The preparation of chromium powders by the thermal decomposition of chromium hydride

Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

461

Short Communications The preparation decomposition

of chromium of chromium

powders by the thermal hydride

In reviewing the methods of production of chromium powders, SULLY ANI) point out that conventional mechanical methods of comminution of

RRANDES~,

chromium powders (e.g. ball milling and crushing) can lead to high impurity pick-up during the lengthy processing time involved, especially when fine particle sizes (I-IO ,u) are produced. To overcome this an attempt was made to produce chromium powders in this range by grinding decomposing

chromium

hydride

for short times and thermally

the ground hydride in vacuum to produce chromium

The chromium

hydride was produced

electrolytically

powders.

by a method

similar to

that used by KN~~DLER”, using a chromium trioxide/sulphuric acid bath. Periodic observation of the deposit showed that the bulk of the deposit formed as nodules on an initial “skin” and that the nodules were particularly abundant in cathodic areas well displaced from the surface of the bath. A typical deposit is shown in Fig. I. A value of 6.092 g/cm” obtained for the pyknometric density of the hydride is in fair agreement

with the X-ray

value

of 6.123

g/cm3 obtained

Fig. I. Deposit of chromium hydride on the copper cathode. ( x - 3.5)

by KN~~DLER~:

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462

the difference of these two values may be attributed to the presence of 0.7 wt.% oxygen in the hydride powder. X-ray analysis of the deposit confirmed that the lattice of the hydride is hexagonal, as reported by GOLDSCHMIDT~. The hydride nodules were ground to powder in a tungsten carbide pestle and mortar: the grinding operation was found to be relatively easy due to the extreme brittleness of the chromium hydride. The hydride was then decomposed in a vacuum of 1.6 x IO-~ torr at various temperatures. The decomposition product was identified as metallic chromium by X-ray powder photographs and the particle sizes of the chromium and the chromium hydride were determined by a sedimentation technique (the Bound Brook Photosedimentometer), see Table I. TABLE

I

PARTICLE

SIZE

OF

Electrolysis run

CHROMIUM

AND

Hydride size [p)

particle

CHROMIUM

POWDERS

Decomposition temperature (“C) 300 70 90 60

17.0

4.5 4.5 3.0 3.0 3.0 8.75 8.75 8.75 3.5 3.5

The particle on the temperature

HYDRIDE

90 115 60 90 115 90 115

Chromium

particle

size (p) 10.5 3.25 3.35 2.25 2.5 2.7 5.6 6.25 6.875 2.9 3.2

size of the chromium powders produced was found to depend both of decomposition (Fig. 2) and the initial particle size of the hydride

(Fig. 3). Electron micrographs of one batch of chromium powder were prepared to check the particle size determinations (Fig. 4). These indicated that the particle size

i

I

;

I

0x .0 run P””

6 10 11 12

!

4 501

01234567 Chromium

particle

size

Fig. 2. Effect of decomposition

123456769 Hydride

(I_L)

temperature

particle

diameter

$11

on the particle size of chromium powder.

Fig. 3. Effect of hydride particle diameter on the size of chromium powders produced temperatures. J. Less-Common

Metals,

16 (1968)

461-464

at various

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403

COMMUNICATIONS

Fig. 4. Electron

micrograph

of chromium

powder

(Run

10,6o”C).

(x 0000)

was finer than that derived from the Photosedimentometer value is an aggregate

size. The surface

was determined by a modified B.E.T. and found to be 2.19 mz/g, equivalent

data, and that the latter

area of one batch

of powder (Run

12,

90°C)

technique using krypton gas as the absorbent to a particle diameter of 0.38 ,u.

It was anticipated that an increase in the impurity content would occur during the handling of the hydride after removal from the bath; chemical analysis of the powders before and after decomposition TABLE CHEMICAL

Run

6 IO IO

I

confirms

this (Table II).

II ANALYSIS

OF

Decomposition wi 90 90 I’5

Undecomposed

* No determinations

POWDERS

PRODUCED

telnp.

hydride

Oxygen (wt.%)

Nitrogen (wt.“&)

Sulphur (wt.%)

Carbon (wt.%)

I.00 I.33 I.31

0.0‘23 0.031 0.038

0.0020 -

0.05 _* _*

0.71

0.021

0.0037

0.0007

of S or C were carried

out on these samples.

It will be observed that although the experiments yielded chromium powders of the desired grain size, these were contaminated by oxygen, and to a lesser extent also by nitrogen, impurities.

sulphur

and carbon.

Further

work is required

to eliminate

these

The high oxygen content of the hydride may be attributed to the low temperature of deposition, since the oxygen content of chromium electrodeposits from chromic acid baths increases as the deposition temperature decreases”. The nitrogen in the hydride and in the chromium powders is almost certainly a pick-up from the plating bath, in which case the nitrogen

content

of the deposit could probably J. Less-Cowman

be reduced by

Metals, 16 (1968) 461-464

464

SHORTCOMMUNICATIONS

eliminating the nitrate ion from the plating bathe. The sulphur content in the powders also originates from the sulphate in the plating bath. The carbon in the hydride is low but that in the chromium is significantly higher and this is probably due to carbon pick up during grinding with tungsten carbide. Department

M. J. WOOLLEY T. J. DAVIES

of Metallwgy,

University of Manchester Institute of Science and Technology, Martchester I 2 3 4 5 6

(Gt. Britain)

A. H. SULLY AND E. A. BRANDES, Chromium, Butterworths, London, 1967, p. 150. A. KNGDLER, Metalloberflache, 17 (1963) 161. A. KNBDLER, Metalloberflache. 17 (1963) 331. H. I. GOLDSCHMIDT, InterstitiaZ Alloys, Butterworths, London, 1967, p. 458. A. BRENNER, P. BURKHEAD AND C. W. JENNINGS, J. Res. Natl. Bur. Std., 40 (1948) N. RYAN AND E. J. LUMLEY, J. Electrochem. Sot., ro6 (1959) 388.

Received April 4th, 1968; revised July qth, J. Less-Common

Metals,

16 (1968)

461-464

1968

31