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Reversal of the densification process in oxides and metals
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Reversal of the densification process in oxides and metals
Normally, when a mass of compacted powder shrinks in the solid-state sintering process, the shrinkage and bulk density increase continuously to a limiting value. The magnitude of the change (i.e. the extent to which porosity is reduced) will obviously depend on time, temperature and the nature of the system, but the direction of change will remain the same. Exceptionally, systems are encountered in which the density increases to a maximum and then decreases; the decrease may be either permanent or may be followed by a second stage of densification. Recent recorded instances of the phenomenon suggest that expansion of trapped gas, vapour or liquid is a dominant cause of such densification reversal. For example, Mansour and White 1, investigating the densification of UO2, invoke a mechanism suggested by Eudier 2 involving transfer of gas by diffusion from highpressure small to low-pressure large pores with consequent expansion of the compact as a whole. Amato and Colombo 3'4 state that reversal in UO2 is more severe with high green density and high soaking temperature, since these conditions are conducive to the trapping of gases evolved from the necessary organic additions. Reversal such as that reported by Rogers 5 could be due to a vapourisable impurity, analogou s to sodium chloride in titanium 6. Alternatively, a liquid phase may have been present, leading to rapid surface sintering, gas entrapment and a lower final density 6. Reversal has been observed in (a) Cr2Oa-Cr mixtures 7 and (b) Cr203-Si mixtures a. In (a) impurity (unspecified) in the chromium powder was responsible, which probably reacted with the oxide matrix, releasing chromium vapour; in (b) it appeared that reversal was linked to the onset of liquation 9' lo, 11. Compacts of laboratory-reagent grade ferric oxide sintered
to 1200°C in oxygen are sound and of high density, but if the same samples are re-heated to 1400°C, severe cracks develop lz. The re-heating increases the grain size markedly and this, in conjunction with the dissociation of ferric oxide at high temperatures13,14,15, would seem to favour an explanation based on isolation of closed pores and subsequent bursting. In all these examples the essential condition for reversal is the prior formation of a consolidated structure upon which subsequent expansions may operate. This differentiates reversal due to liquid expansion from normal liquid-phase sintering. H. E. N. STONE
University of Surrey, London (Gt. Britain) REFERENCES 1 N.A.L. MANSOURANDJ. WHrrE, Powder Met., 12 (1963) 108. 2 M. EUDmR, Syrup. Powder Met. 1954, (Sp. Rep. 58), Iron & Steel Inst., London, 1956, p. 59. 3 I. AMATO AND R. L. COLOMBO, Powder Met., 7 (14) (1964) 327. 4 I. AMATO, J. Am. Ceram. Soc., 48 (1965) 53. 5 S. E. ROGERS, Powder Met., 12 (1963) 122. 6 S. E. ROGERS, Powder Met., 7 (1961) 249. 7 H. E. N. STONE AND N. A. LOCKINGTOr~, Powder Met., 8 (15) (1965) 81. 8 H. E. N. STONE, Ph.D. Thesis, London, 1966. 9 Y. I. OL'SHANSKIIAND V. K. SHLEPOV,Doklady Akad. Nauk S.S.S.R., 91 (1953) 563. l0 Phase Diagrams for Ceramists, Am. Ceram. Soc., Columbus, Ohio, 1964, p. 130. 11 N. S. KURNAKOV, Compt. Rend. (Doklady) Acad. Sci. U.S.S.R., 34 (1942) ll0. 12 H. E. N. STONE, unpublished work. 13 J. WHITE, R. GRAHAMAND R. HAY, J. Iron Steel Inst., 131 (1935) 91. 14 J. WHITE, I.S.L Carnegie Schol. Memoirs, 27 (1938) 1. 15 O. N. SALMON,J. Phys. Chem., 65 (1961) 550.
Received December 16, 1966
Materials Science and Enoineerino - Elsevier Publishing Company, Amsterdam - Printed in the Netherlands
Reversal of the densification process in oxides and metals
Normally, when a mass of compacted powder shrinks in the solid-state sintering process, the shrinkage and bulk density increase continuously to a limiting value. The magnitude of the change (i.e. the extent to which porosity is reduced) will obviously depend on time, temperature and the nature of the system, but the direction of change will remain the same. Exceptionally, systems are encountered in which the density increases to a maximum and then decreases; the decrease may be either permanent or may be followed by a second stage of densification. Recent recorded instances of the phenomenon suggest that expansion of trapped gas, vapour or liquid is a dominant cause of such densification reversal. For example, Mansour and White 1, investigating the densification of UO2, invoke a mechanism suggested by Eudier 2 involving transfer of gas by diffusion from highpressure small to low-pressure large pores with consequent expansion of the compact as a whole. Amato and Colombo 3'4 state that reversal in UO2 is more severe with high green density and high soaking temperature, since these conditions are conducive to the trapping of gases evolved from the necessary organic additions. Reversal such as that reported by Rogers 5 could be due to a vapourisable impurity, analogou s to sodium chloride in titanium 6. Alternatively, a liquid phase may have been present, leading to rapid surface sintering, gas entrapment and a lower final density 6. Reversal has been observed in (a) Cr2Oa-Cr mixtures 7 and (b) Cr203-Si mixtures a. In (a) impurity (unspecified) in the chromium powder was responsible, which probably reacted with the oxide matrix, releasing chromium vapour; in (b) it appeared that reversal was linked to the onset of liquation 9' lo, 11. Compacts of laboratory-reagent grade ferric oxide sintered
to 1200°C in oxygen are sound and of high density, but if the same samples are re-heated to 1400°C, severe cracks develop lz. The re-heating increases the grain size markedly and this, in conjunction with the dissociation of ferric oxide at high temperatures13,14,15, would seem to favour an explanation based on isolation of closed pores and subsequent bursting. In all these examples the essential condition for reversal is the prior formation of a consolidated structure upon which subsequent expansions may operate. This differentiates reversal due to liquid expansion from normal liquid-phase sintering. H. E. N. STONE
University of Surrey, London (Gt. Britain) REFERENCES 1 N.A.L. MANSOURANDJ. WHrrE, Powder Met., 12 (1963) 108. 2 M. EUDmR, Syrup. Powder Met. 1954, (Sp. Rep. 58), Iron & Steel Inst., London, 1956, p. 59. 3 I. AMATO AND R. L. COLOMBO, Powder Met., 7 (14) (1964) 327. 4 I. AMATO, J. Am. Ceram. Soc., 48 (1965) 53. 5 S. E. ROGERS, Powder Met., 12 (1963) 122. 6 S. E. ROGERS, Powder Met., 7 (1961) 249. 7 H. E. N. STONE AND N. A. LOCKINGTOr~, Powder Met., 8 (15) (1965) 81. 8 H. E. N. STONE, Ph.D. Thesis, London, 1966. 9 Y. I. OL'SHANSKIIAND V. K. SHLEPOV,Doklady Akad. Nauk S.S.S.R., 91 (1953) 563. l0 Phase Diagrams for Ceramists, Am. Ceram. Soc., Columbus, Ohio, 1964, p. 130. 11 N. S. KURNAKOV, Compt. Rend. (Doklady) Acad. Sci. U.S.S.R., 34 (1942) ll0. 12 H. E. N. STONE, unpublished work. 13 J. WHITE, R. GRAHAMAND R. HAY, J. Iron Steel Inst., 131 (1935) 91. 14 J. WHITE, I.S.L Carnegie Schol. Memoirs, 27 (1938) 1. 15 O. N. SALMON,J. Phys. Chem., 65 (1961) 550.
Received December 16, 1966
Materials Science and Enoineerino - Elsevier Publishing Company, Amsterdam - Printed in the Netherlands