2.1 INTRODUCTION

The stability of dimensions may be maintained actively or passively. In the first approach, actuators are used to restore dimensions disturbed by stress or environmental influences. The passive approach is to use materials that do not respond to these influences in the first place. The latter approach is generally the less complicated or costly and hence is preferred. Since any stress or environmental change imposed on a material will cause it to grow or shrink, the problem for an engineer or designer is to determine what the acceptable distortion limits are and to choose appropriate dimensionally stable materials. In this chapter, we review the options that an engineer has in terms of materials selection. By way of introduction, and somewhat arbitrarily, we use insensitivity to temperature change as a guide to dimensionally stable materials. One justification is that melting points, interatomic binding energies, resistance to solute absorption or diffusion and stiffness tend to increase as the dimensional sensitivity to temperature decreases. The search for a zero-expansion coefficient (CTE) is therefore likely to produce materials that are dimensionally stable in other respects as well. Unfortunately, a truly zero CTE material over a substantial temperature range does not exist, although all the materials discussed in this chapter can be modified in this direction. Low expansion may be defined as an average CTE within 1.8 ppm/K ( 1.8 10 ^?6/ C or 1 10 ^?6/ F) over a substantial temperature range, which includes room temperature, the most practical operating temperature...