Thermal conductivity of organic semi-conducting materials using 3omega and photothermal radiometry techniques

Organic semiconductors for opto-electronic devices show several defects which can be enhanced while increasing the operating temperature. Their thermal management and especially the reduction of their temperature are of great interest. For the heat transfer study, one has to measure the thermal conductivity of thin film organic materials. However the major difficulty for this measurement is the very low thickness of the films which needs the use of very specific techniques. In our work, the 3-omega and photothermal radiometric methods were used to measure the thermal conductivity of thin film organic semiconducting material (Alq3). The measurements were performed as function of the thin film thickness from 45 to 785 nm and also of its temperature from 80 to 350 K. With the 3 omega method, a thermal conductivity value of 0.066 W.m−1K−1 was obtained for Alq3 thin film of 200 nm at room temperature, in close agreement with the photothermal value. Both techniques appear to be complementary: the 3 omega method is easier to implement for large temperature range and small thicknesses down to a few tens of nanometers whereas the photothermal method is more suitable for thicknesses over 200nm since it provides additional information such as the thin film volumetric heat capacity

Assessment of modulated hot wire method for thermophysical characterization of fluid and solid matrices charged with (nano)particle inclusions

Recently we reported on simultaneous thermal conductivity k and thermal diffusivity a measurement of liquids and in particular of nanofluids in a configuration using an ac excited hot wire combined with lock-in detection of the third harmonic (3ω method) [1]. The conductive wire is used as both heater and sensor. The requirements for the asymptotic validity of the line heat source model are fulfilled at low modulation frequencies below a few Hz. The study of the relative sensitivity of signal amplitude and phase to changes in k and a indicates that there is an optimum frequency range for accurate and stable results. We extend by up to two decades the feasible frequency range for 3ω measurements by considering various more elaborate models for the heat transfer between the wire and the fluid. Finally we show that the same ac hot wire method can be applied to soft solid, composite materials. We measured the k enhancement of a poly(ethylene vinyl acetate) EVA polymer matrix charged with various fractions of graphite

Spectrally resolved modulated infrared radiometry of photothermal, photocarrier, and photoluminescence response of CdSe crystals: Determination of optical, thermal, and electronic transport parameters

International audienceSpectrally resolved modulated infrared radiometry (SR-MIRR) with super-band gap photoexcitation is introduced as a self-consistent method for semiconductor characterization (CdSe crystals grown under different conditions). Starting from a theoretical model combining the contributions of the photothermal (PT) and photocarrier (PC) signal components, an expression is derived for the thermal-to-plasma wave transition frequency ftc which is found to be wavelength-independent. The deviation of the PC component from the model at high frequency is quantitatively explained by a quasi-continuous distribution of carrier recombination lifetimes. The integral, broad frequency band (0.1 Hz–1 MHz) MIRR measurements simultaneously yielded the thermal diffusivity a, the effective IR optical absorption coefficient βeff, and the bulk carrier lifetime τc. Spectrally resolved frequency scans were conducted with interchangeable IR bandpass filters (2.2–11.3 μm) in front of the detector. The perfect spectral match of the PT and PC components is the direct experimental evidence of the key assumption in MIRR that de-exciting carriers are equivalent to blackbody (Planck) radiators. The exploitation of the β spectrum measured by MIRR allowed determining the background (equilibrium) free carrier concentration n0. At the shortest wavelength (3.3 μm), the photoluminescence (PL) component supersedes the PC one and has distinct features. The average sample temperature influences the PC component but not the PT one

Three omega Probe with Auto-zeroing

The aim of this study is to build an active auto-zeroing circuit for the three omega thermal conductivity measurement method to overcome the disadvantage of the method. For this purpose an auto-zeroing algorithm is implemented which consists of two stages; simply reading the voltage drops on lower hand side resistances with high resolution and setting the resistance of the digital potentiometer to the value calculated through proposed algorithm using the difference of acquired voltages. As a result, a probe with auto-zeroing capability is developed successfully. Usage of the new probe will increase the reliability of three omega measurement results by reducing the effects of uncertainties arising from manual zeroing routines

Polymer matrix composites reinforced with expanded and unexpended graphite Particles for electronic packaging applications

Polymer composites with high thermal conductivity are used more frequently in thermal management of electronic packaging systems. In this study, conductive polymer composites were prepared by melt mixing of ethylene-vinyl acetate (EVA) copolymer with graphite at different volumetric concentrations up to 29.3%. Two kinds of graphite were used as reinforcement to prepare composites: untreated natural graphite (UG) having particle sizes ranging from 20 mu m to 25 mu m and expanded graphite (EG) having originally particle sizes ranging from 5 to 6 mu m in length. Upon mixing at high shear forces EG exfoliates in thin sheets of a few nanometers in thickness. Due to this high aspect ratio of graphite sheets, nanocomposites filled with expanded graphite have a lower percolation threshold for electrical conductivity, about (5 to 6) vol.% compared to the composites filled with untreated graphite (UG) which have a percolation threshold of (15 to 17) vol.%. Thermal diffusivity of the samples was measured by photothermal radiometry. At similar concentrations, thermal diffusivity values for the nanocomposites, EG-filled EVA, were significantly higher than those composites filled with UG

PREPARATION AND CHARACTERIZATION OF NANOFLUIDS CONTAINING ALUMINA PARTICLES

This paper reports an experimental work on the measurements of thermal conductivity and viscosity of Al2O3-water and Al2O3-ethylene glycol nanofluids. Results show that thermal conductivity values are within the limits of Maxwell model while viscosity increases dramatically with increasing particle concentration and decreases with increasing temperature. However, the relative viscosity is independent of temperature. Also pH, particle size and zeta potential of nanofluid samples are characterized

Photothermal imaging of localized delamination between organic coatings and metallic substrates using a scanning photopyroelectric microscope

We introduce the scanning photopyroelectric microscope and demonstrate its application for the investigation of delamination sites below organic coatings applied onto metallic substrates. The technique is based on a photothermal method, i.e., the photopyroelectric one, and uses a flexible polyvinylidene-difluoride sensor in contact with the sample surface. It is suitable for studying with high spatial resolution subsurface features like air gaps in multilayer systems and determining their depth below the surface. Using a one-dimensional approach, we derived an analytical expression for the signal contrast arising from subsurface thermal barriers. We also developed a three-dimensional finite element model that allows describing the thermal response of more complex systems and the lateral resolution in the actual measurement. The theoretical models are validated using both, model and real-life samples, and we show that it is possible to detect air gaps with thickness down to 1 mum below organic coatings applied onto metallic substrates. (C) 2003 American Institute of Physics.status: publishe

PREPARATION AND PHOTOTHERMAL CHARACTERIZATION OF NANOCOMPOSITES BASED ON HIGH DENSITY POLYETHYLENE FILLED WITH EXPANDED GRAPHITE: PARTICLE SIZE AND SHAPE EFFECTS

This work aimed at thermal transport characterization of high density polyethylene (HDPE) filled with two sizes (5 and 50 mu m) of expanded graphite (EG) particles. Sample platelets were produced by melt mixing followed by compression molding. Thermal conductivity k was determined by combining measurements of density, specific heat capacity and thermal diffusivity. For the latter, we used the self-checking, non-contact method of photothermal radiometry (PTR) in back detection configuration. Starting from an effective medium approximation model, we derived a simple linearized expression for the effective k of composites with low particle charge. It explains the unusually high experimental k values (up to four-fold increase) as the effect the strongly non-spherical EG particles (aspect ratio 1/p = 110 - 290). Larger particle sizes produce higher k enhancement, while the interfacial thermal resistance (R-bd = 2.1.10(-7) m(2).K/W) has an opposite effect on k. The eventual deviation of experimental k from the model at high particle charge is possibly due to limitation of interparticle free space preventing random orientation of high aspect ratio particles