An experimental investigation of heat transfer in thermally unstable polymer systems

Abstract

An experimental study is made into the process of heat transfer from a pulse-superheated probe to a solidified polymer system at probe temperatures above the temperature T d∞ of the beginning of thermal destruction of matter in a quasi-static process. Use is made of the procedures of thermal stabilization of the pulse-superheated probe (with the characteristic time of constancy of superheated probe temperature of 1 ms) and of shock heating (with the characteristic time of increasing the probe temperature of 1 μs and that of monitoring its cooling-down of 1 ms). The effect of short-term thermal stability of polymers in the region of T > T d∞ is revealed. A procedure is developed for identifying the signs of thermal destruction of polymers in the pulsed process. The maximal values are estimated of the density of heat flux through samples without their thermal destruction.

Heat transfer under high-power heating of liquids. 3. Threshold decrease of heat conduction in supercritical region

“…According to the specificity of heat exchange of ''isothermal'' probe with an immovable medium [13], the power values P(t) along a portion of temperature plateau are monotonically decreasing. The process of spontaneous boiling-up at a certain time t > t 1 at subcritical pressures is physically reasonable exception [13,14]. Obviously, there is no reason to expect that the signs of convection onset will be as confined in time as signs of boiling-up.…”

“…Upon reaching the chosen temperature T pl at a certain timing t 1 , the heating power P(t) varies so that the probe temperature is maintained constant, T(t P t 1 ) % T pl , t 1 $ 0.1 ms, from that time until the end of the pulse. The prospect of technique is that the T pl value can be selected (and can be changed with an arbitrary step in a series of measurements) in the temperature range that meets the conditions of the tasks, including the region of thermally unstable and superheated states of a liquid [1,[13][14][15][16]. In accordance with the first aim of our study, the selection of T pl values was made in the areas of both below and above the isopropanol critical temperature T c and was coordinated with the parameters of basic experiment [3], see Fig.…”

“…[13,14]), samples with a higher value of thermal effusivity require a higher level of power P(t), other variables being equal. In experiments with liquids, increasing pressure at a given T pl value is accompanied by increase in P(t) in accordance with the character of the dependence of thermal effusivity (k Á q Á c p ) 0.5 on pressure [19].…”

“…For this purpose, it is important to resolve the appearance of characteristic signs caused by free convection of a substance near the probe surface. In this regard, we have chosen the technique of thermostabilization of the pulse-heated probe at a given temperature value T pl > T 0 , or, in short, the technique of temperature plateau, see [13,14] references therein. This approach allows for changing the characteristic duration of experiment by orders of magnitude under strictly specified probe temperature, which is equivalent to a ''zero'' rate of heating at the temperature plateau time period.…”

Volatile products formed during electrical breakdown of polyethylene terephthalate and polypropylene films in high vacuum

“…This will make it possible to draw conclusions regarding the phase composition of the material ejected from the polymer upon breakdown. Additionally, it is expedient to obtain mass-spectra of the thermodestruction of polymer films and compare them with breakdown spectra at a constant electron energy in the ion source, since under conditions of ultrahigh exposure speed the physical and mechanical properties of polymers can change [7,8]. On the basis of the data obtained and with the involvement of earlier results propose a picture of the development of the process of the electrical breakdown of the polymer and support it with calculations of the accumulation of charge…”