SU782494A1

SU782494A1 - Method for measuring temperature conductivity of materials

Info

Publication number: SU782494A1
Application number: SU792787565A
Authority: SU
Inventors: В.М. Баранов; А.Н. Быков; В.А. Гаранов; А.Н. Самохвалов
Original assignee: Московский Ордена Трудового Красного Знамени Инженерно-Физический Институт
Priority date: 1979-06-25
Filing date: 1979-06-25
Publication date: 1982-01-23

Description

Изобретение относится к измерительной технике и может быть использовано при определении теплофизических свойсть. материалов в широком интервале температур, в частности при воздействии ионизирующих излучений,The invention relates to measuring technique and can be used to determine thermophysical properties. materials in a wide temperature range, in particular when exposed to ionizing radiation,

Известен способ определения температуропроводности, основанный на измерении изменения температуры поверхности образца с помощью термопар при соэда— нии в нем нестационарного температурного поля [1]· ’Однако этот способ не позволяет проводить надежные и точные измерения в широком интервале температур из-за инерционности термопар и погрешности, обусловленной 'изменением поля температуры в месте заделки термопар в образец и изменением свойств материала термопар.A known method for determining thermal diffusivity, based on measuring changes in the surface temperature of a sample using thermocouples when a non-stationary temperature field is connected in it [1] · 'However, this method does not allow reliable and accurate measurements in a wide temperature range due to inertia of thermocouples and error due to a change in the temperature field at the place where the thermocouples are embedded in the sample and a change in the properties of the thermocouple material.

Известен способ определения температуропроводности твердых тел £2], заключающийся в том, что в исследуемом образце возбуждают ультразвуковые коле2 бания вблизи одной из резонансных частот и одновременно с этим осуществляют нестационарный нагрев поверхности образца. Изменение амплитуды колебаний, обусловленное изменением резонансной частоты образца в результате нагрева, регистрируют с помощью приборов, нахо·*· дяг постоянную времени изменения амплитуды и по известным, соотношениям опре-. деляют температуропроводность образца.A known method for determining the thermal diffusivity of solids £ 2] is that ultrasonic vibrations are excited in the sample under study near one of the resonance frequencies, and at the same time, nonstationary heating of the surface of the sample is carried out. A change in the amplitude of oscillations caused by a change in the resonant frequency of the sample as a result of heating is recorded with the help of instruments, finding the time constant of the change in the amplitude and, according to the known, determinations. divide the thermal diffusivity of the sample.

Недостатком способа является большая погрешность при измерениях на образцах материалов, обладающих большим коэффициентом затухания ультразвуковых колебаний, например, полимерных материалов.The disadvantage of this method is the large error in measurements on samples of materials having a large attenuation coefficient of ultrasonic vibrations, for example, polymeric materials.

Цель изобретения - повышение точности и надежности измерений температуропроводности материалов с большим коэффициентом затухания ультразвуковых колебаний.The purpose of the invention is to increase the accuracy and reliability of measurements of thermal diffusivity of materials with a large attenuation coefficient of ultrasonic vibrations.

Цель достигается тем, что исследуемый образец механически соединяют с эталонным образцом с известными физи— вескими свойствами, изготовленным из материала с малым коэффициентом ватухания ультразвуковых колебаний, возбуждают в системе резонансные колебания на .. частоте, вблизи которой'наблю.г' 5 дается наиболее сильная зависимость амплитуды резонансных колебаний от часто··²ты, и по скорости изменения амплитуды колебаний системы, состоящей из исследуемого и эталонного образцов, нахо— ю дяг температуропроводность материала доследуемого образца.The goal is achieved by the fact that the test sample is mechanically connected to a reference sample with known physical properties made of a material with a low coefficient of attenuation of ultrasonic vibrations, excite resonance vibrations in the system at .. frequency, near which the most intense the amplitude of the resonant oscillations often you ·· ^2, and the rate of change of amplitude oscillations of the system consisting of the test and reference samples naho- w dyag thermal diffusivity material dosleduem th sample.

На фиг. 1 изображена система опрел· деления температуропроводности,' на фиг. 2 - резонансные кривые изменения амплитуды колебаний образца при изменении частоты возбуждения.In FIG. 1 shows a system for determining the thermal diffusivity, 'in FIG. 2 - resonance curves of changes in the amplitude of oscillations of the sample with a change in the excitation frequency.

Систему, состоящую из исследуемого и эталонного 2 образцов, размещают между звукопроводамй 3, через которые ₂₀осуществляется возбуждение и регистрация колебаний. Механическое соединение образцов осуществляют с помощью пружинящего элемента 4. Нестационарное температурное поле в исследуемом образ—25 це создают путем воздействия на его плоскую поверхность импульсного теплового потока.The system, consisting of the studied and reference 2 samples, is placed between the sound ducts 3, through which ₂₀ the excitation and registration of vibrations are carried out. The mechanical connection of the samples is carried out using a spring element 4. An unsteady temperature field in the test sample — 25 ° C is created by exposing it to a flat surface with a pulsed heat flow.

Условие применимости такой схемы измерения заключается в том, чтобы время переходного теплового процесса в эталонном образце было много меньше, чем время переходного теплового процесса в исследуемом образце.The condition for the applicability of such a measurement scheme is that the time of the transitional thermal process in the reference sample is much shorter than the time of the transitional thermal process in the test sample.

Постоянная времени переходного тепнового процесса исследуемого образца определяется по формуле гдеThe time constant of the transitional heat process of the test sample is determined by the formula where

11^- толщина исследуемого образца, '1С=3,14,11 ^ is the thickness of the test sample, '1C = 3.14,

С^- температуропроводность материала исследуемого образца. 'C ^ is the thermal diffusivity of the material of the test sample. ''

Постоянная времени переходного теплового процесса эталонного образца определяется по формулеТ^ - Q₄ , гдеThe time constant of the transitional thermal process of the reference sample is determined by the formula T ^ - Q ₄ , where

5^2“ толщина эталонного образца,’ 01^- температуропроводность материа— ла эталонного образца. Тогда условие применимости способа можно записать в виде 7 7 Такое условие можно выполнить путем выбора эталонного образца с малой толщиной (4l_z= 2-3 мм) из материала с большим коэффициентом температуропроводности, например из алюминия (0^= 8,9..10”^м/с).5 ^ 2 “is the thickness of the reference sample, '01 ^ is the thermal diffusivity of the material of the reference sample. Then the condition of applicability of the method can be written in the form 7 7 Such a condition can be fulfilled by choosing a reference sample with a small thickness (4l _z = 2-3 mm) from a material with a large thermal diffusivity, for example, from aluminum (0 ^ = 8.9..10 ”^ M / s).

<5 .50<5 .50

Известно, что изменение температуры задней поверхности исследуемого плоского образца при импульсном тепловом воздействии на его переднюю поверхность описывается формулой А Т= СОЯЛ'/Г’^где, ΔΤ — приращение температуры задней поверхности исследуемого образца;It is known that a change in the temperature of the back surface of the investigated flat sample under pulsed thermal action on its front surface is described by the formula A T = SOYAL '/ G ’^ where, ΔΤ is the temperature increment of the back surface of the studied sample;

•fe - время, отсчитываемое с момента начала теплового воздействия.• fe - time counted from the moment of the beginning of the heat exposure.

Поскольку тепловая постоянная времени эталонного образца существенно меньше, чем исследуемого, его температура меняется по тому же закону, что и температура задней поверхности исследуемого образца. Изменение температуры приводит к изменению параметров колебаний эталонного образца, которые измеряются в эксперименте.Since the thermal time constant of the reference sample is significantly less than that of the sample, its temperature changes according to the same law as the temperature of the back surface of the sample under study. A change in temperature leads to a change in the vibration parameters of the reference sample, which are measured in the experiment.

Пусть £р^ — резонансная частота эталонного образца, а I - соответствующая резонансная кривая, характеризующая изменение амплитуды колебаний образца при изменении частоты возбуждения. При возбуждении образца на частоте£р_аБ)вблизи которой наблюдается наиболее сильная зависимость амплитуды резонансных колебаний от частоты, амплитуда колебаний будет равна . Если теперь провести нагрев одной стороны образца, то в результате изменения температуры эталонного образца и соответствующего изменения модуля упругости материала резонансная частота эталонного образца изменится и станет равной некоторому новому значению £р^ , а соответствующая резонансная кривая будет иметь вид II.Let £ p ^ be the resonance frequency of the reference sample, and let I be the corresponding resonance curve characterizing the change in the amplitude of the sample oscillations with a change in the excitation frequency. When a sample is excited at a frequency £ p _aB) near which the strongest dependence of the amplitude of the resonant oscillations on the frequency is observed, the amplitude of the oscillations will be equal. If we now heat one side of the sample, then as a result of a change in the temperature of the reference sample and a corresponding change in the modulus of elasticity of the material, the resonant frequency of the reference sample will change and become equal to some new value £ p ^, and the corresponding resonance curve will have the form II.

При неизменных прочих условиях амплитуда ;колебаний образца примет новое значение . При правильном выборе рабочей частоты £ _рС)5 на участке, где наблюдается наиболее сильная зависимость амплитуды резонансных колебаний от частоты, изменение амплитуды ΔΑ= =А^—будет линейно связано с изменением резонансной частоты эталонного образца Δ £р = fр - £р^ . В случае отклонения рабочей частоты от указанного участка связь между изменением резонансной частоты и амплитуды будет нелинейной. В свою очередь, изменение резонансной частоты эталонного образца линейно зависит от изменения температуры АТ, ввиду малости ΔΤ . Поэтому изменение амплитуды колебаний эталонного образца будет происходить по такому же закону, что и изменение Δ.Τ A A -Conste -ΦΙ'Κί . Таким образом, определяя изменение во времени амплитуды ультразвуковых колебаний эталонного образца, можно рассчитывать 5 температуропроводность исследуемого об—' разца по формуле дТдД =сопвБ е^-4'' *t.Under other conditions, the amplitude; the oscillations of the sample will take on a new value. With the right choice of the operating frequency £ _pC) 5 in the area where the most pronounced dependence of the amplitude of the resonant oscillations on the frequency is observed, the change in the amplitude ΔΑ = A ^ - will be linearly related to the change in the resonant frequency of the reference sample Δ £ p = fp - £ p ^. If the operating frequency deviates from the indicated section, the relationship between the change in the resonant frequency and amplitude will be nonlinear. In turn, the change in the resonant frequency of the reference sample linearly depends on the change in temperature AT, due to the smallness of ΔΤ. Therefore, the change in the amplitude of oscillations of the reference sample will occur according to the same law as the change in Δ.Τ AA -Conste -ΦΙ'Κί. Thus, by determining the time variation of amplitude of ultrasonic oscillation of the reference sample, we can calculate the thermal diffusivity of the test ob- 5 'samples were dTdD formula e = sopvB ^-4' '* t.

Сигнал высокой частоты от генераторов -5 или 6(см. фиг. 3) частота которого измеряется частотомером 7, подает- 10 ся на пьезопреобразователь 8, где преобразуется в механические колебания, Через звукопровоц 9 механические колебания передаются системе исследуемого 10 и эталонного 11 образцов, механически со— 15 уединенных с помощью пружинящего элемента 12. Колебания эталонного образца через звукопровод 13 передаются пьезопреобразователю- 14, где преобразуются в электрический сигнал. Импульс- ₂о ный нагрев системы образцов осуществляют с помощью лампы-вспышки 15. Электрический сигнал с преобразователя усиливается усилителем 16, фильтруется фильтром 17 и наблюдается на экране ₂$ осциллографа 18. Низкочастотная составляющая сигнала, соответствующая процессу изменения амплитуды колебаний эталонного образца в результате нагрева, выделяется с помощью детектора 19 и выводится на ленту самописца 20. Для исключения влияния исследуемого образца на ультразвуковые колебания эталонного образца сцепление между ними осуществ—. ляюг с незначительными усилиями (^ 310 Г/см^), кроме того, используют ради-35 альные типы колебаний эталонного образца. При указанных условиях погрешность определения температуропроводности сос~ тавляет<у 5—6%:The high-frequency signal from the generators -5 or 6 (see Fig. 3) whose frequency is measured by the frequency meter 7, is supplied 10 to the piezoelectric transducer 8, where it is converted into mechanical vibrations. Mechanical sound vibrations are transmitted through sound transducer 9 to the system of the test 10 and the reference 11 samples, mechanically co-15 secluded with the help of a spring element 12. Oscillations of the reference sample through the sound pipe 13 are transmitted to the piezoelectric transducer-14, where they are converted into an electrical signal. ₂ to pulse heating of the samples is performed using the system of the flash lamp 15. The electrical signal from the transducer is amplified by amplifier 16, filtered by a filter 17 and is observed on the screen ₂ $ oscilloscope 18. The low-frequency signal component corresponding to the process changes the oscillation amplitude reference sample by heating , is extracted using the detector 19 and displayed on the tape recorder 20. To exclude the influence of the test sample on the ultrasonic vibrations of the reference sample, coupling between them is carried out—. with slight efforts (^ 310 G / cm ^), in addition, they use radial-35 types of oscillations of the reference sample. Under these conditions, the error in determining the thermal diffusivity is <5–6%:

Измерения, проведенные на известных ⁴® образцах из резины, текстолита и фторопласта совпали с известными данными в пределах погрешности измерений.The measurements carried out on known ⁴ ® samples of rubber, textolite and fluoroplastic coincided with the known data within the measurement error.

Применение эталонного образца для определения температуропроводности ма- ⁴⁵териалов с большим коэффициентом затухания ультразвуковых колебаний позволя ет повысить точность ультразвуковых измерений в 5—10 раз надежность получаемых результатов.Use of a reference sample to determine the thermal diffusivity Ma- materials under ⁴⁵ with a high coefficient of damping ultrasonic vibrations allows to increase the accuracy of ultrasound measurements in 5-10 times the reliability of the results.

Применение способа позволяет проводить измерения температуропроводности малых образцов материала простой формы (пластины толщиной 2—4 мм, диаметром ^10—60 мм) в различных условиях эксперимента.The application of the method allows measurements of the thermal diffusivity of small samples of a simple-shaped material (plates with a thickness of 2-4 mm, diameter ^ 10-60 mm) under various experimental conditions.

Claims

(54) METHOD FOR DETERMINING TEMPERATURE CONDUCTIVITY The invention relates to a meter. This technique can be used to determine thermal properties of materials in a wide range of temperatures, in particular when exposed to ionizing radiation. A known method for determining thermal diffusivity is based on measuring the temperature change of a sample surface using thermocouples when creating a non-stationary temperature field in it. However, this method does not allow reliable and accurate measurements in a wide range of temperatures due to the inertia of thermocouples and errors due to a change in the temperature field at the place where thermocouples are embedded in the sample and changes in the properties of the material of the thermocouples. . A known method for determining the thermal diffusivity of solids C2, is that ultrasound coils of MATERIALS are excited in the sample under test near one of the resonant frequencies and at the same time, non-stationary heating of the sample surface is carried out. The change in the amplitude of oscillations due to the change in the resonant frequency of the sample as a result of heating is recorded with the help of instruments, Haxof is the time constant of the amplitude change and, according to limestone, the ratios are determined by the thermal diffusivity of the sample. The disadvantage of the method is a large error in measurements on samples of materials with a large attenuation coefficient of ultrasonic vibrations, for example, polymeric materials. The purpose of the invention is to improve the accuracy and reliability of measurements of the thermal diffusivity of materials with a large attenuation coefficient of ultrasonic vibrations. The goal is that the test sample is mechanically connected to a reference sample with a PWRT-TABE 378 Good Properties, prepared from a material with a small damping factor of ultrasonic vibrations, excites resonant oscillations in the system at a frequency close to which the observed The amplitudes of the resonant oscillations from the frequency, and according to the rate of change of the amplitude of oscillations of the system consisting of the test to the reference one, find the temperature conductivity of the material} 1 of the sample under study. FIG. 1 shows a system for determining the temperature accuracy; in fig. 2 shows the resonance curves of the variation of the amplitude of oscillations of the sample as the excitation frequency changes. The system, consisting of the test 1 and reference 2 samples, is placed between the chimneys 3, through which the oscillations are excited and recorded. The mechanical connection of the samples is carried out with the help of the spring element 4. The unsteady temperature field in the test sample is created by applying a pulsed heat flux to its flat surface. The condition for the applicability of such a measurement scheme is that the time of the transition thermal process in the reference sample is much less than the time of the transition thermal process in the test sample. The time constant of the transient heat loss process of the sample under study is determined by the formula 1}. The thickness of the sample under study,, 14 ,. O is the thermal diffusivity of the material of the sample under study. The time constant of the transition process of the reference process is determined by the formula T.e. where 10l is the thickness of the reference sample, 01 is the thermal diffusivity of the material of the reference sample. Then the condition of applicability of the method can be written as tr f. Such a condition can be fulfilled by selecting a reference sample with a small thickness (2. material with a greater thermal diffusivity, for example, from aluminum (c (2. 8.9.10 m / 4). It is known that the change in temperature of the back surface of a flat sample under investigation its front surface is described by the formula 4G with / 75/1 gae, DT is the increment of the temperature of the back surface of the sample under study, fc is the time counted from the onset of the thermal effect. Because the thermal time constant is The sample is substantially smaller than the sample under study, its temperature varies according to the same law as the back surface temperature of the sample under study. A change in temperature leads to a change in the oscillation parameters of the reference sample measured in the experiment. Let p be the resonant frequency of the reference sample and I is the corresponding resonance curve characterizing the change in the amplitude of oscillations of the sample with a change in the frequency of the excitation. When the sample is excited at a frequency gp, near which the amplitude of resonance is observed, the oscillation versus frequency, the oscillation amplitude will be A. If you now heat one side of the sample, then the temperature of the reference sample and the corresponding change in the elastic modulus of the material will resonate the frequency of the reference sample will change and become equal to some new value of pn, and the corresponding resonance curve will look like P. .. Under unchanged other conditions, the amplitude; -samples of the sample will take the new value of L2. With the correct choice of the operating frequency in the area where the amplitude of the resonant oscillations depends on the frequency, the change in the amplitude of the DL A –A is most pronounced. will be linearly related to the change in the resonant frequency of the reference sample A p –pc. In the event of a deviation of the operating frequency from the indicated section, the connection between the change in the resonant frequency and the amplitude will be non-linear. In turn, the change in the resonant frequency of the reference sample is linearly dependent on the change in the LT temperature, in view of the smallness of LT. Therefore, a change in the amplitude of oscillations of a reference sample will occur according to the same law, and a change in T dA - ICi. Thus, we determine the temporal variation of the amplitude of ultrasonic oscillations of a reference sample. We can calculate the temperature of the sample under study using the formula dTdA coinst e t. The high frequency signal from the oscillators -5 or 6 (see Fig. 3) whose frequency is measured by a frequency meter 7, delivers to the piezoelectric transducer 8, where it transforms into mecanic vibrations. After the sound conductor 9, the mechanical vibrations are transmitted to the system of the studied Yun reference 11 samples, mechanically from; integral with the help of the spring element 12. The oscillations of the reference sample are transmitted through a twist duct 13 to a piezoelectric transducer-14, where they are converted into an electrical signal. Pulsed heating of the sample system is carried out using a flash lamp -15. The electrical signal from the transducer is amplified by the amplifier 16, filtered by the filter 17 and observed on the oscilloscope screen 18. The low frequency component of the signal corresponding to the process of changing the amplitude of oscillations of the reference sample as a result of heating is extracted using the detector 19 and output to the tape recorder 2O. In order to eliminate the effect of the sample under study on the ultrasonic vibrations of the reference specimen, the adhesion between them was carried out with minor efforts (G / cm2). In addition, radial types of the reference sample & h are used. Under these conditions, the error in determining the temperature and temperature of the measurements measured on known samples of rubber, textolite, and fluoroplastic material coincided with known values within the limits of measurement error. The use of a reference sample for determining the thermal diffusivity of materials with a large attenuation coefficient of ultrasonic oscillations will allow an increase in the accuracy of ultrasonic measurements by 5–10 times the reliability of the results obtained. The application of the method makes it possible to measure the thermal diffusivity of small samples of simple-shaped material (plates 2-4 mm thick, with a diameter of -60 mm) under various experimental conditions. Claims of Invention A method for determining the thermal diffusivity of materials by creating non-stationary thermal pop in a test. taking a sample and measuring the temporal variation of the resonant frequency and amplitude of the mechanical oscillations of the sample, about 10 and so that, in order to increase the accuracy and reliability of measurements of the thermal diffusivity of materials with a large oscillation damping factor, with a reference sample with known physical properties, made of a material with a small attenuation coefficient of oscillations, excite resonant oscillations in the system at a frequency near which the maximum The amplitude of resonance oscillations versus frequency, and the rate of change of the amplitude of oscillations of the system, consisting of the test and reference samples, are determined by the temperature conductivity of the material of the test sample. Sources of information taken into account in the examination: 1. Petrunin PI. and P. Yurchak. Installation for measuring the thermal diffusivity of materials by the method of plane temperature waves, TVT, No. 3. 1971.

2. Authors certificate of the USSR N 342117, cl. Q01N 25/20, 1972 (prototype).

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