Thermal-wave Cavity Research Articles

Environmental Synthesis of Few Layers Graphene Sheets Using Ultrasonic Exfoliation with Enhanced Electrical and Thermal Properties.

In this paper, we report how few layers graphene that can be produced in large quantity with low defect ratio from exfoliation of graphite by using a high intensity probe sonication in water containing liquid hand soap and PVP. It was founded that the graphene powder obtained by this simple exfoliation method after the heat treatment had an excellent exfoliation into a single or layered graphene sheets. The UV-visible spectroscopy, FESEM, TEM, X-ray powder diffraction and Raman spectroscopy was used to analyse the graphene product. The thermal diffusivity of the samples was analysed using a highly accurate thermal-wave cavity photothermal technique. The data obtained showed excellent enhancement in the thermal diffusivity of the graphene dispersion. This well-dispersed graphene was then used to fabricate an electrically conductive polymer-graphene film composite. The results demonstrated that this low cost and environmental friendly technique allowed to the production of high quality layered graphene sheets, improved the thermal and electrical properties. This may find use in the wide range of applications based on graphene.

A simplified model for thermal-wave cavity self-consistent measurement of thermal diffusivity

A simplified theoretical model was developed for the thermal-wave cavity (TWC) technique in this study. This model takes thermal radiation into account and can be employed for absolute measurements of the thermal diffusivity of gas and liquid samples without any knowledge of geometrical and thermal parameters of the components of the TWC. Using this model and cavity-length scans, thermal diffusivities of air and distilled water were accurately and precisely measured as (2.191 ± 0.004) × 10(-5) and (1.427 ± 0.009) × 10(-7) m(2) s(-1), respectively, in very good agreement with accepted literature values.

Dimensionality considerations of thermal transport mechanisms in a Thermal-Wave Cavity

The dimensionality of the photopyroelectric signal and the importance of radiation heat transfer mechanisms in a thermal-wave cavity (TWC) are closely investigated in the current study. Radiation components were added to the purely conduction model by linearizing the radiation heat transfer at the brass film-air boundary. A 3-D theoretical approach based on the Hankel integral was developed and implemented. Experimental results indicate that a 3-D model is necessary to describe the PPE signal, especially at low frequencies. Radiation is found to be the major contributor of the photopyroelectric (PPE) signal when high frequencies and large cavity lengths are used.

Theoretical analysis of PPE measurements in liquids using a thermal-wave cavity

The photopyroelectric measurements in a thermal-wave cavity (PPE) were analyzed with a conventional one-dimensional approach and a three-dimensional approach. The calculations show that the dimensionality of the thermal-wave field in the cavity depends on the boundary conditions and the beam size of the applied laser. The study resulted in identifying ranges of heat transfer rates and cavity configurations for which accurate quantitative measurements of the thermal diffusivity of intracavity fluids can be made within the far simpler, but only approximate, one-dimensional approach conventionally adopted by users of thermal-wave cavities.

Photothermal measurement of thermal diffusivity in carbonyl iron powder suspensions

We have measured the thermal diffusivity of micron size Carbonyl Iron Powder suspension in a silicone oil base fluid, at various Carbonyl Iron Powder concentrations by means of a photothermal technique called Thermal Wave Cavity. Using literature data (density and specific heat capacity) we can determine the effective thermal conductivity. We compare our experimental results with various theoretical models for the effective thermal conductivity in heterogeneous materials, previously deployed in the literature.

Optimally accurate thermal-wave cavity photopyroelectric measurements of pressure-dependent thermophysical properties of air: Theory and experiments

An experimental technique for the measurement of thermal properties of air at low pressures using a photopyroelectric (PPE) thermal-wave cavity (TWC) was developed. In addition, two theoretical approaches, a conventional one-dimensional thermal-wave model and a three-dimensional theory based on the Hankel integral, were applied to interpret the thermal-wave field in the thermal-wave cavity. The importance of radiation heat transfer mechanisms in a TWC was also investigated. Radiation components were added to the purely conductive model by linearizing the radiation heat transfer component at the cavity boundary. The experimental results indicate that the three-dimensional model is necessary to describe the PPE signal, especially at low frequencies where thermal diffusion length is large and sideways propagation of the thermal-wave field becomes significant. Radiation is found to be the dominant contributor of the PPE signal at high frequencies and large cavity lengths, where heat conduction across the TWC length is relatively weak. The three-dimensional theory and the Downhill Simplex algorithm were used to fit the experimental data and extract the thermal diffusivity of air and the heat transfer coefficient in a wide range of pressures from 760 to 2.6 Torr. It was shown that judicious adjustments of cavity length and computational best fits to frequency-scanned data using three-dimensional photopyroelectric theory lead to optimally accurate value measurements of thermal diffusivity and heat transfer coefficient at various pressures.

Quantitative one-dimensional thermal-wave cavity measurements of fluid thermophysical properties through equivalence studies with three-dimensional geometries

The thermal-wave field in a photopyroelectric thermal-wave cavity was calculated with two theoretical approaches: a computationally straightforward, conventional, one-dimensional approach and a three-dimensional experimentally more realistic approach. The calculations show that the dimensionality of the thermal-wave field in the cavity depends on the lateral heat transfer boundary conditions and the relation between the beam size of the laser impinging on the thermal-wave generating metallic film and the diameter of the film itself. The theoretical calculations and the experimental data on the photopyroelectric signal in the cavity were compared. The study resulted in identifying ranges of heat transfer rates, beam sizes, and cavity radii for which accurate quantitative measurements of the thermal diffusivity of intracavity fluids can be made within the far simpler, but only approximate, one-dimensional approach conventionally adopted by users of thermal-wave cavities. It was shown that the major parameters affecting the dimensionality of thermal-wave cavities are the laser beam spot size and the Biot number of the medium comprising the sidewalls of the (cylindrical) cavity.

Ultrahigh-resolution pyroelectric thermal-wave technique for the measurement of thermal diffusivity of low-concentration water-alcohol mixtures

Thermal diffusivities of water-methanol and water-ethanol mixtures were measured using a thermal-wave cavity with two techniques: conventional single-pulse photopyroelectric frequency scans and the common-mode-rejection demodulation dual-pulse scheme. The frequency-scan measurements showed maximum resolution of the photothermal signal in water at the level of 0.5% by volume in mixtures of methanol and ethanol. The common-mode-rejection demodulation method improved the resolution up to the level of 0.2% by volume, which is the highest thermophysical resolution of water-methanol and water-ethanol mixtures reported to date, to our best knowledge. The ultrahigh sensitivity of the method can be especially useful in environmental applications, specifically in real-time water pollution monitoring.

High-Precision and High-Resolution Measurements of Thermal Diffusivity and Infrared Emissivity of Water–Methanol Mixtures Using a Pyroelectric Thermal Wave Resonator Cavity: Frequency-Scan Approach

The thermal diffusivity and effective infrared emissivity of water–methanol mixtureswere measured at atmospheric pressure and ambient temperature using a pyroelectric thermal-wave resonator cavity. The applied frequency-scan method allows keeping the cavity length fixed, which eliminates instrumental errors and substantially improves the precision and accuracy of the measurements. A theoretical model describing conduction and radiation heat transfer in the cavity was developed. The model predictions and the frequency-scan experimental data were compared, showing excellent agreement. The measurements were performed for methanol volume fractions of 0, 0.5, 1, 2, 5, 10, 20, 40, 75, and 100%. The fitted thermal diffusivity and effective emissivity vs. concentration results of the mixtures were compared to literature theoretical and experimental data. The maximum resolution of 0.5% by volume of methanol in water by means of the thermal-wave cavity method is the highest reported to date using thermophysical techniques. Semi-empirical expressions for the mixture thermal diffusivity and infrared emissivity as functions of methanol concentration have been introduced. The expression for infrared emissivity is consistent with the physical principle of detailed balance (Kirchhoff’s law). The expression for thermal diffusivity was found to describe the data satisfactorily over the entire methanol volume-fraction range.

Thermal diffusivity measurements in liquids using signal common-mode-rejection demodulation in a thermal-wave cavity

A recently developed signal common-mode rejection demodulation signal methodology [Mandelis et al., Rev. Sci. Instrum. 71, 2440 (2000)] has been used to make direct absolute thermal diffusivity measurements in liquids using a thermal-wave cavity. This methodology combines the precision of the thermal-wave cavity scan and the flexibility of modulation-frequency scan modes, along with baseline suppression, yielding a high-resolution technique for thermal diffusivity measurements in liquids. The thermal diffusivity of two pure liquids (distilled water and ethylene glycol) has been measured and good agreement has been obtained with values reported in the literature.

Simple, accurate, and precise measurements of thermal diffusivity in liquids using a thermal-wave cavity

A simple methodology for the direct measurement of the thermal wavelength using a thermal-wave cavity, and its application to the evaluation of the thermal diffusivity of liquids is described. The simplicity and robustness of this technique lie in its relative measurement features for both the thermal-wave phase and cavity length, thus eliminating the need for taking into account difficult-to-quantify and time-consuming instrumental phase shifts. Two liquid samples were used: distilled water and ethylene glycol. Excellent agreement was found with reported results in the literature. The accuracy of the thermal diffusivity measurements using the new methodology originates in the use of only difference measurements in the thermal-wave phase and cavity length. Measurement precision is directly related to the corresponding precision on the measurement of the thermal wavelength.

Deconvolution and measurement of bulk and surface optical absorptions in Ti:Al2O3 crystals using photopyroelectric interferometry

The extension of our earlier single-layer (monolithic) photopyroelectric (PPE) interferometric theory to include surface and bulk optical absorptions has allowed the measurement of both bulk absorption coefficient and surface absorptance in one single experiment. Based on purely thermal-wave interferometry, the thermal-wave cavity lengths of a PPE interferometer were scanned using pairs of Ti: sapphire crystals with appropriate combinations of figure of merit, surface polish, and thickness. In the conventional single-ended (noninterferometric) PPE technique, the surface reflectivity, surface absorptance, and bulk absorption coefficient are always coupled together. However, PPE destructive interferometry provides a method for extracting highly precise values of one of these optical parameters, without the requirement of equally precise knowledge of the values of the others.

Purely thermal-wave photopyroelectric interferometry

A thermal-wave interferometric technique based on pure thermal-wave interference using two-cavity photopyroelectric (PPE) detection has been developed. The salient features of this interferometry are investigated with a general theory, which describes the dependence of the PPE signal on the optical, thermal, and geometric parameters of the thermal-wave cavity configuration. Preliminary experimental results are presented. A major feature of the technique is the efficient suppression of the background PPE signal base line. The physical origins of the interferometry are also discussed.

Measurement of thermal diffusivity of air using photopyroelectric interferometry

A new photopyroelectric (PPE) method for measuring the thermal diffusivity of ambient gases with optimal precision is introduced. The technique is based on destructive PPE interferometric detection inside a thermal-wave resonant cavity with an optically transparent wall. A new PPE sensor design generating purely thermal-wave interference is used to improve the signal-to-noise ratio and the measurement dynamic range and precision by suppressing the large optically transmitted base line signal. Cavity-length scanning is employed and the thermal diffusivity of air is measured with third-significant-figure precision. Comparisons of measurement precision are made with experimental results obtained using a transparent-wall PPE thermal-wave cavity in the single-ended (noninterferometric) mode. The physical signal generation mechanisms are also discussed.