This is a preliminary description of a new method of determining absolute pressure which is based on the simultaneous measurement of length and ultrasonic transit times. This method has been used to determine the transition pressure of pure mercury at 0 °C at 7571·2±1·6 bar. It is particularly advantageous to use for calibrating secondary gauges such as manganin gauges inasmuch as it gives a highly accurate measurement of nonlinearities in such secondary gauges. The gauge calibrated in the present study reaches a maximum deviation from linearity of −11·6 bar at a pressure midway between atmospheric pressure and the 0 °C mercury melting pressure.
Second and third order elastic constants of NaCI, NaBr, KF, KCI, KBr, and RbBr have been calculated using the Born model of ionic solids. Short range repulsive interactions have been included up to second nearest neighbors using an expression of the form A exp ( -r / b ) where b is the repulsive parameter. A distinction is made between the like and unlike ion interactions. Van der Wads terms are also included. When the room temperature lattice parameter is used, the elastic constants satisfy the Cauchy relations. Experimental data show that many of the alkali halides fail to satisfy the Cauchy relations. This aspect is examined by using the quasi-harmonic approximation as developed by Leibfried and his associates. Linear temperature coefficients of the elastic constants are also calculated in the high temperature limit. I n order to assess the contributions of the van der Waals terms, elastic constants are also calculated by dropping the van der Waals terms in the expressions for the elastic constants. Calculated values of the elastic constants are compared with the available experimental data.Die elastischen Konstanten zweiter und dritter Ordnung werden mit dem Bornschen Modell fur Ionenfestkorper fur NaCI, NaBr, KF, KCI, KBr und RbBr herechnet. Kurzreichende Abstohngswechselwirkung wird bis zu den zweiten nachsten Nachbarn durch einen Ausdruck der Form A exp (-r/b), mit b als Abstohngsparameter, berucksichtigt. Es wird zwischen der Wechselwirkung gleicher und ungleicher Ionen unterschieden. Van der Waals-Terme werden ebenfalls berucksichtigt. Wenn die Gitterkonstante bei Zimmertemperatur benutzt wird, erfullen die elastischen Konstanten die Cauchy-Relationen. Experimentelle Ergebnisse zeigen, daB viele Alkalihalogenide die Cauchy-Relationen nicht erfullen. Diese Erscheinung wird mit der von Leibfried und Mitarbeitern entwickelten quasi-harmonischen Naherung untersucht. I m Grenzfall hoher Temperaturen werden auch die linearen Temperaturkoeffizienten der elastischen Konstanten berechnet. Urn den Beitrag der Van der Waals-Terme abzuschatzen, werden die elastischen Konstanten berechnet, indem die Van der Waals-Terme in die Ausdrucke fur die elastischen Konstanten einbezogen werden. Die berechneten Werte fur die elastischen Konstanten werden mit den zur Verfugung stehenden experimentellen Werten verglichen.
A length measurement system is described which can make absolute length change measurements of a meter long specimen within a pressure vessel (to pressures of 8 kbar) with a fractional error -8 of 3x10 even when changes in length of 4 cm occur. Possible uses of such a system for measuring the volume thermal expansivity (at pressure) and the isothermal bulk modulus (at pressure) are described. Moreover it is noted that when this system is combined with an ultrasonic velocity measurement system, the adiabatic bulk modulus (at pressure) and GrGneisen parameter as a function of Pressure can be directly determined and the absolute pressure itself can be measured.
A new technique is proposed for measuring thermal diffusivities of materials at high temperature. A thin-slab specimen is heated with a steady flux of energy on one or both faces. The flux to one face is then interrupted momentarily during which time radiative heat losses cause a negative temperature pulse to develop in the specimen and to propagate through it. The thermal diffusivity is determined from an analysis of the temperature excursion of the opposite face. A theoretical analysis of this heat-transfer problem for which one-dimensional heat flow and a general linearized form of surface heat loss are assumed is presented. Data analysis techniques are discussed, and the results are given for some measurements on AXM-9Q graphite to 3250 K. A 250-W CO2 continuous-wave laser was used as the heat source.
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