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DD10.6.1

Negative Thermal Expansion Arthur W. Sleight* Department of Chemistry Oregon State University Corvallis, OR 97331-4003 ABSTRACT Negative thermal expansion behavior has been found in many oxides where oxygen or a cation has a coordination number of two. The MO2, AM2O7, A2M3O12, AMO5, and AO3 families, where A is an octahedral cation, M a tetrahedral cation, and the oxygen coordination is two, have been investigated for their thermal expansion properties. Negative thermal expansion has been found in all families except the AO3 family, where very low thermal expansion was found in the case of TaO2F. Open networks are necessary to allow free transverse thermal motion of oxygen, which is the apparent cause negative thermal expansion in these families. This openness leads to two problems. One is that structure collapse transitions tend to occur as the temperature is lowered. There is little or no thermal expansion below this transition. A solution to this problem is to maintain sufficient ionic character in the bonds holding the network together. The other problem is that when the networks become sufficiently open, they tend to hydrate. This hydration destroys the negative thermal expansion of the network. ––––––––––––––––––––– *[email protected] INTRODUCTION Prior to 1996 there were no examples of strong, three-dimensional negative thermal expansion (NTE) over extended temperature ranges that included room temperature. The discovery [1] of this behavior in ZrW2O8 led to the development of qualitative rules for NTE behavior. These rules have in turn led to the discovery of NTE behavior in other oxides (Table I). The most studied of NTE materials is ZrW2O8, but similar and even stronger NTE behavior has been found in other oxides [2-17]. This paper presents a summary of our research on NTE materials, including some previously unpublished results.

MECHANISMS FOR NEGATIVE THERMAL EXPANSION More than one mechanism can give NTE behavior. Increasing symmetry with increasing temperature is one mechanism for NTE, but there are few good examples of this. The best example is PbTiO3 where NTE behavior is observed in the tetragonal form. The a and b cell edges always show positive thermal expansion, but the c cell edge shows strong NTE behavior giving an overall negative value for the average linear thermal expansion [18]. However, once PbTiO3 becomes cubic at 465 K it exhibits normal positive thermal expansion at higher temperatures.

DD10.6.2

Table I. Oxides Showing Negative Thermal Expansion at Room Temperature Compound SiO2(chabazite) SiO2(ITQ-4) SiO2(faujasite) AlPO2-17 Sc2W3O12 Lu2W3O12 Y2W3O12 Sc2Mo3O12 Zr2WP2O12 TaVO5 ZrW2O8 ZrMo2O8 Ag2O

α(× 10-6/K) -0.5 -3.0 -4.2 -11.0 -2.2 -6.8 -7.0 -1.7 -3.0 -5.1 -8.7 -5.0 -6.7

references 2 2 3 4 5,6 7 8 5,9 5,10 11 1,12 13 14

The mechanism for NTE that now has many examples is based on transverse thermal motion of O in M-O-M linkages or of M in O-M-O linkages. The transverse motion of oxygen in the M-O-M linkage pulls the metal atoms together as temperature increases