A study is made of the viscoelastic behavior of the polyimide DFO and polybenzoxasol (PBZ) Some relatively new polymers --polyimides and polyoxymethylenes --have recently been used as matrices in composites. These polymers are stable at elevated temperatures: However, an analysis of their chemical structure shows (especially due to the presence of large low-mobility groups in the main chain) that these materials should also have a large free volume at very low temperatures as well. Thus, polyimides and polyoxymethylenes should have good service properties at both low and high temperatures. Nevertheless, there has been very little study of the viscoelastic behavior of these polymers at very low (helium) temperatures.In light of this, we measured the velocities of longitudinal and transverse ultrasonic waves at frequencies of 1 and 5 MHz in polyimide DFO and polybenzoxasol (PBZ) within the temperature range 4.2-240 K. As is known [1], the results of such measurements can be used to calculate parameters that characterize the viscoelastic behavior of polymers: dynamic elastic modulus E', shear modulus G', compressive bulk modulus Ks', Poisson's ratio and the corresponding loss moduli.The experimental unit, cryostat, iustnunentation, and method of measuring ultrasonic velocity were described in [2]. The relative error of the velocity measurement was 1%. Temperature was measured with a copper--comtantin thermocouple to within 0.2 K. Specimens of DFO and PBZ were used for the acoustic measurements. The specimens were in the form of cylinders with a diameter of 22 ram, height of 5 ram, and densities of 1.392 and 1.308 g/cm 2, respectively. The chemical strucutre of the polymers was as follows:Polyimide DFO Polybenzoxazolr /o~ c
It is well known that a nonhomogeneous state of stress occurs in compounds of dissimilar materials upon heating. Uon the assessment of the strength of the joints, it is necessary to factor in the physical specifications of the soldered elements, the geometric dimensions and temperature conditions of their operation. The purpose of the research was to perform the stress-strain state analysis of the contact zone of a semiconductor upon electrodegradation of a metallization track on its surface. Thin-film metal-semiconductor structures were researched. As substrates, it was used phosphorus-doped silicon single-crystal wafers oriented in the (111) and (100) directions, with a resistivity in the range p = 1 ... 0.01 Ω.cm, and a 30 ... 50 μm n-epitaxial layer was deposited on a part of the wafers. As a conductive metal film, aluminum 1 ... 2 μm thick was used. The test structure was formed by optical photolithography. The oscillograms of the U(t) inclusion in the process of passage of the current pulse were taken by the corresponding probes from potential sites and recorded by a digital storage oscilloscope. An estimation procedure for the semiconductor stressed region at local surface heating of a metallized surface area with electric pulse was given. The calculated size of deformed silicon substrate region was compared with the experimental one under passage of square electric pulses. It was estimated the deflected region that depends on duration and amplitude of electric pulse. A considerable nonuniformity of the metallization track after electric pulse passage was fixed experimentally.
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