X-ray sources are used for both scientific instrumentation and inspection applications. In X-ray photoelectron spectroscopy (XPS), aluminum Kα X-rays are generated through electron beam irradiation of a copper-based X-ray anode incorporating a thin surface layer of aluminum. The maximum power operation of the X-ray anode is limited by the relatively low melting point of the aluminum. Hence, optimization of the materials and design of the X-ray anode to transfer heat away from the aluminum thin film is key to maximizing performance. Finite element analysis (FEA) has been employed to model the heat transfer of a water-cooled copper-based X-ray anode with and without the use of a chemical vapor deposited (CVD) diamond heat spreader. The modeling approach was to construct a representative baseline model, and then to vary different parameters systematically, solving for a steady-state thermal condition, and observing the effect on the maximum temperature attained. The model indicates that a CVD diamond heat spreader (with isotropic thermal properties) brazed into the copper body reduces the maximum temperature in the 4 μ m aluminum layer from 613 °C to 301 °C. Introducing realistic anisotropy and inhomogeneity in the thermal conductivity (TC) of the CVD diamond has no significant effect on heat transfer if the aluminum film is on the CVD diamond growth face (with the highest TC). However, if the aluminum layer is on the CVD diamond nucleation face (with the lowest TC), the maximum temperature is 575 °C. Implications for anode design are discussed.

Enhancing of boiling heat transfer by combining the electrohydrodynamic (EHD) effect and surface wettability has been shown to remove the high heat fluxes from electrical devices such as laser diodes, light emitting diodes, and central processing units. However, this phenomenon is not well understood. Our previous studies on the critical heat flux (CHF) of pool boiling have shown that CHF greatly increases with the application of an electric field and that the wall temperature can be decreased to a level with the safe operation of the electrical devices by using a low contact angle with the boiling surface. To verify the earlier prediction model, CHF enhancement by changing the contact angle with the boiling surface and by the application of an electric field was investigated. A fluorinated dielectric liquid (Asahi Glass Co. Ltd, Tokyo, Japan, AE-3000) was selected as the working fluid. To allow the contact angle between the boiling surface and the dielectric liquid to be changed, several different materials (Cu, Cr, NiB, Sn) and a surface coated with a mixture of 1.5 and 5 μ m diamond particles were used as boiling surfaces. The CHFs at different contact angles were 20.5–26.9 W/cm 2 , corresponding to 95–125% of that for a polished Cu surface (21.5 W/cm 2 ). Upon application of a −5 kV/mm electric field to the microstructured surface (the mixture of 1.5 μ m and 5 μ m diamond particles), a CHF of 99 W/cm 2 at a superheat of 33.5 K was obtained. Based on this experimental evidence, we normalized the CHF and contact angle using our previously developed hydrodynamic instability model and semi-empirical model derived from the interfacial area density close to the boiling surface. This procedure allowed us to develop a general model that predicted CHF well, including the CHF for the de-ionized (DI) water.

Smooth DLC (diamond-like carbon) coated surfaces can profoundly mitigate scaling during pool boiling of calcium sulphate solutions. Previous investigations though carried out mostly for the smooth surfaces rather than structured, i.e., finned tubes. This study compares experimental results of DLC coated smooth and finned tubes at clean and fouling conditions. Fouling runs were conducted during pool boiling of saturated CaSO 4 solution of 1.6 g/L at 300 kW/m 2 . The substrate of the attempted tubes was stainless steel and finned tubes of 19 and 40 fins per inch were used. The DLC coated smooth tube showed an enhanced clean heat transfer up to 50% and reduced fouling resistance compared to the uncoated smooth tube. After a short operating time, though, the coated smooth tube reached an asymptotic fouling resistance of 0.00005 m 2 K/W whereas for the uncoated smooth tube, it was 4.8 times higher. DLC coating of the finned tubes with the physical vapor deposition (PVD) technique implicated difficulties. The base surface of the finned tubes was defectively coated. The defectiveness of the coating was attributed to the limitation of the PVD for coating of structured surfaces.

Boiling heat transfer enhancement via compound effect of the electrohydrodynamic (EHD) effect and microstructured surfaces has been experimentally and analytically investigated. A fluorinated dielectric liquid (Asahi Glass Co., Ltd., AE-3000) was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. Microstructured surfaces, which are mainly used for cutting tools, were developed with diamond particles using electrodeposition technique. Four different particle diameters were prepared: 5, 10, 15, and a mixture of 5 and 1.5 μm. The critical heat flux (CHF) for diamond particle surfaces showed 27–30 W/cm2 which was 26–40% increase for comparing with a noncoated surface (21.5 W/cm2). Upon application of a −5 kV/mm electric field to the microstructured surface (a mixture of 5 and 1.5 μm particles), a CHF of 70.2 W/cm2 at a superheat of 21.7 K was obtained. The previous theoretical equation of pool boiling predicted the CHF with electric field and without the electrode within 10%. Also, the CHF enhanced by the diamond coated surfaces was correlated well with the contact angle.

Silicon films of submicrometer thickness play a central role in many advanced technologies for computation and energy conversion. Numerous thermal conductivity data for silicon films are available in the literature, but they are mainly for the lateral, or in-plane, direction for both polycrystalline and single crystalline films. Here, we use time-domain thermoreflectance (TDTR), transmission electron microscopy, and semiclassical phonon transport theory to investigate thermal conduction normal to polycrystalline silicon (polysilicon) films of thickness 79, 176, and 630 nm on a diamond substrate. The data agree with theoretical predictions accounting for the coupled effects of phonon scattering on film boundaries and defects related to grain boundaries. Using the data and the phonon transport model, we extract the normal, or cross-plane thermal conductivity of the polysilicon (11.3 ± 3.5, 14.2 ± 3.5, and 25.6 ± 5.8 W m−1 K−1 for the 79, 176, and 630 nm films, respectively), as well as the thermal boundary resistance between polysilicon and diamond (6.5–8 m2 K GW−1) at room temperature. The nonuniformity in the extracted thermal conductivities is due to spatially varying distributions of imperfections in the direction normal to the film associated with nucleation and coalescence of grains and their subsequent columnar growth.

Thermal analysis of planar and near-square semiconductor device chips employing angular Fourier-series (AFS) expansion is presented for the first time. The determination of the device peak temperature using AFS requires only a single two-dimensional computation, while full three-dimensional temperature distribution can be obtained, if desired, by successively adding higher-order Fourier terms, each of which requires a separate 2D computation. The AFS method is used to compare the heat spreading characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on silicon, silicon carbide, and synthetic diamond. We show that AlGaN/GaN HEMTs built using GaN/diamond technology can offer better than half the thermal resistance of GaN/SiC HEMTs under worst-case cooling conditions. Furthermore, we show that, if left unmanaged, an inherent and non-negligible thermal boundary resistance due to the integration of semiconductor epilayers with non-native substrates will dampen the benefits of highly conductive substrates such as SiC and diamond.

This paper quantifies the influence of diamond nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a roughened, horizontal, and flat surface. Nanofluids are liquids that contain dispersed nanosize particles. A lubricant based nanofluid (nanolubricant) was made by suspending 10 nm diameter diamond particles in a synthetic ester to roughly a 2.6% volume fraction. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) up to 129%. A similar enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 91% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in boiling heat transfer degradation of approximately 19% for the best performing tests. It was speculated that the poor quality of the nanolubricant suspension caused the performance of the (99.5/0.5), and the (98/2) nanolubricant mixtures to decay over time to, on average, 36% and 76% of the of pure R134a/polyolester performance, respectively. Thermal conductivity and viscosity measurements and a refrigerant\lubricant mixture pool-boiling model were used to suggest that increases in thermal conductivity and lubricant viscosity are mainly responsible for the heat transfer enhancement due to nanoparticles. Particle size measurements were used to suggest that particle agglomeration induced a lack of performance repeatability for the (99.5/0.5) and the (98/2) mixtures. From the results of the present study, it is speculated that if a good dispersion of nanoparticles in the lubricant is not obtained, then the agglomerated nanoparticles will not provide interaction with bubbles, which is favorable for heat transfer. Further research with nanolubricants and refrigerants are required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.

Continued reduction in characteristic dimensions in nanosystems has given rise to increasing importance of material interfaces on the overall system performance. With regard to thermal transport, this increases the need for a better fundamental understanding of the processes affecting interfacial thermal transport, as characterized by the thermal boundary conductance. When thermal boundary conductance is driven by phononic scattering events, accurate predictions of interfacial transport must account for anharmonic phononic coupling as this affects the thermal transmission. In this paper, a new model for phononic thermal boundary conductance is developed that takes into account anharmonic coupling, or inelastic scattering events, at the interface between two materials. Previous models for thermal boundary conductance are first reviewed, including the diffuse mismatch model, which only considers elastic phonon scattering events, and earlier attempts to account for inelastic phonon scattering, namely, the maximum transmission model and the higher harmonic inelastic model. A new model is derived, the anharmonic inelastic model, which provides a more physical consideration of the effects of inelastic scattering on thermal boundary conductance. This is accomplished by considering specific ranges of phonon frequency interactions and phonon number density conservation. Thus, this model considers the contributions of anharmonic, inelastically scattered phonons to thermal boundary conductance. This new anharmonic inelastic model shows improved agreement between the thermal boundary conductance predictions and experimental data at the Pb/diamond and Au/diamond interfaces due to its ability to account for the temperature dependent changing phonon population in diamond, which can couple anharmonically with multiple phonons in Pb and Au. We conclude by discussing phonon scattering selection rules at interfaces and the probability of occurrence of these higher order anharmonic interfacial phonon processes quantified in this work.

It is shown that addition of alumina, zinc-oxide, and diamond particles can enhance the critical heat flux (CHF) limit of water in flow boiling. The particles used here were in the nanometer range ( < 100 nm ) and at low concentration ( ≤ 0.1 vol % ) . The CHF tests were conducted at 0.1 MPa and at three different mass fluxes ( 1500 kg / m 2 s , 2000 kg / m 2 s , and 2500 kg / m 2 s ). The thermal conditions at CHF were subcooled. The maximum CHF enhancement was 53%, 53%, and 38% for alumina, zinc oxide, and diamond, respectively, always obtained at the highest mass flux. A postmortem analysis of the boiling surface reveals that its morphology is altered by deposition of the particles during boiling. Additionally, the wettability of the surface is substantially increased, which seems to correlate well with the observed CHF enhancement.

An experimental investigation of a nanofluid oscillating heat pipe (OHP) was conducted to determine the nanofluid effect on the heat transport capability in an OHP. The nanofluid consisted of HPLC grade water and 1.0 vol % diamond nanoparticles of 5 - 50 nm . These diamond nanoparticles settle down in the motionless base fluid. However, the oscillating motion of the OHP suspends the diamond nanoparticles in the working fluid. Experimental results show that the heat transport capability of the OHP significantly increased when it was charged with the nanofluid at a filling ratio of 50%. It was found that the heat transport capability of the OHP depends on the operating temperature. The investigated OHP could reach a thermal resistance of 0.03 ° C ∕ W at a heat input of 336 W . The nanofluid OHP investigated here provides a new approach in designing a highly efficient next generation of heat pipe cooling devices.

This work is a numerical and experimental investigation of the effect of the use of a metallic absorption layer on the laser-based measurements of the thermal conductivity of dielectric, semiconductor, and highly-conductive materials. The specific experimental studies, which were carried out on silicon dioxide samples, were used to validate the numerical approach and to support the findings of this investigation. The numerical and supporting experimental results reveal the presence of behaviors associated with thermally thin and thermally thick absorption layers, depending on the ratio between the thickness of the absorption layer and the heat penetration depth. It is concluded that the TTR method performs optimally when the thickness of the metalization layer falls in the transition range between the identified thermally thin and thermally thick layers.

Effects of surface-mounted obstacles on the local heat transfer enhancement of a base plate are investigated by using transient liquid crystal thermograph technique. To explore the geometry effects of short obstacles, the height less than one hydraulic diameter (d), three cross-sectional shapes of obstacles, i.e., circular, square and diamond, with variations in number of obstacles, obstacle spacing, and free-stream Reynolds number are considered. The maximum number of the obstacles in tandem array is 3 and the spacing between obstacles is 1d, 2d, or 4d. The free-stream Reynolds number ranges from 2100 to 4200. The experimental results reveal that the local heat transfer enhancement in front of leading circular and square obstacles are better than the diamond one, while the influenced area by the obstacle of the diamond shape is most remarkable. The present results disclose that an intermediate height (0.5d) of the protruding elements is more beneficial to the heat transfer enhancement in wake of the obstacle. With the sweepback leading edge of the top surface, the diamond and circular obstacles produce vortical flow across the obstacles and thus enhance heat transfer downstream in wake. Increasing Reynolds number leads to an enhancement in heat transfer performance. The number of and the spacing between the obstacles in tandem array are also influential factors to the flow structure and heat transfer enhancement on the basic plate.

The measured thermal resistance across a thin film deposited on a substrate often includes the internal thermal resistance within the film and the thermal boundary resistance (TBR) across the film-substrate interface. These two resistances are frequently lumped and reported as an equivalent thermal conductivity of the film. Two fundamental questions should be answered regarding the use of this equivalent thermal conductivity. One is whether it leads to the correct temperature distribution inside the film. The other one is whether it is applicable for thin films with internal heat generation. This paper presents a study based on the Boltzmann transport equation (BTE) to treat phonon heat conduction inside the film and across the film-substrate interface simultaneously, for the cases with and without internal heat generation inside the film. Material systems studied include SiO 2 and diamond films on Si substrates, representative of thin-film materials with low and high thermal conductivity. It is found that for a SiO 2 film on a Si substrate, the film thermal conductivity and TBR can be treated independently, while for a diamond film on a Si substrate, the two are related to each other by the interface scattering. When the free surface behaves as a black phonon emitter, the TBR for thin diamond films with internal heat generation is the same as that without the internal heat generation. When the free surface is adiabatic, however, the TBR increases and approaches the value of the corresponding black surface as the film thickness increases. Results of this study suggest that great care must be taken when extending the effective thermal conductivity measured for thin films under one experimental condition to other application situations.

Results from an experimental study on the rapid measurement of thermal conductivity of chemical vapor deposited (CVD) diamond films are presented. The classical thermal comparator method has been used successfully in the past for the measurement of thermal conductivity of bulk materials having high values of thermal resistance. Using samples of known thermal conductivity, a calibration curve is prepared. With this calibration curve, the comparator can be used to determine thermal conductivity of unknown samples. We have significantly modified and extended this technique for the measurement of materials with very low thermal resistance, i.e., CVD diamond films with high thermal conductivity. In addition to the heated probe, the modified comparator employs a thermoelectric cooling element of increase conductive heat transfer through the film. The thermal conductivity measurements are sensitive to many other factors such as the thermal contact resistances, anisotropic material properties, surrounding air currents and temperature, and ambient humidity. A comprehensive numerical model was also developed to simulate the heat transfer process for the modified comparator. The simulations were used to develop a “numerical” calibration curve that agreed well with the calibration curve obtained from our measurements. The modified method has been found to successfully measure the thermal conductivity of CVD diamond films. [S0022-1481(00)00804-5]

Diamond heat spreaders are very attractive for lowering the temperature of laser diodes and computer chip components. The use of diamonds as heat spreaders is very advantageous because its thermal conductivity is so large, about four times that of copper. The diamond heat spreader is mounted on a semi-infinite heat sink. The purposes of this paper are to present (a) a surface element method for the analysis of such composite systems, (b) a set of convenient algebraic equations for the maximum temperatures, (c) optimal geometry conditions and (d) some accurate numerical results. The analysis method is an adaptation of the unsteady surface element method but is different because the present problem is a steady-state one. The surface element method with one node gives a relatively simple algebraic solution, which contains all the important dimensionless groups. The one-node solution is simple, accurate, and has a form that can give deep insight into the effects of various parameters; for example, it premits derivation of the optimal geometry corresponding to the minimal temperature at the hot spot. The study shows that the optimal geometry of the thickness to radius aspect ratio of the diamond heat spreaders is about 0.4. In addition to the single-node analysis, a multinode analysis is developed and very accurate results are presented. These results show that the single-node analysis is generally within 2 percent.