Cerumics hl/ernathmal 18 (1992) 301-305

Growth of Magnesia Whiskers

Akira Yamaguchi & Shinobu Hashimoto

Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan

(Received 26 August 1991; accepted 11 December 1991)

Abstract: Magnesia whiskers were grown by heating a mixed powder compact composed of AIMg alloy, carbon, AI203 and MgO in a furnace in which the partial pressure of oxygen was controlled by CO(g) and CO2(g), and a mechanism of whisker growth was discussed from the point of view of chemical thermodynamics.

The internal pressure, PM~, of the compact was maintained high by Mg(l) and ml4Ca(s ). On the other hand, the outside of the compact was maintained at a lower PM~ by CO(g) and CO2(g ). As a result, excess Mg(g) diffusing from the inside condensed near the surface of the compact as whiskers.

The whiskers obtained were several to 40 pm in diameter and several to 20 mm in length.

1 INTRODUCTION

Magnesia whiskers were grown by vapour phase reactions between MgO and tungsten, hydrogen, halide and carbon. 1-3

In this investigation, magnesia whiskers were grown by heating mixed powder compacts com- posed of A1Mg alloy, carbon, A120 3 and MgO in a furnace in which the partial pressure of oxygen was controlled by CO(g) and CO2(g), and the mechanism of whisker growth was discussed from the point of view of chemical thermodynamics.

500°C. Above this temperature CO(g) was flowing at 200ml/min and, at the same time, CO2(g ) was injected intermittently, as shown in Fig. 1. CO2(g) was first passed into oil at 10-60 bubbles/min through a glass tube 5 mm in diameter. Each bubble contained 0-14 ml of CO2(g), which was added near the surface of the compact through a hole in an alumina pipe set in the alumina tube. After heating at 1500°C for 2h, the compact was cooled at 600°C/h.

Whiskers formed near the surface were investi- gated by SEM and X-ray analysis.

2 EXPERIMENTAL PROCEDURE

The A1Mg alloy used in this study contained equivalent amounts of AI and Mg, was identified as the 7-phase and under 350 mesh in size. Graphite, MgO and A120 3 were reagent grade powders. The alloy and these powders were mixed in various ratios. The mixed powders were pressed uniaxially at 78.5 MPa to form compacts of 20 x 20 × 10 mm. The compact was placed on an alumina boat which was inserted into an alumina tube set in an electric furnace. The compact was heated at 600°C/min up to 1500°C. Argon gas was flowing into the tube up to

~ ( A ) (D) (E) (F)

Fig. 1. Apparatus for whisker growth: (A) glass pipe, 5 mm; (B) machine oil; (C) silicon rubber hose; (D) alumina pipe, 5 mm; (E) furnace; (F) silicon rubber stopper; (G) alumina tube, 34 mm; (H) alumina boat; (I) compact (sample); (J) gas burner; (K) flow

meter. 301

Ceramics International 0272-8842/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

302 Akira Yamaguchi, Shinobu Hashimoto

Fig. 2. Magnesia whiskers grown on the compact after heating.

3 RESULTS AND DISCUSSION

Figure 2 shows the whiskers formed near the surface of the compact, composed of 36wt% A1Mg alloy, 51wt% A1203, 4wt% MgO and 10wt% C. The whiskers were several to 40#m in diameter and several to 20 mm in length, and were MgO crystals

which grew along the (200) face. Table 1 shows the degree of the whisker production for the amount of CO2(g) passed in. As the amount of CO2(g) was 20 bubbles/min, namely 2.8 cm3/min, the whiskers were abundantly formed. When CO2(g) was not passed in and passed in at 60 bubbles/min, no whiskers were formed.

Under the coexistence of carbon and oxygen, the main gaseous species are CO(g), CO2(g ) and O2(g), which are in equilibrium according to the following equations:

2C(s) + O2(g ) = 2CO(g)

C(s) + O2(g) = CO2(g)

2CO(g) + O2(g ) = 2CO2(g)

(1) (2) (3)

Accordingly, since Pco., Pco, and Po2 are related, we discuss under the basis Of Pco.-

Under the coexistence of carbon, oxygen and magnesium, there are Mg(g), Mg2(g) and MgO(g) as gaseous species. Figure 3 shows the equilibrium partial pressures of these gas species for logpco under the existence of Mg(1) and/or MgO(s) with C(s) at 1027°C according to the following equations:

Mg(l) = Mg(g) (4)

MgO(s) + C(s) = Mg(g) + CO(g) (5)

2Mg(1) = Mg2(g) (6)

2MgO(s) + 2C(s) = Mg2(g) + 2CO(g) (7)

Mg(1) + CO(g) = MgO(g) + C(s) (8)

MgO(s) = MgO(g) (9)

Equilibrium constants for these equations are obtained on the basis of JANAF thermodynamic data 4 listed in Table 2.

The equilibrium partial pressure of Mg(g) is the highest among the partial pressure of the gaseous species of the system Mg-O. Accordingly, the mechanism of whisker growth was discussed on the basis of the behaviour of Mg(g).

Since CO(g) flows into the tube under atmospheric pressure in this experiment, I atm pressure in the tube is considered to be 1 atm of CO(g) when CO2(g ) is not passed in. Mg(1) changes to MgO(s), because MgO(s) is a stable condensed phase under this condition, as shown in Fig. 3. As far as Mg(1) exists, however, Pco is maintained at the value at which Mg(1) and MgO(s) coexist by the next reaction, and thenps~ in the compact is considered to be a value of the point A:

Mg(1) + CO(g) = MgO(s) + C(s) (10)

Growth of magnesia whiskers 303

Table 1. State near the surface of the compact heated in an atmosphere of CQ(g) mixed intermit tent ly wi th CO=(g)

Amount of CO=(g) used

None

6-12 bubbles/min (0.8-1.7 cm 3)

20 bubbles/min (2.8 cm 3)

60 bubbles/min (8.4cm 3)

Magnesia whiskers were not generated Colour of the surface of the compact is black

A few magnesia whiskers were generated

Many magnesia whiskers were grown

Magnesia whiskers were not generated Colour of the surface of the compact is white

Starting compact is composed of 36wt% AIMg, 51 wt% AlaO 3, 4wt% MgO and 9wt% C.

1 og Poz(a t m) -35 -30 -25 -20 -15

i I I , I C(s)

~ : MgO(s) -

0 t (I027°C)

~ - 1 5 -

o. A I C(9)

-25

i

I I I i

-10 -5 0 IogPco(atm)

Fig. 3. Stable condensed species and equilibrium partial pressure of gas species in the system AI-Mg-C-O for logPco or

logpo ~ at 1027°C.

The curve 'a' in Fig. 4 shows the Pug corresponding to the point A in Fig. 3 for temperature. The partial pressure of the Mg(g) in the surface of the compact is assumed to be almost the same as that in the interior.

On the other hand, Pco outside of the compact is 1 atm, and then the equilibrium partial pressure of

Mg(g) is the value at point C shown in Fig. 3 and considerably lower than that inside the compact.

As a result, Mg(g) diffusing from the inside of the compact condenses as MgO(s) near the surface according to the following equation:

Mg(g) + CO(g) = MgO(s) + C(s) (11)

However, in an atmosphere of CO(g) only, whiskers were not formed, and the colour of the surface of the compact became black. These facts suggest that carbon precipitated according to eqn (11) and checked the growth of MgO whiskers.

When CO 2 gas was intermittently passed in, carbon did not precipitate. It is considered that CO2(g ) reac t s with carbon precipitated by reaction (11) to form CO(g) as follows:

C(s) + CO2(g ) = 2CO(g) (12)

As a result, the following equation is obtained from eqns (11) and (12), and then whiskers seem to grow:

Mg(g) + CO2(g) = MgO(s) + CO(g) (13)

The equilibrium constant, Kp, of this equation is expressed as follows:

Kp - aMgoPco PMgPco2

Assumingpco2 is almost the same aSpco as CO2(g) was used, and aMg O = 1,

Kp = 1/pMg viz. logpMg =-- logKp

Table 2. Thermochemical data 4 at 1300 K (1027°C)

Species log Kp Species log Kp Species log Kp

O=(g ) 0.000 Mga(g ) -3.494 Al,C3(s) 5.687 CO(g) 9.099 MgO(g) 1.474 Al=O3(s) 50.712 CO=(g) 15.920 Mg(s) -0.144 Al(g) -6.408 C(s) 0.000 MgO(s) 18.401 AI20 (g) 9.508 Mg(g) -0.291 Al(s) -0.159 Ale(g) -18.244

304 Akira Yamaguchi, Shinobu Hasttimoto

Temperature (°C) 700 900 1100 1300 1500

= i I =

I

o_

c / o~ 10 ¢~"" / / I / ~

~.Z //" "D (:a j O

- #

/ 20 .,,fit

o l /,,.

I | I I I I

a 0 0 100o 1200 I~00 1600 ls0o

Temperature(K)

Fig. 4. Equilibrium partial pressure of Mg(g) under various conditions.

TABLE 2

Curve Coexisting condensed

species

Equation for calculation ofpM =

a Mg(l), C(s), MgO(s), AI4Ca(s), A1203(s)

b C(s), MgO(s), AI4C3(s), A1203(s)

c C(s), MgO(s), AI203(s)

d MgO(s)

Mg(l) = Mg(g)

A14C3(s ) + 6MgO(s) = 2AI203(s ) + 6Mg(g) + 3C(s)

MgO(s) + C(s) = Mg(g) + CO(g) Pco = 1 atm

Mg(g) + CO2(g) = MgO(s) + CO(g) Pco = Pco2

The PM, obtained is shown as curve 'd' in Fig. 4 and lower than that in the case of pc o = 1 atm. That is, the amount of MgO(s) condensed from Mg(g) increases.

On the other hand, when the amount of CO2(g) was in excess no whiskers were formed. It seems that the condensation of Mg(g) as MgO(s) occurs inside the compact.

If AI(I) or ml4C3(s) does not coexist as Mg metal has become extinct, PM~ decreases and takes a value obtained from the following equation:

MgO(s) + C(s) = Mg(g) + CO(g) (14)

As Pco = 1 atm, PM= is shown at point C in Fig. 3 and in curve 'c' in Fig. 4. It is considerably lower than that coexisting under Mg(1).

Fig. 5. SEM photographs of magnesia whisker with a lump on its tip.

However, when A1 coexists with MgO(s) and carbon, PM, increases. A1 reacts with carbon to form A14C 3, which reduces Pco. Stable condensed species and the partial pressures of the main gaseous species in the system A1-C-O are shown for log Pco in Fig. 3.

As far as A14C 3 exists, Pco is maintained at point B, and then PMg is higher than that at point C. PMg corresponding to the point B for various tempera- tures is shown as curve 'b' in Fig. 4.

A fundamental mechanism of whisker growth in this experiment is that Mg(g) diffusing outside from the compact condenses near the surface as MgO(s) whiskers. As shown in Fig. 2, there were two kinds of whisker with and without a lump on its tip, as shown in Fig. 5. The whisker with a lump is considered to grow by the VLS mechanism. On the other hand, a whisker without a lump seems to grow by the VL mechanism.

Growth of magnesia whiskers

4 CONCLUSIONS

Magnesia whiskers were obtained by heating a mixed powder compact composed of A1Mg alloy, carbon, A120 3 and MgO in an atmosphere con- trolled by CO(g) and CO2(g).

(1) The internal pressure of the compact is maintained at a high PMg by the existence of Mg(1) and A14C3(s ).

(2) Outside of the compact is maintained at a lower PM, by CO(g) and CO2(g ).

(3) Excess Mg(g) diffusing f rom the inside condenses near the surface of the compact as MgO whiskers.

305

(4) The MgO whiskers obtained were several to 40 #m in diameter and several to 20 mm in length.

REFERENCES

1. WOLFF, E. G. & COSKREN, T. D., Growth and morphol- ogy of magnesium oxide whisker. J. Am. Ceram. Sot., 48(6) (1965) 279-85.

2. HAYASHI, S. & SAITO, H., Growth of magnesia whiskers by vapor phase reactions. J. Crystal Growth, 24/25 (1974) 345-9.

3. BUDNIKOV, P. P. & SANDULOV, D. B., Magnesium oxide single crystal whiskers. Kristall und Technik, 2(4) (1967) 549-53.

4. JANAF Thermochemical Data, 2nd edn. National Standard Reference Data System, US National Bureau of Standards, June 1971.