Measurements of CTE: The specimens were cut off from the C/C composite samples, and machined into dimensions of 60-10-5mm, then placed in a dialatometer equipped with silica tube and push rods. The measurements were performed under a nitrogen atmosphere and at a heating rate of 2C per min. The nine temperature segments from 0 to 900C are selected. Tests are limited in the X-Y direction, and CTE is calculated by the formula α=△L/L(To)﹒ △T’ where L(To) is the length of the samples at the room temperature (25C), △L and △T are the changes of the length of the samples and the temperature, respectively, where △L=L(T)-L(To), △T=T-To. The accuracy of the measurements is ±4%.
Measurement of thermal diffusivity: The laser flash method is used to measure the thermal diffusivity of the C/C composites. The measurement system consists of laser equipment of red gem, vacuum furnace with a pressure of less than 1.3 Pa and back face temperature collecting system. Under the condition of heat insulation, the front face of the wafer-shape sample is heated by a short laser pulse and one dimension heat flow comes into being in the back face. The process is monitored by the back face collecting system to record the temperature curves through which thermal diffusivity is calculated. The uncertainty of the measurements is ±5%. The measured temperature points are ten from 25 to 900C. The X-Y and Z directions of the samples are all tested. The samples are disc-shaped with a diameter of 10mm and a thickness of 2.5mm.
Measurement of specific heat: Mixture method is used to measure the specific heat of the C/C composites. The equipment is composed of the furnace, calorimeter, heat insulation system and temperature measurement system. In the experiment, the sample is suspended by a nickel thread with a diameter of 0.1mm in the furnace and heated. After the sample temperature reaches the required value T1 and is kept stable, then it is put into heat insulation copper calorimeter with a temperature of Tc. The emitted heat lets the colorimeter temperature go up to the terminal value Tn, and by calculating the changes of temperature, the mean specific heat is obtained, and furthermore the real specific heat can also be achieved. The whole experiment process was performed under a vacuum condition and the room temperature of 25C, and the accuracy of the measurements is ±3%. The samples are column-shaped with a diameter of 14mm and a height of 15mm.
Calculation of TC: The thermal conductivity of the C/C composites is calculated by the formula:
α=c·λ·ρ
Where λ and c is thermal diffusivity and specific heat under different temperatures, andρis density of the composite at 25C.
Analysis of X-ray diffraction: The microstructure is analyzed using D/M-3C X-ray diffraction analyzer. Data of diffraction peaks (002) and (110) were obtained. The interlayer spacing and crystallite size (La, Lc) were calculated. The analytical methods have been introduced in (6). The microstructures of PyC were examined using polarized light microscope model Neophot 21.