RU2073231C1 - Method for coefficient of thermal expansion determining - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: for coefficient of thermal expansion veritable determining sampling material with constant cross-section and well known density and specific heat capacity is treated by adiabatic compression or stretching with setting on tension and known starting temperature. Caused by this treatment sample material temperature deviation is measuring and coefficient of thermal expansion is calculating by corresponding formula using. Method gives possibility to dilatometer constructing without sample elongation metering. EFFECT: higher productivity of solid sample testing and higher accuracy of metering. 1 cl

Description

 The invention relates to the field of thermophysical measurements and can be used in the experimental determination of the coefficient of thermal expansion of solids.

Такой способ принимаем за прототип.Known methods for determining the coefficient of thermal expansion (KTR) [1] are that the sample is heated (cooled) by a given value of the temperature increment ΔT, after which the relative change in its linear size ΔL / L _o or volume ΔV / V _o is measured, and KTR calculated by the formulas:

This method is taken as a prototype.

 It should be noted that the whole variety of existing methods for determining the CTE lies in the difference in either the method of measuring the elongation of the test sample or the method of heating it.

So, for example, an absolute method for determining the CTE [2] is known in which an interferometer is used to measure the elongation of a sample. The main disadvantage of this method should be considered the need for stepwise heating of the sample with a step of at least 5 ^o C, i.e. by such a significant amount that when calculating the KTP there is a problem of relating the found value to a certain temperature. It should be noted and low measurement performance by this method, associated with the need for long-term aging of the sample with any change in its temperature in order to achieve its uniformity in volume of the sample. These shortcomings are fundamentally irreparable, since an attempt to reduce the step size of temperature changes in experiments leads to a significant decrease in the measurement accuracy (see O. Mazurin et al. Thermal expansion of glass. L. Nauka, 1969, p.7).

 There is a way to determine KTR on dilatometers called differential. In devices of this type, the difference in the expansion of the standard and the sample is measured with the same heating. (see A. Amatuni. Methods and devices for determining the temperature coefficients of linear expansion of materials. M. Iz-in standards, 1972). Measurement of the difference in the stroke of the sample and the reference is carried out by a photoelectric device. Knowing the thermal expansion of the standard, calculate the CTE of the sample.

 Certain structural improvements of this method, however, do not allow to get rid of the above disadvantages, fully inherent in the differential methods for determining the CTE.

 The problem to which the invention is directed, obtaining the true value of the CTE of the material at a given temperature and a significant reduction in time spent on related tests.

 The solution to this problem is possible using the effect of coupled thermoelasticity of solids. Its essence lies in the fact that with an adiabatic change in the internal energy of a solid, by doing external work on it, the temperature of the latter changes (see I. Goldenblat, Nonlinear Problems of the Theory of Elasticity. M. Nauka, 1969, pp. 169-173.

,
где a_т коэффициент линейного термического расширения, К^-1;
Т_o начальная температура образца, К;
ρ плотность образца, кг/м³;
C_σ удельная теплоемкость, Дж/кг•К;
s₁ напряжение, H/м²;
θ=T-T_o адиабатическое приращение температуры, К.In the particular case, when the body is subjected to uniaxial compression or extension, the temperature change caused by a step-like voltage change is expressed by the formula

,
where a _{t is the} coefficient of linear thermal expansion, K ^-1 ;
T _{o the} initial temperature of the sample, K;
ρ density of the sample, kg / m ³ ;
C _σ specific heat, J / kg • K;
s ₁ voltage, N / m ² ;
θ = TT _o adiabatic temperature increment, K.

Сущность изобретения заключается в том, что на образец постоянного сечения (призма, цилиндр и т.п.) с известными плотностью и удельной теплоемкостью при данной начальной температуре воздействуют путем адиабатического сжатия (растяжения) известным напряжением, измеряют вызванное этим воздействием изменение температуры образца и вычисляют коэффициент термического расширения по указанной выше формуле.The above formula allows us to express the coefficient of thermal expansion in terms of parameters measurable in the experiment as

The essence of the invention lies in the fact that a sample of constant cross section (prism, cylinder, etc.) with known density and specific heat at a given initial temperature is affected by adiabatic compression (tension) with a known voltage, the change in temperature of the sample caused by this effect is measured, and calculated coefficient of thermal expansion according to the above formula.

 In the study of materials related to the claimed object, no objects were found containing features identical to those of the claimed technical solution, on the basis of which it is concluded that it meets the criterion of "inventive step". Since no solutions were found that would contain the full totality of the features of the proposed solution, it is concluded that it meets the criterion of "novelty."

The proposed method for determining the KTR of solids has the following advantages over the prototype. Since the determination of the coefficient of thermal expansion by the proposed method is accompanied by a very small deviation from the initial temperature (at least 50 ^o C100 times less than in the counter-comparable methods), the calculated value of the KTP is really true, related to the initial temperature of the experiment. By virtue of this proposed method can be effectively used in the radiation of KTP In the region of its strong changes, for example, in the zone of phase transitions.

 In addition, if in opposed methods the duration of a single experiment is of the order of an hour, which is associated with the requirement of uniformity of the temperature of the sample, then for the experiment according to the proposed method no more than 2 3 seconds, i.e. about 1000 times less.

 To test the possibility of determining the coefficient of thermal expansion of solids by the proposed method and comparing the results obtained with the available reference data, we tested samples of aluminum and organic glass (PMMA copolymer grade 2-55). Samples of aluminum with dimensions of 190x11.5x1.8 mm and samples of organic glass with dimensions of 190x11x3.1 mm were subjected to tension at a temperature of 297 K and normal atmospheric pressure. The duration of the experiment was 2 3 seconds, and during this time the heat transfer of the sample with the surrounding air was negligible, which allows us to consider the conditions of the experiment very close to adiabatic. As temperature sensors, nickel film resistance thermometers were used, which were included in the bridge measuring circuit so that an electrical signal was generated at its output, proportional to the difference in temperature between the sample and the environment.

The table below presents the results of these experiments, from which it follows that the found values of the coefficient of thermal expansion are in satisfactory agreement with the reference data (see Tables of physical quantities. Edited by academician IK Kikoin. M. Atomizdat, 1976 . - 1008 pp. Gudimov M.M. Petrov B.V. Organic glass.M. Chemistry, 1981. - 216 p.)
The given examples confirm the practical feasibility of implementing the method for determining the thermal expansion coefficient of solids, in which this parameter can really be considered a true value that relates practically to the temperature point, and not to the temperature range.

 In addition, the proposed method for determining the CTE is very productive, because the time of the experiment is determined exclusively by the mechanical speed of the loading system and the inertia of the temperature sensors.

Claims (1)

The method of determining the coefficient of thermal expansion of solids, which consists in acting on a sample of constant cross section with known density and specific heat at a given initial temperature, measuring the temperature change before and after exposure and determining the coefficient of thermal expansion, characterized in that the effect on the sample is by adiabatic compression (tension) of a known stress, and the coefficient of thermal expansion is determined by the formula

where ρ is the density kg / m ³ ;
C _σ specific heat, D / kg K;
T is the initial temperature of the sample, K;
s axial stress in the sample, n / m ² ;
q adiabatic increment of sample temperature, K.

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