METHOD FOR PRODUCING A SILICON CARBIDE SHAPED BODY

Abstract

The invention relates to a method for producing a shaped body that contains at least 85 vol. % crystalline silicon carbide and at most 15 vol. % silicon, comprising the following steps: a) providing a powdered mixture that contains at least 60 vol. % amorphous silicon carbide and at most 40 vol. % silicon having a crystallite size of 3-50 nm; b) shaping the mixture by b1) hot pressing, or b2) compacting at room temperature and subsequent sintering or hot pressing, wherein the sintering or hot pressing is carried out in an inert gas or vacuum at a temperature of at least 1400 C. The volume ratio SiC:Si in the powdered mixture is 95:5-99:1.

Claims

1. A process for producing a shaped body, the method comprising the steps of: a) providing a pulverulent mixture comprising at least 60% by volume of amorphous silicon carbide and not more than 40% by volume of silicon having a crystallite size of from 3 to 50 nm, and b) shaping the pulverulent mixture by either b1) hot pressing or b2) compacting at room temperature and subsequent sintering or hot pressing, wherein the shaped body comprise at least 85% by volume of crystalline silicon carbide and not more than 15% by volume of silicon, and the sintering or hot pressing is carried out under inert gas or reduced pressure at a temperature of at least 1400 C.

2. The process of claim 1, wherein the pulverulent mixture comprises at least 90% by volume of amorphous silicon carbide and not more than 10% by volume of silicon.

3. The process of claim 1, wherein the temperature is from 1900 to 2100 C.

4. The process of claim 1, wherein a volume ratio of SiC:Si in the pulverulent mixture is from 95:5 to 99:1 and the temperature is from 2000 to 2100 C.

5. The process claim 1, wherein the pulverulent mixture consists to an extent of at least total 98% by weight of amorphous silicon carbide and silicon.

6. The process of claim 1, wherein no sintering aids are used.

7. The process of claim 1, wherein a BET surface area of the pulverulent mixture is from 10 to 100 m.sup.2/g.

8. The process claim 1, wherein the mixture is in the form of fused aggregates.

9. The process of claim 8, wherein an average diameter of the fused aggregates is from 50 to 500 nm.

10. The process of claim 1, wherein the pulverulent mixture is provided by, in a hot-wall reactor, reacting a gas stream comprising at least one starting compound of silicon selected from the group consisting of SiH.sub.4, Si.sub.2H.sub.6 and Si.sub.3H.sub.8, and a gas stream comprising at least one starting compound of carbon selected from the group consisting of methane, ethane, propane, ethylene and acetylene in a molar ratio of silicon:carbon of from 1.5:1 to 1:3 at a temperature of from 900 to 1200 C., thereby forming a reaction mixture, and cooling the reaction mixture or allowing the reaction mixture to cool and separating the pulverulent mixture from gaseous substances.

11. The process of claim 10, wherein hydrogen is introduced into the hot-wall reactor.

Description

EXAMPLES

Starting Material

Example 1: Production of a Pulverulent SiC/Si Mixture in a Hot-Wall Reactor

[0032] The present gas phase reactor has a coaxial nozzle system for introduction of the process gases. 5 slm (slm=standard liters per minute)

[0033] SiH.sub.4 and 2.5 slm of acetylene are introduced into the core of a tubular hot-wall reactor as a homogeneous mixture via a nozzle. In addition, 35 slm of hydrogen are introduced as blanketing gas. There is a laminar flow through the hot wall reactor.

[0034] A temperature of 985 C. is measured at the outer reactor wall. The solids are separated from gaseous substances in a filter and dispensed under inert conditions via an airlock system.

[0035] The solids have a BET surface area of 67 m.sup.2/g, a volume ratio of SiC:Si of 96:4.

[0036] The crystallite size of Si is (84) nm. The SiC phase is characterized by three broad reflections. These broad reflections, also known as a halo, are characteristic of an amorphous phase (FIG. 1, x-axis: 2 theta [ ], y-axis: intensity [a.u.]). The composition of the amorphous phase may either correspond to stoichiometric SiC or differ from the SiC stoichiometry.

Production of Shaped Bodies

Example 2: Pressing at Room Temperature

[0037] The pulverulent Si/SiC mixture produced in example 1 is introduced into a compression die made of steel and pressed at room temperature. Two different dies are used: a cylindrical die having a diameter of 13 mm which is laden with about 0.3 g of pulverulent Si/SiC mixture, and a cuboidal die having dimensions of 1016 mm, which is laden with about 0.5 g of pulverulent Si/SiC mixture. Sufficiently stable compacts were obtained after pressing at pressures of 20 to 80 MPa. The height of the compacts is about 3 mm.

Example 3: Sintering at 2100 C.

[0038] The compacts produced in example 2 are introduced into a high-temperature oven. The oven is evacuated to a pressure of 10.sup.2-10.sup.3 mbar. The sintering is executed under an argon atmosphere to the temperature program detailed in table 1.

[0039] The oven is additionally equipped with an optical and thermogravimetric measurement system which allows in situ observation of the sintering characteristics.

[0040] In the course of heating, shrinkage of the compacts of 1% to 2% is observed at (82040) C. It is noted that this temperature corresponds to 0.6T.sub.m where T.sub.m is the melting temperature of pure silicon. The shrinkage at this temperature may be caused by a reaction between nanoscale Si and amorphous SiC, and also by SiC crystallization.

[0041] At a temperature of (140050) C., further significant shrinkage of the compacts of 23% to 30% is observed. Above 1410 C., silicon in the Si/SiC mixture is liquid and therefore enables liquid phase sintering of SiC without addition of sintering aids. In addition, a loss of mass totaling 10% is observed above 1400 C. This may be due to formation of volatile species.

[0042] From the Rietveld refinement of the x-ray diffractograms (FIG. 2, bottom; x-axis: 2 theta [ ], y-axis: intensity [a.u.]), it is possible to calculate the proportions and lattice constants of the phases formed in the course of sintering in the end product (see table 2). According to the Rietveld refinement, the end product contains predominantly SiC-3C phase (86% by volume) and 13.9% by volume of hexagonal SiC phases. The phase proportion of Si of (0.10.4)% by volume is negligibly small.

[0043] Table 2 shows the results of the Rietveld refinement of the x-ray diffractograms: phase proportions, lattice constants and average crystallite size of SiC-3C in the end product. The error in the determination of phase proportions is 0.4% by volume.

Example 4: Sintering at 1600 C.

[0044] The compacts produced in example 2 are introduced into a high-temperature oven. The oven is evacuated to a pressure of 10.sup.2-10.sup.3 mbar. The sintering is executed under an argon atmosphere to the temperature program detailed in table 1.

[0045] In the course of heating, shrinkage of the compacts of 2.2% is observed at (82040) C. At a temperature of (140050) C., further significant shrinkage of the compacts of 19% is observed. In addition, a loss of mass totaling 0.2% is observed above 1400 C. This loss of mass is distinctly smaller than the loss of mass in example 2.

[0046] From the Rietveld refinement of the x-ray diffractograms (FIG. 2, top), it is possible to calculate the proportions and lattice constants of the phases formed in the course of sintering in the end product (see table 2). According to the Rietveld refinement, the end product contains predominantly SiC-3C phase (80% by volume) and 18.9% by volume of hexagonal SiC phases. The phase proportion of Si is (1.10.4)% by volume. This phase proportion of silicon is higher than in example 3, which may firstly be due to an incomplete reaction between nanoscale Si and amorphous SiC and a lesser degree of formation of volatile species.

[0047] By contrast with the end product in Example 3, the crystallite size of SiC-3C after sintering at 1600 C. is within the nanoscale range (table 2). This is advantageous for the strength of the end product.

Example 5. Hot Pressing by Means of Spark Plasma Sintering (SPS) at 1400 C.

[0048] The pulverulent Si/SiC mixture produced in example 1 is introduced into a compression die made of graphite and pressed according to the temperature program detailed in table 3. SPS is conducted under reduced pressure. A pressure of 5 MPa is applied at room temperature. When the temperature of 1200 C. is attained, the pressure is increased continuously from 5 MPa to 24 MPa. The pressure of 24 MPa is kept constant at 1400 C. for 2 min. Subsequently, the temperature is reduced to 300 C. within a period of 10 min. The die, ram and material produced by means of SPS are subsequently cooled down to room temperature at the chamber cooling rate.

[0049] From the Rietveld refinement of the x-ray diffractograms, it is possible to calculate the proportions and the lattice constants of the phases formed in SPS in the end product (see table 4). According to the Rietveld refinement, the end product contains predominantly SiC-3C phase (97% by weight) and 3% by weight of hexagonal SiC phase. More particularly, no Si phase is observed. Thus, the material produced by SPS consists of 100% SiC. The crystallite size of SiC-3C after SPS at 1400 C. is within the nanoscale range (table 4).

[0050] The density of the shaped body determined by the Archimedes method is 3.05 g/cm.sup.3 and is thus above 96% of the theoretical density of 3CSiC.

Example 6. Hot Pressing by Means of Spark Plasma Sintering (SPS) at 1550 C.

[0051] The pulverulent Si/SiC mixture produced in example 1 is introduced into a compression die made of graphite and pressed according to the temperature program detailed in table 3. SPS is conducted under reduced pressure. A pressure of 6.5 MPa is applied at room temperature. When the temperature of 1200 C. is attained, the pressure is increased continuously from 6.5 MPa to 24 MPa. The pressure of 24 MPa is kept constant at 1550 C. for 2 min. Subsequently, the temperature is reduced to 300 C. within a period of 12 min. The die, ram and material produced by means of SPS are subsequently cooled down to room temperature at the chamber cooling rate.

[0052] From the Rietveld refinement of the x-ray diffractograms, it is possible to calculate the proportions and the lattice constants of the phases formed in SPS in the end product (see table 4). According to the Rietveld refinement, the end product contains predominantly SiC-3C phase (96% by weight) and 4% by weight of hexagonal SiC phase. More particularly, no Si phase is observed. Thus, the material produced by SPS consists of 100% SiC. The crystallite size of SiC-3C after SPS at 1550 C. is within the nanoscale range (table 4).

[0053] The density of the shaped body determined by the Archimedes method is 2.97 g/cm.sup.3 and is thus above 93% of the theoretical density of 3CSiC.

TABLE-US-00001 TABLE 1 Temperature program for sintering Example 3 Example 4 Heating from room temp. to 200 C. 5 K/min 5 K/min Heating from 200 to 1300 C. 20 K/min 20 K/min Heating from 1300 to 1600 C. 2 K/min 2 K/min Hold time at 1600 C. 0 min 30 min Heating from 1600 to 2100 C. 10 K/min Hold time at 2100 C. 30 min Cooling to 1100 C. 10 K/min 10 K/min Cooling below 1100 C. Oven Oven cooling rate cooling rate

TABLE-US-00002 TABLE 2 Phase proportions, lattice constants and grain size <D> of the phases formed in the course of sintering Example 3 4 SiC-3C vol % 86.0 80.0 a () 4.3613 4.3608 Si vol % 0.1 1.1 a () 5.421 5.430 SiC-21R vol % 4.3 18.9 a () 3.079 30.805 c () 54.77 53.73 SiC-18H vol % 1.6 0 a () 3.085 c () 44.1 SiC-6H vol % 8.0 0 a () 30.877 c () 15.160 <D>.sub.SiC-3C nm microcrystalline 28 2

TABLE-US-00003 TABLE 3 Temperature program for SPS Example 5 Example 6 Heating from room temp. to 400 C. 60 K/min 60 K/min Heating from 400 to 950 C. 100 K/min 110 K/min Heating from 950 to 1400 C. 40 K/min Heating from 950 to 1550 C. 85 K/min Hold time at T.sub.max = 1400 C. 2 min Hold time at T.sub.max = 1550 C. 2 min Cooling from T.sub.max to 300 C. 10 min 12 min Cooling below 300 C. Chamber Chamber cooling rate cooling rate

TABLE-US-00004 TABLE 4 Phase proportions, lattice constants and grain size <D> of the phases formed in SPS Example 5 6 SiC-3C % by wt. 97 96 a () 4.359 4.360 Si % by wt. 0 0 SiC-2H % by wt. 3 4 a () 3.046 3.074 c () 5.212 5.238 <D>.sub.SiC-3C nm 40 5 30 5