or particles of the low expansion phase material have an aspect ratio in the range of from about 1 1 to about 4 1 preferably from about 1 1 to about 2 2 so as to create a thermal path length through the matrix from one side of the composite material to a second side of the composite material opposed to the first side it has been found that a particularly useful composite material is one having a continuous matrix of copper and a low expansion phase of molybdenum particles copper may be present in the composite in an amount from about 5 wt to about 50 wt preferably from about 10 wt to about 25 wt while molybdenum is present in the composite in an amount from about 50 wt to about 95 wt preferably from about 75 wt to about 90 wt of course the molybdenum is present in the form of discrete particles having the aforesaid aspect ratios the composite strip material is formed by first blending the low expansion phase material in powder form such as the uncoated molybdenum particles shown in fig1 with an oxide of the matrix material when the low expansion phase material is molybdenum and the matrix material is copper a high purity molybdenum powder having a particle size in the range of from about 20 u03bc to about 200 u03bc preferably from about 50 u03bc to about 150 u03bc is blended with cuprous oxide powder having a particle size in the range of from about.