Probe calibration of the scanning thermal microscope in the AC mode

Scanning thermal microscope based DC measurements of local thermal conductivity are relevant to insulating (<20 W m⁻¹ K⁻¹) materials only. We aim at using the 3ω method to enhance the sensitivity of the device to a larger range. In this paper we present both a thermal model and experimental results from the calibration procedure to study the thermal behaviour of stand-alone probes. The two approaches provide data in very good agreement on the full measured frequency domain. Several geometric and thermal parameters are deduced from the comparison. Those quantities are key inputs for future heat transfer modeling of the tip–sample contact.     

Semianalytical analysis of thermal effect in LD double-side-pumped rectangular laser crystal

A semianalytical method to analyze the thermal effect in a LD double-side-pumped rectangular laser crystal is put forward. Through the analysis of working characteristics of the laser crystal, a thermal model that matches actual situations of the laser crystal is established. General expressions of temperature field and thermal distortion field in the laser crystal can be obtained by a novel method to solve the heat conduction equation of orthotropic material. This semianalytical method can be used to calculate the temperature field and thermal distortion field in other LD double-side-pumped laser crystals and is applied to Nd:YVO₄ crystal in detail in this paper, and two methods of effectively reducing thermal distortion in the laser crystal are offered. Results show that a maximum temperature rise of 362.2 °C and a maximum thermal distortion of 5.55 μm are obtained in Nd:YVO₄ crystal when the output power of the two laser diodes are both 30 W. When the off-center distance is 0.6 mm, the maximum thermal distortion is reduced by 37.7%; when the thickness of the crystal is reduced from 2.0 mm to 1.4 mm, the maximum thermal distortion is reduced by 31.7%. Results in this paper can offer theoretical base for better solving thermal problems in laser system.     

Thermal transport in multiwall carbon nanotube buckypapers

A steady-state electro-Raman-thermal (SERT) technique is developed to characterize the thermal transport in multiwall carbon nanotube (MWCNT) buckypapers. This SERT technique involves steady-state joule heating of a suspended sample and measuring its middle point temperature based on Raman shift intensity. The thermal conductivity is determined from linear fitting of the temperature against heating power. Combined with the transient-electro-thermal technique, the thermophysical properties of two MWCNT buckypapers are characterized as 1.19 and 2.92 W/(m K) for thermal conductivity, 3.65×¹⁰−6 and 7.58×¹⁰−6 m²/s for thermal diffusivity, 459 and 543 kg/m³ for density. Detailed discussion and analysis are provided about the uncertainty of the SERT technique and its capacity for measuring wires down to sub-μm length. The low thermal conductivity of the buckypaper indicates its thermal transport is determined by the thermal contact resistance between MWCNTs. These contact points feature low thermal conductance. The mean distance between two adjacent contact points is estimated in the order 45-450 μm and 93-933 μm for the two samples, indicating low-density contacts within the buckypaper.     

Peculiar behavior of structure rearrangement in nanofiber of intermetallic Ni<Subscript>3</Subscript>Al,containing long-period paired thermal anti-phase boundaries,under high-rate tensile uniaxial loading along <001>

Using molecular dynamics simulations, peculiarities of structure rearrangement in nanofiber of intermetallic Ni₃Al containing long-period, paired, thermal (nonconservative) anti-phase boundarties (APBs is investigated in the course of high-rate, tensile uniaxial loading along <001>. Four main deformation stages are determined (quasi-elastic, plastic, material flow and rupture), with each stage revealing particular features of structure transformations and energy transfer. The presence of periodic thermal planar defects in the long-period nanostructure (combined thermal anti-phase boundaries) significantly affects the onset of plastic deformation. A change in the type of thermal APBs in the long-period structure in turn affects the time to total rupture of nanofiber under plastic deformation condition. For the thermal AA 1/2 < 110 > {001}APBs, the time to total nanofiber rupture is slightly decreased, while that for the thermal AB 1/2 < 110 > {001} APBs is considerably increased.     

Thermal conductivities of solid and liquid phases for neopentylglycol, aminomethylpropanediol and their binary alloy

The variations of thermal conductivities of solid phases versus temperature for neopentylglycol (NPG), 2-amino-2-methyl-1,3-propanediol (AMPD) and AMPD-42.2 mol% NPG alloy were measured with a radial heat flow apparatus. From the graphs of the solid phases thermal conductivity variations versus temperature, the thermal conductivities of the solid phases at their melting temperature and temperature coefficients for same materials were also found to be 0.22±0.01, 0.45±0.02 and 0.32±0.02 W/Km and 0.0047, 0.0031 and 0.0043 K⁻¹, respectively. The thermal conductivity ratios of liquid phase to solid phase for the same materials at their melting temperature are found to be 1.07, 1.12 and 0.74 with a Bridgman type directional solidification apparatus, respectively. Thus, the thermal conductivities of liquid phases for pure NPG, pure AMPD and AMPD-42.2 mol% NPG alloy at their melting temperature were evaluated to be 0.24, 0.50 and 0.23 W/Km, respectively, by using the values of solid phase thermal conductivities and the thermal conductivity ratios of liquid phase to solid phase.     

Molecular alloys as phase change materials (MAPCM) for energy storage and thermal protection at temperatures from 70 to 85 °C

Molecular alloys, that combine a relatively high heat of melting with a suitable melting temperature adapted to the application temperature, are excellent materials for thermal protection and for thermal energy storage. Of special interest is the fact that, by making alloys of molecular materials; the range of melting can be adjusted over a range of temperatures. The present paper reports on the design of MAPCMs to be used for energy storage and thermal protection at temperatures from 70 to 85 °C. The aim is to use these materials for thermal protection in the catering sector in order to avoid proliferation of micro organisms; the minimal temperature required is higher than 65 °C. The work illustrates how some fundamental studies are helpful in choosing the right composition that is able to work at the temperature required for an application. Several molecular alloys using the n-alkanes are elaborated and characterized. The preparation of mixed crystals, their crystallographic and thermodynamic properties and stability, phase change behaviour, and their use in practical applications are reported.     

Size dependent ultrasonic properties of InN nanowires

The ultrasonic properties of single crystalline indium nitride nanowires (InN NWs) are studied for wire size (diameter) 6–100 nm at 300 K following the interaction potential model. Ultrasonic attenuation, ultrasonic velocity, acoustic coupling constant and thermal relaxation time are calculated using higher order elastic constants and thermal conductivity of the nanowires. The analysis of size dependent thermal relaxation time and ultrasonic properties shows that above the 20 nm diameter, InN nanowire tends towards its bulk material property. The third order polynomial is found to be best fit for size variation of thermal relaxation time. The ultrasonic attenuation as a function of size of the nanowires is found to be mainly affected by the thermal conductivity of the nanowires of different sizes.     

Energetic and thermal performance of high-gain diode-side-pumped Nd:YAG rods

We investigated the energetic and thermal performance of a diode-side-pumped Nd:YAG rod laser with up to 50 W power deposited as excess heat into a 3-mm-diameter, 10-cm-length rod. The rod design produces an extremely flat gain profile resulting in “textbook” expressions of thermal lensing and birefringence. Thermal and energetic measurements are compared to corresponding “textbook” theoretical expressions. Discrepancies between various published thermo-mechanical YAG parameters are resolved by a self-consistent set of measured and calculated data for rod thermal lens focal lengths, birefringence depolarization and ratio of heat to stored energy (χ). Measured thermal and energetic performance under lasing and nonlasing conditions are presented, which agree with published theoretical expressions and measurements. Compensation of rod thermal lensing with simple spherical concave lenses is demonstrated. In addition various methods for compensating birefringence depolarization are theoretically and experimentally analyzed and compared. Received: 19 July 1999 / Revised version: 22 October 1999 / Published online: 23 February 2000     

Thermal conductivity of ozone-safe C10M1 refrigerant in liquid and gaseous phases

Thermal conductivity of ozone-safe refrigerant C10M1 in liquid (303.9–342.4 K, 1.23–4.257 MPa) and gaseous (324–398.15 K; 0.672–2.107 MPa) states was studied by the methods of high-frequency thermal waves and coaxial cylinders. The estimated measurement errors for the temperature, pressure, and thermal conductivity are ±0.02 K, ±1.5 kPa, and ±1.5–2.5 %, correspondingly. Approximation dependencies for thermal conductivity were obtained over the studied range of temperatures and pressures as well as on the dew and bubble lines. It is shown that thermal conductivity in the liquid state is additive relative to mass concentrations of components. The work was financially supported by the Russian Foundation for Basic Research (grant No. 04-02-16355).     

Investigation of thermal transport in colloidal silica dispersions (nanofluids)

Thermal conductivity enhancement in colloidal silica dispersions (nanofluids) is investigated experimentally using a novel optical technique. The effects of nanoparticle size, concentration, and state of aggregation are examined. New data on well dispersed systems are compared to published data obtained using the more conventional transient hot-wire technique and good agreement was found. Experimental results are also compared with model predictions for relative thermal conductivity based on effective medium theory. For systems composed of larger diameter nanoparticles (~30 nm), good agreement was found between the measured thermal conductivity enhancement and that predicted by the classical Maxwell-Garnett model. For systems composed of smaller nanoparticles (∼10 and 20 nm), thermal conductivity enhancement was reduced by as much as 10%, presumably because interfacial thermal resistance effects become important. Measurements on two systems that were induced to form gels exhibited an increase in thermal conductivity of approximately 5% relative to the well-dispersed systems. The observed increase in thermal conductivity is larger than that predicted by a recently proposed model for aggregated nanofluids.     

Influence of ordering change on the optical and thermal properties of inflation polyethylene films

Changes of thermal diffusivity inside femtosecond laser-structured volumes as small as few percent were reliably determined (with standard deviation less than 1%) with miniaturized sensors. An increase of thermal diffusivity of a crystalline high-density polyethylene (HDPE) inflation films by 10-20% from the measured (1.16 ± 0.01) × 10⁻⁷ m² s⁻¹ value in regions not structured by femtosecond laser pulses is considerably larger than that of non-crystalline polymers, 0-3%. The origin of the change of thermal diffusivity are interplay between the laser induced disordering, voids’ formation, compaction, and changes in molecular orientation. It is shown that laser structuring can be used to modify thermal and optical properties. The birefringence and infrared spectroscopy with thermal imaging of CH₂ vibrations are confirming inter-relation between structural, optical, and thermal properties of the laser-structured crystalline HDPE inflation films. Birefringence modulation as high as Δn ∼ ± 1 × 10⁻³ is achieved with grating structures.     

Photothermal deflection spectroscopy PDS investigation of optical and thermal properties of BGaAs/GaAs alloys

Optical and thermal properties of metal organic chemical vapor deposition (MOCVD) grown BGaAs/GaAs alloys with boron composition of 3% and 8% are investigated by photothermal deflection spectroscopy (PDS). The band gap energy, absorption spectrum and thermal conductivities are evaluated by comparing the experimental and the theoretical PDS signals amplitude and phase. The boron effects in these parameters have been shown. In fact, it was found that gap energy and thermal conductivity of BGaAs/GaAs alloys with 3% of boron are respectivelly 1376 mev and 4.7 W/m K, these values decrease to 1360 mev and 3.5 W/m K for 8% of boron composition.     

Analytically derived conversion of spectral band radiance to brightness temperature

Simple analytic expressions for brightness temperature have been derived in terms of band response function spectral moments. Accuracy measures are also derived. Application of these formulas to GOES-12 Sounder thermal infrared bands produces brightness temperature residuals between −5.0 and 2.5 mK for a 150-400 K temperature range. The magnitude of residuals for the five ASTER Radiometer thermal infrared bands over the same temperature range is less than 0.22 mK.     

A peak structure in isothermal luminescence signals in quartz: Origin and implications

Isothermal heating is commonly used in luminescence dosimetry and trap parameter studies. It is often observed that the isothermal luminescence signal has a peak shape instead of a monotonous decay form. We provide here evidence that this peak shape in quartz may equally result from a ‘thermal lag’ problem in contrast to the earlier propositions of non first-order kinetics. Temperature modelling suggests that the peak-shaped signal contains elements of both the ramped — and isothermal — thermoluminescence (TL). The modelled changes in the peak movement as a function of isothermal temperature and the ramp-rate show an excellent agreement with those obtained experimentally. This understanding of thermal lag is extended to optically stimulated luminescence (OSL) measurements for which the effects of isothermal TL contamination and changing thermal assistance during thermal equilibration are discussed. Appropriate methods are suggested to identify thermal lag on the basis of the peak structure, and to circumvent this effect in isothermal methods.     

Use of an open photoacoustic cell for the thermal characterisation of liquid crystals

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4^′-octyloxybiphenyl (8OCB) and 4-cyano-4^′-heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases. Received: 28 March 2001 / Revised version: 8 June 2001 / Published online: 18 July 2001     

Thermal diffusivity of electrodeposited Ni and Co nanowires using infrared thermography

One-dimensional nanostructures such as Ni and Co nanowires (NWs) show anisotropic thermal properties in a direction parallel and perpendicular to the NW axis. Thermal diffusivity of Ni and Co NWs embedded in a 100-nm pore anodic alumina (AAO) template has been measured in a direction perpendicular to the NW axis, using an infrared thermography-based non-contact approach. The measured thermal diffusivity values in the radial direction are 0.728×10⁻⁶ and 0.732×10⁻⁶ m²s⁻¹, respectively, for the Ni and Co nanocomposites. The changes in the thermal diffusivity of the synthesized NWs alone were estimated using a first-order lower bound model (FOLBM). A nearly seven- and sixfold reduction, respectively, of thermal diffusivity in a direction perpendicular to the NW axis is estimated for the synthesized Ni and Co NWs.     

Fabrication and characterization of 1.55 μm single transverse mode large diameter electrically pumped VECSEL

We report on the design, fabrication, and characterization of InP-based 1.55 μm wavelength large diameter (50 μm) electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). The hybrid device consists of a half vertical cavity surface emitting laser (1/2-VCSEL) structure assembled with a concave dielectric external mirror. The 1/2-VCSEL is monolithically grown on InP substrate and includes a semiconductor Bragg mirror and a tunnel junction for electrical injection. Buried (BTJ) and ion implanted (ITJ) tunnel junction electrical confinement schemes are compared in terms of their thermal and electrical characteristics. Lower thermal resistance values are measured for BJT, but reduced current crowding effects and uniform current injection are evidenced for ITJ. Using the ITJ technique, we demonstrate Room-Temperature (RT) continuous-wave (CW) single transverse mode laser operation from 50-μm diameter EP-VECSEL devices. We show that the experimental laser optical output versus injected current (L–I) curves are well-reproduced by a simple analytical thermal model, consistent with the thermal resistance measurements performed on the 1/2-VCSEL structure. Our results indicate that thermal heating is the main mechanism limiting the maximum CW output power of 50-μm diameter VECSELs, rather than current injection inhomogeneity.     

Influence of thermal treatment on the performance of copper phthalocyanine thin-film transistors

Organic thin-film transistors (OTFTs) with bottom-gate and bottom-contact configuration based on copper phthalocyanines (CuPc) as active layer were fabricated. The performance of CuPc OTFTs was studied before and after thermal treatment on CuPc layer. The values of the threshold voltage before and after thermal treatment are −6.3 and −5.7 V, respectively. The field-effect mobility values in saturation regime of CuPc thin-film transistors before and after thermal treatment are 0.014 cm²/Vs and 0.0068 cm²/Vs, respectively. The experimental results indicate that there is a heavy decay on the mobility of CuPc based OTFTs mostly due to the crystalline morphology change induced by the thermal treatment, and absolute value of the threshold voltage after thermal treatment decreases with the decrease of the CuPc film thickness and the roughness.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.