a sintering aid, and mixtures thereof.

19. The aqueous suspension of claim 18, wherein the base is sodium hydroxide solution, the defoamer is a fatty alcohol polyalkylene glycol ether, and/or the sintering aid is cobalt, silicon, or a mixture thereof.

20. The aqueous suspension of claim 16, wherein: the proportion of metal carbide particles is in the range from 40% to 90% by weight based on the total weight of the suspension; and/or the proportion of the dispersant is in the range from 0.05% to 5% by weight based on the total weight of the suspension; and/or the proportion of the at least one additive is in the range from 0% to 10% by weight based on the total weight of the suspension.

21. A process for coating a substrate comprising the following steps: i) providing a substrate; ii) providing the aqueous suspension of claim 16; iii) applying the suspension from step ii) to the surface of the substrate from step i); and iv) drying the applied suspension, resulting in the formation of a coating on the surface of the substrate.

22. The process of claim 21, wherein the substrate is selected from the group consisting of graphite, materials having an adjusted coefficient of thermal expansion, and mixtures thereof.

23. The process of claim 21, further comprising: v) pretreating the substrate provided in step i) prior to step iii), with the pretreatment effected by a measure selected from the group consisting of mechanical roughening of the surface, thermal pretreatment of the surface, chemical treatment of the surface, and combinations thereof, and subsequent cleaning, in particular by means of sonication; and vi) sintering of the coating obtained after step iv).

24. The process of claim 21, wherein the suspension in step iii) is applied by painting, dipping, or spraying.

25. The process of claim 21, wherein step iv) is carried out at a temperature in the range from 100 to 600 C.

26. The process of claim 25, comprising carrying out step iv) over a period of 5 to 40 hours.

27. The process of claim 26, wherein, in step iv), the temperature is increased in stages.

28. The process of claim 27, wherein, in step iv) the temperature is increases as follows: (1) 140 to 160 C. for 2.5 to 3.5 hours; then (2) 180 to 220 C. for 1.5 to 2.5 hours; then (3) held for 2 hours at the temperature from (2); then (4) 200 to 250 C. for 1.5 to 2.5 hours; then (5) 310 to 350 C. for 4.5 to 5.5 hours; then (6) 330 to 350 C. for 1.5 to 2.5 hours; then (7) held for 2 hours at the temperature from (6); then (8) 380 to 420 C. for 3.5 to 4.5 hours; then (9) 430 to 470 C. for 1.5 to 2.5 hours.

29. The process of claim 23, wherein step vi) is carried out at a temperature in the range from 2000 to 2600 C. over a period of 1 to 10 hours; step vi) is carried out at a pressure in the range from 500 to 900 torr; and/or step vi) is carried out under inert gas.

30. The process of claim 23, wherein the green density of the coating after step iv) and/or prior to step vi) is at least 50%; and/or the coating after step iv) or vi) has an impurity content of less than 300 ppm; and/or the coating after step iv) or vi) has an open porosity of less than 5%.

31. The process of claim 21, wherein the thickness of the coating after step iv) or vi) is in the range from 20 to 500 m; and/or the pH of the aqueous suspension prior to step iii) is in the range from 5 to 10.

32. A coated substrate produced by the process of claim 21.

33. The coated substrate of claim 32, wherein the thickness of the coating is in the range from 20 to 500 m.

34. A process of growing crystals comprising utilizing the coated substrate of claim 33 as a carbidic material.

35. The process of growing crystals according to claim 34, comprising a physical vapor-phase process, an epitaxy process, or crucible formation.

Description

[0056] Preferred embodiments of the process according to the invention are indicated herein below.

[0057] In a preferred embodiment of the process according to the present invention, the substrate is selected from the group consisting of graphite, materials having an adjusted coefficient of thermal expansion, preferably graphite having an adjusted coefficient of thermal expansion in the range from 6.5 to 7.5*10.sup.6 K.sup.1, and mixtures thereof.

[0058] In a further preferred embodiment of the process according to the invention, it may be the case that the process includes the following further steps: [0059] v) pretreating the substrate provided in step i) prior to step iii), with the pretreatment effected preferably by a measure selected from the group consisting of mechanical roughening of the surface, thermal pretreatment of the surface, chemical treatment of the surface, and mixtures thereof, and subsequent cleaning, in particular by means of sonication; [0060] vi) sintering of the coating obtained after step iv).

[0061] In order to achieve good adhesion of the coating on the substrate, it is particularly advantageous to first mechanically roughen the substrate and then create a hydrophilic surface through appropriate cleaning steps by means of sonication. Particularly in the case of a graphite substrate, care must be taken to ensure elimination of, or at least a reduction in, the number of loose particles on the surface.

[0062] In another preferred embodiment, the suspension in step iii) is applied by painting, dipping or spraying.

[0063] When applying by spraying, the substrate is preferably positioned in the centre of a rotatable turntable and fixed in place by means of special holders. The tilt angle of the turntable and the spray angle of the spray gun are additionally adjusted according to the geometry of the substrate to be coated by means of a specially designated holder. The substrate is then coated with the aqueous suspension under firmly defined spray parameters (including the atomizer air pressure, throttling of the material supply via the needle lift, and distance from the nozzle opening to the substrate surface). The speed of rotation of the turntable during the spraying process is guided by the desired layer thickness of the subsequent coating.

[0064] In a further preferred embodiment of the present invention, step iv) is carried out at a temperature in the range from 100 to 600 C., preferably from 120 to 550 C., and more preferably from 145 to 455 C., with preference given to carrying out step iv) over a period of 5 to 40 hours and more preferably of 20 to 30 hours.

[0065] In order to obtain crack-free layers, it has proven advantageous to anneal the coating obtained in step iv) over several temperature steps. The inclusion of hold phases is particularly preferable in order to prevent overly rapid drying and consequent cracking. The specific hold phases are guided by the evaporation behavior of the dispersant used. When cobalt is used as sintering aid, it is advantageous to carry out the drying process under an inert gas atmosphere.

[0066] In a preferred drying process iv), the temperature ranges and time intervals may be as follows: [0067] (1) 140 to 160 C. for 2.5 to 3.5 hours; then [0068] (2) 180 to 220 C. for 1.5 to 2.5 hours; then [0069] (3) held for 2 hours at the temperature from (2); then [0070] (4) 200 to 250 C. for 1.5 to 2.5 hours; then [0071] (5) 310 to 350 C. for 4.5 to 5.5 hours; then [0072] (6) 330 to 350 C. for 1.5 to 2.5 hours; then [0073] (7) held for 2 hours at the temperature from (6); then [0074] (8) 380 to 420 C. for 3.5 to 4.5 hours; then [0075] (9) 430 to 470 C. for 1.5 to 2.5 hours.

[0076] In another preferred embodiment of the present invention, step vi) (=sintering) is carried out at a temperature in the range from 2000 to 2600 C., preferably from 2100 to 2500 C., and more preferably from 2200 to 2300 C. Particular preference is given to carrying out step vi) over a period of 1 to 10 hours, more preferably of 3 to 5 hours. It is further preferable to carry out step vi) at a pressure in the range from 500 to 900 torr, preferably from 600 to 800 torr, and more preferably from 680 to 720 torr.

[0077] In another preferred embodiment of the present invention, step vi) is carried out under inert gas, with the inert gas particularly preferably being selected from the group consisting of helium, argon, nitrogen, and mixtures thereof.

[0078] The addition of sintering aids such as cobalt or silicon boosts the flow behavior during the sintering process and increases the achievable coating end density.

[0079] In another preferred embodiment of the present invention, the green density of the coating prior to step v) is at least 50% and preferably at least 60%.

[0080] In a further preferred embodiment, it may be the case that the coating after step iv) or vi) has a content of individual elemental impurities of less than 300 ppm and preferably of less than 1 ppm.

[0081] In another preferred embodiment, the coating after step iv) or vi) has an open porosity of less than 5% and preferably of less than 1%. This is determined preferably by Hg porosimetry.

[0082] In another preferred embodiment of the present invention, the thickness of the coating after step iv) or vi) is in the range from 20 to 500 m, preferably from 50 to 400 m, and more preferably from to 100 to 300 m.

[0083] In a further preferred embodiment, it may be the case that the pH of the aqueous suspension prior to step iii) is in the range from 5 to 10 and preferably from 7 to 8, particularly for polyacrylic acid as dispersant.

Coated Substrate

[0084] The present invention further relates to a coated substrate producible by the process according to the invention.

[0085] In another preferred embodiment, the thickness of the coating is in the range from 20 to 500 m, preferably from 50 to 400 m, and more preferably from 100 to 300 m.

[0086] In a further preferred embodiment, it may be the case that the coating after step iv) or vi) has an impurity content of less than 300 ppm and preferably of less than 1 ppm.

[0087] In another preferred embodiment, the coating after step iv) or vi) has an open porosity of less than 5% and preferably of less than 1%.

Use

[0088] The coated substrates according to the invention are used as carbidic materials.

[0089] Preference is given here to uses in applications for crystal growing, in particular applications for PVT (physical vapor phase) processes, epitaxy processes, and for crucibles.

[0090] The object of the invention is elucidated in more detail with reference to the examples that follow, without intending to restrict it to the specific embodiments shown here.

Preparation of Aqueous Suspension 1

[0091] An aqueous tantalum carbide suspension was prepared using a dispersing agitator. This was done by adding the tantalum carbide powder (70% by weight, total impurity content: 300 ppm, H. C. Starck), polyacrylic acid (0.5% by weight, M.sub.w 5000 g/mol, Polyscience Europe GmbH), sintering aid (0.7% by weight of silicon, H. C. Starck), defoamer (2 drops of Contraspum, Zschimmer and Schwarz) one step at a time to distilled water (28.8% by weight). Between the addition of each individual component, the suspension was processed with a stirrer unit for up to 15 minutes at 4000 revolutions per minute to ensure that the metal carbide powder, the dispersant, and the additives used were homogeneously dispersed in the suspension. The pH of the suspension was adjusted with sodium hydroxide solution to pH 8. The proportion of tantalum carbide was 70% by weight based on the total weight of the aqueous suspension.

Preparation of Aqueous Suspension 2

[0092] An aqueous tantalum carbide suspension was prepared using a dispersing agitator. This was done by adding the tantalum carbide powder (70% by weight, total impurity content: 300 ppm, H. C. Starck), tetrabutylammonium hydroxide (0.5% by weight, Sigma Aldrich), sintering aid (0.7% by weight of silicon, H. C. Starck), defoamer (2 drops of Contraspum, Zschimmer and Schwarz) one step at a time to distilled water (28.8% by weight). Between the addition of each individual component, the suspension was processed with a stirrer unit for up to 15 minutes at 4000 revolutions per minute to ensure that the metal carbide powder, the dispersant, and the additives used were homogeneously dispersed in the suspension. The pH of the suspension was 7. The proportion of tantalum carbide was 70% by weight based on the total weight of the aqueous suspension.

[0093] Aqueous suspensions 1 and 2 were used to coat a graphite substrate.

[0094] Coating was carried out on a coating stand having a rotatable and tiltable turntable with the aid of a spray gun. The spray gun was operated with 2 bar of compressed air and was mounted on a holder that allows both the angle and the distance from the specimen to be varied. For coating the graphite substrate (FIG. 1 depicts a graphite cylinder by way of example), a distance of 19 cm and a spray angle of 90 were chosen. On account of the cylindrical geometry, the interior of the graphite cylinder was coated in accordance with the assembly shown in FIG. 1. For this, the cylinder was fixed to the rotary table and tilted at an angle of 50. The mouth of the gun was positioned at a distance of 21 cm and at a horizontal tilt angle of 70 (FIG. 1). The exterior was coated manually by making up and down movements perpendicular to the cylinder wall with simultaneous rotation of the table.

[0095] To determine the sintered density of the corresponding layers, after the actual sintering step the mass, thickness, and area of the layer thus obtained were determined, the sintered density calculated from the thickness and area the layer volume and from the volume and mass and related to the maximum theoretical density of TaC (14.3 g/cm.sup.3). The coating from suspension 1 afforded a sintered density of 54% and for the coating from suspension 2 the sintered density was 56%.

[0096] Through analyses of cross sections of the coated and sintered substrates, the layer thickness in both cases could be determined by scanning electron microscopy and incident light microscopy and was 100 m.