Chromium metal powder

Abstract

A metal powder has a chromium content of at least 90 Ma %, a nanohardness according to EN ISO 14577-1 of ≤4 GPa and/or a green strength measured according to ASTM B312-09 of at least 7 MPa at a compression pressure of 550 MPa.

Claims

1. A metal powder, comprising: a chromium content of at least 90 Ma %; and a nanohardness .sub.HIT 0.005/5/1/5 according to EN ISO 14577-1 of ≤4 GPa.

2. The metal powder according to claim 1, wherein the metal powder is chromium powder having a metallic purity ≥99.0 Ma %.

3. The metal powder according to claim 1, wherein the metal powder is an alloyed powder or composite powder.

4. The metal powder according to claim 1, wherein the metal powder is granulated.

5. The metal powder according to claim 1, which further comprises a surface area according to BET of ≥0.05 m2/g with or without a surface-enlarging operation.

6. A method for producing a metal powder, the method comprising the following steps: reducing at least one compound of the group consisting of chromium oxide and chromium hydroxide, optionally with an admixed solid carbon source, under at least temporary action of hydrogen and hydrocarbon to produce a metal powder having: a chromium content of at least 90 Ma %; and a nanohardness .sub.HIT 0.005/5/1/5 according to EN ISO 14577-1 of ≤4 GPa.

7. The method according to claim 6, which further comprises: heating the compound of the group consisting of chromium oxide and chromium hydroxide, optionally with an admixed solid carbon source, to a temperature TR with 1100° C.≤TR≤1550° C.; optionally holding the temperature at 1100° C.≤TR≤1550° C.; and at least temporarily setting the hydrocarbon partial pressure at 5 to 500 mbar at least during the heating step.

8. The method according to claim 6, wherein the action of hydrogen and hydrocarbon occurs at least in a temperature range of 800 to 1050° C.

9. The method according to claim 8, which further comprises setting the hydrocarbon partial pressure at 5 to 500 mbar at least in the temperature range of 800 to 1050° C.

10. The method according to claim 8, which further comprises setting a sum of heating time and holding time in the temperature range of 800° C. to 1050° C. to be at least 45 minutes.

11. The method according to claim 6, which further comprises setting a total pressure at 0.95 to 2 bar.

12. The method according to claim 6, which further comprises reducing the compound of the group consisting of chromium oxide and chromium hydroxide under at least temporary action of a H.sub.2—CH.sub.4 gas mixture.

13. The method according to claim 12, which further comprises setting a H.sub.2/CH.sub.4 volume ratio at 1 to 200 or 1.5 to 20.

14. The method according to claim 6, which further comprises admixing a solid carbon source having at least one component selected from the group consisting of carbon black, activated carbon, graphite, carbon-releasing compound and mixtures thereof.

15. The method according to claim 14, which further comprises using between 0.75 and 1.25 mol or between 0.90 and 1.05 mol of carbon per mol of oxygen in the chromium oxide or chromium hydroxide.

16. The method according to claim 6, which further comprises at least partially reacting at least one compound selected from the group consisting of chromium oxide and chromium hydroxide under the action of hydrogen and hydrocarbon to form a chromium carbide selected from the group consisting of Cr.sub.3C.sub.2, Cr.sub.7C.sub.3 and Cr.sub.23C.sub.6.

17. The method according to claim 16, which further comprises at least partially reacting the chromium carbide with at least one compound selected from the group consisting of chromium oxide and chromium hydroxide to form chromium.

18. The method according to claim 6, wherein the hydrocarbon is CH.sub.4.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) Further advantages and details of the invention are explained hereafter on the basis of examples and figures.

(2) FIG. 1 shows a picture of the powder morphology of chromium metal powder produced from chromium oxides by aluminothermic method

(3) FIG. 2 shows a picture of the powder morphology of chromium metal powder produced from chromium oxides by an electrolytic method

(4) FIG. 3 shows an SEM picture of Cr.sub.2O.sub.3 (pigment quality).

(5) FIGS. 4; 5a,b show SEM pictures of metal powders obtainable according to the method according to the invention.

(6) FIG. 6 shows the green strength of powder according to the invention (CP—181) in comparison to aluminothermically produced chromium powder (Cr—standard).

(7) FIG. 7 shows the relative compression density of powder according to the invention in comparison to aluminothermically (A-Cr) and electrolytically (E-Cr) produced chromium of differing purity (specification in % by weight) and powder particle size.

(8) FIG. 8 shows the time curve of the reduction of Cr.sub.2O.sub.3 to chromium at different temperatures according to the invention.

(9) FIG. 9 shows the specific surface area of various chromium powders according to the invention.

DESCRIPTION OF THE INVENTION

Example 1

(10) 500 g Cr.sub.2O.sub.3 in pigment quality (Lanxess Bayoxide CGN-R) having a mean particle size d.sub.50 of 0.9 μm measured by means of laser diffraction (powder morphology see FIG. 3) was heated in H.sub.2 (75 vol. %)-CH.sub.4 (25 vol. %) (flow rate 150 l/h, pressure approximately 1 bar) in 80 min. to 800° C. In the further procedure, the reaction mixture was slowly heated to 1200° C., wherein the reaction mixture was in the temperature range from 800 to 1200° C. for 325 minutes. The reaction mixture was then heated in 20 minutes to T.sub.R with T.sub.R=1400° C. The holding time at 1400° C. was 180 min. Heating from 1200° C. to T.sub.R and holding at T.sub.R were performed with supply of dry hydrogen with a dew point <−40° C., wherein the pressure was approximately 1 bar. The furnace cooling was also performed under H.sub.2 with a dew point <−40° C. A metallic sponge was obtained, which could be deagglomerated very easily to form a powder. The chromium metal powder thus produced is shown in FIG. 4. The degree of reduction was >99.0%, the carbon content was 80 μg/g, and the oxygen content was 1020 μg/g. An x-ray diffraction analysis only delivered peaks for body centred cubic (BCC) chromium metal. The specific surface area was determined by means of the BET method (according to ISO 9277:1995, measurement range: 0.01-300 m.sup.2/g; device: Gemini II 2370, heating temperature: 130° C., heating time: 2 hours; adsorptive: nitrogen, volumetric analysis via five-point determination) and was 0.14 m.sup.2/g, the bulk density was 1.2 g/cm.sup.3. The nanohardness .sub.HIT 0.005/5/1/5 was determined according to EN ISO 14577-1 and was 3 GPa. The green strength was determined according to ASTM B 312-09. As a compression additive, 0.6 Ma % LICOWAX® Micropowder PM (supplier Clariant, product number 107075, CAS—No. 00110-30-5) was used. At a compression pressure of 550 MPa, the green strength was 23.8 MPa, at 450 MPa 18.1 MPa, at 300 MPa 8.5 MPa, at 250 MPa 7.2 MPa, and at 150 MPa 3.0 MPa.

Example 2

(11) Cr.sub.2O.sub.3 in pigment quality (Lanxess Bayoxide CGN-R) having a mean particle size d.sub.50 of 0.9 μm measured by means of laser diffraction was well mixed with amorphous carbon black (Thermax ultra-pure N908—Cancarb). The carbon content of the mixture thus produced was 0.99 mol/mol oxygen in Cr.sub.2O.sub.3. 12500 g of this mixture was heated in 80 minutes to 800° C. and then in 125 minutes to 1050° C. The heating was performed under the action of H.sub.2, wherein the H.sub.2 pressure was set so that in the temperature range of 800° C. to 1050° C., the CH.sub.4 partial pressure measured by mass spectrometry was >15 mbar. The total pressure was 1.1 bar in this case. The reaction mixture was then heated in 20 min. to T.sub.R with T.sub.R=1200° C. The holding time at 1200° C. was 540 min. Heating from 1000° C. to T.sub.R and holding at T.sub.R were performed with supply of dry hydrogen with a dew point <−40° C., wherein the pressure was approximately 1 bar. The furnace cooling was also performed under H.sub.2 with a dew point <−40° C. A metallic sponge was obtained, which could be deagglomerated very easily to form a powder. The chromium metal powder thus produced is shown in FIGS. 5a, b. The carbon content and oxygen content are shown in Table 1. The x-ray diffraction analysis only delivered peaks for body centred cubic (BCC) chromium metal. The green strength was determined according to ASTM B 312-09. As a compression additive, 0.6 Ma % LICOWAX® Micropowder PM (supplier Clariant, product number 107075, CAS—No. 00110-30-5) was used. In this case, 550 MPa, 450 MPa, 350 MPa, 250 MPa, and 150 MPa were applied as compression pressures. FIG. 6 shows the measured green strength values in comparison to samples which were compressed using aluminothermically produced powder (Cr-standard). The powder according to the invention (CP181) displayed a green strength at least five times higher in this case.

(12) The powder batch (with 0.6 Ma % LICOWAX® Micropowder PM compression additive) was furthermore compressed at various pressures to form pill-shaped samples. In FIG. 7, the relative compression densities are shown as a function of the compression pressure in comparison to standard chromium metal powder (E-Cr: electrolytically produced; A-Cr: aluminothermically produced) with different particle sizes.

(13) Furthermore, the specific surface area was determined according to BET (ISO 9277:1995, measurement range: 0.01-300 m.sup.2/g; device: Gemini II 2370, heating temperature: 130° C., heating time: 2 hours; adsorptive: nitrogen, volumetric analysis via five-point determination) and the nanohardness .sub.HIT 0.005/5/1/5 was determined according to EN ISO 14577-1. These features are listed in Table 1 and compared to the properties of chromium powder produced electrolytically. The significantly lower nanohardness of the powder according to the invention is noteworthy. The particle size calculated from the BET surface area was 8.3 μm.

(14) TABLE-US-00001 TABLE 1 Properties of chromium powder according to the invention in comparison to electrolytically produced chromium powder BET surface area O C Nanohardness powder type [m.sup.2/g] [μg/g] [μg/g] [GPa] Chromium powder 0.10 1064 114 2.92 according to the invention (example 2) Electrolytically produced 0.11 736 87 5.32 chromium powder, particle size <45 μm

Example 3

(15) In each case 20 g of a mixture according to example 2 was heated in a molybdenum crucible in 80 min. to 800° C. and then in 125 min. to 1050° C. The heating was performed under the action of H.sub.2, wherein the H.sub.2 was set so that in the temperature range of 800° C. to 1050° C., the CH.sub.4 partial pressure measured by mass spectrometry was >15 mbar. The total pressure was 1.1 bar in this case. The reaction mixture was then heated at a heating speed of 10 K/min to T.sub.R. In this case, 1150° C., 1250° C., 1300° C., 1350° C., 1400° C., 1450° C., and 1480° C. were applied as T.sub.R. The holding times at T.sub.R were 30 min, 60 min, 90 min, 120 min, and 180 min. Heating from 1000° C. to T.sub.R and holding at T.sub.R were performed with supply of dry hydrogen with a dew point <−40° C., wherein the pressure was approximately 1 bar. The furnace cooling was also performed under H.sub.2 with a dew point <−40° C. The degree of reduction was determined as described in the description. As is apparent from FIG. 8, an advantageous degree of reduction of >95% at 1400° C., 1450° C., and 1480° C. was already significantly exceeded at a holding time of 30 minutes. At 1350° C. it required approximately 80 min. for this purpose, at 1300° C. approximately 160 min. At 1250° C. and 1150° C. it required approximately 260 minutes and 350 minutes, respectively, for this purpose (extrapolated values). SEM studies showed that the powders thus produced have a sponge-like morphology in conjunction with a very high BET surface area (see FIG. 9).