Thermal Wave Measurements with a Mirage Detection for Investigation of Thermal Diffusivity of GdCa4O(BO3)3 Single Crystals

Single crystals of GdCa4O(BO3)3 were examined to determine their thermal properties. Samples were grown by the Czochralski pulling technique. There were three types of samples: a pure crystal, the crystal doped with neodymium (4 at.% of Nd), and the third one doped with ytterbium (7 at.% of Yb). All samples were rectangular prisms with edges parallel to the axes of the optical indicatrix X, Y , Z (principal axes). The thermal diffusivity was determined by means of the thermal wave method with the optical detection of the temperature disturbance based on a mirage effect. Experimental results showed anisotropy of the thermal diffusivity. The thermal diffusivity along Y direction has the highest value while values obtained in X and Z axes are much lower. Dopants cause decrease in the thermal diffusivity for all investigated directions

AC/DC Thermal Nano-Analyzer Compatible with Bulk Liquid Measurements

Nanocalorimetry, or thermal nano-analysis, is a powerful tool for fast thermal processing and thermodynamic analysis of materials at the nanoscale. Despite multiple reports of successful applications in the material sciences to study phase transitions in metals and polymers, thermodynamic analysis of biological systems in their natural microenvironment has not been achieved yet. Simply scaling down traditional calorimetric techniques, although beneficial for material sciences, is not always appropriate for biological objects, which cannot be removed out of their native biological environment or be miniaturized to suit instrument limitations. Thermal analysis at micro- or nano-scale immersed in bulk liquid media has not yet been possible. Here, we report an AC/DC modulated thermal nano-analyzer capable of detecting nanogram quantities of material in bulk liquids. The detection principle used in our custom-build instrument utilizes localized heat waves, which under certain conditions confine the measurement area to the surface layer of the sample in the close vicinity of the sensing element. To illustrate the sensitivity and quantitative capabilities of the instrument we used model materials with detectable phase transitions. Here, we report ca. 106 improvement in the thermal analysis sensitivity over a traditional DSC instrument. Interestingly, fundamental thermal properties of the material can be determined independently from heat flow in DC (direct current) mode, by using the AC (alternating current) component of the modulated heat in AC/DC mode. The thermal high-frequency AC modulation mode might be especially useful for investigating thermal transitions on the surface of material, because of the ability to control the depth of penetration of AC-modulated heat and hence the depth of thermal sensing. The high-frequency AC mode might potentially expand the range of applications to the surface analysis of bulk materials or liquid-solid interfaces

Thermal diffusivity, effusivity and conductivity of CdMnTe mixed crystals

Cd1-xMnxTe mixed crystals belong to a class of materials called ‘‘semimagnetic semiconductor’’ or diluted magnetic semiconductor (DMS) with addition of magnetic ions like Mn2+ implemented into crystal structure. The crystals under investigation were grown from the melt by the high pressure high temperature modified Bridgman method in the range of composition 0 < x < 0.7. Thermal properties of these compounds have been investigated by means of photopyroelectric (PPE) calorimetry in both, back and front detection configuration. The values of the thermal diffusivity and effusivity were derived from experimental data. Thermal conductivity of the specimens was calculated from the simple theoretical dependencies between thermal parameters. The influence of Mn concentration on thermal properties of Cd1-xMnxTe crystals have been presented and discussed

Investigation of thermal properties of SiC using photothermal method

A photothermal measurements were carried out for a few samples of silicon carbide (SiC). Thickness of the SiC plates was about 1 mm. Samples were highly inhomogeneous and differed in crystal structure. Experimental setup was typical for such investigation. The photothermal signal was measured using mirage detection. Experiment was performed for two configurations: probing beam was running over and under the sample surface. Analysis of experimental data was done according to one dimensional model of thermal wave propagation in multilayer system. Thermal diffusivities and optical absorption coefficients of the samples were determined based on multiparameter fitting procedure

New approach to data analysis in modified Ångström's method

The conditions required for the accurate measurement of the thermal diffusivity of solids by the modified Ångström's method are studied. Authors know that this method is suitable for determination of the thermal diffusivity of thick samples with low thermal diffusivity and it is difficult to determine thermal diffusivity of relatively thin samples of good thermal conductors. Up till now thermal properties of the transitional layers – layers between the thermocouple and the sample - were not taken into account in data analysis. It is shown that thermal resistances of the intermediate layers and noises should be taken into account in analysis of measured signals. Based on that new methodology for analysis of experimental data is proposed

Correlation between the thermal diffusivity and the velocity of ultrasound in YVO

Four single crystals of YVO4 were examined to determine their thermal and elastic properties. The thermal diffusivity was investigated by photothermal method using the mirage effect. The velocity of ultrasound in crystals was measured using the pulse echo method. Two of investigated samples were doped with neodymium (1 at.% and 2 at.% of Nd), one with calcium and thulium (0.4 at.% of Ca and 5 at.% of Tm), and one was pure crystal. Experimental results showed anisotropy of the thermal diffusivity and the velocity of ultrasound. The thermal diffusivity as well as the sound velocity is lower in (001) crystallographic plane than in [001] direction (c-axis). Both quantities decrease with growing concentration of dopants for all crystallographic direction, but the influence of dopants is more distinct in the case of the thermal diffusivity. This fact allows to draw the conclusion that impurity atoms create scattering centres in crystal structure which result in shortening of phonon mean free paths and lowering of the thermal conductivity

Determination of the thermal diffusivity of pure and doped yttrium orthovanadate by Ångström's method

Thermal diffusivities of pure YVO4 single crystal and single crystals doped with Nd, Tm and Ca ions are measured using a modified Ångström's method. Measurements were carried out for main crystallographic directions ([100], [010] and [001]). Obtained results show that the thermal diffusivity in [001] direction is considerably higher than in (001) plane. Decrease of the thermal diffusivity is observed with growing concentration of dopants. For the heavier doped sample (5% at. of Tm + 0.4% at. of Ca) a drop of the thermal diffusivity is about 35%