Laser processing of bulk Al–12Si alloy: influence of microstructure on thermal properties

The feasibility of enhancing the thermal conductivity of an alloy via microstructural refinement was examined using Al–12%Si alloy as a model material. Al–12Si alloy samples were fabricated at different process parameters using laser engineered net shaping (LENS?) and the effect of microstructural features on the thermal conductivity was studied and compared with conventionally cast alloy. The large difference in melting points and laser light absorptivities of Si and Al as well as the low melt viscosity of Al–12Si alloy resulted in a very small process window to successfully fabricate bulk Al–12Si alloy samples using LENS?. Comparison of microstructural features of laser-processed samples with cast Al–12Si alloy showed significant refinement in eutectic Si for laser processed samples. Microstructural refinement not only improved the thermal conductivity of Al–12Si alloy but also compensated the detrimental effect of porosity on thermal conductivity. The thermal conductivity of cast alloy varied between 82 and 93?W/mK, which is ～21–76% lower than the values exhibited by laser-processed samples in the range 103–153?W/mK. The results of LENS? fabrication, microstructural evolution and thermal properties of Al–12Si alloy bulk structures can be extended to other immiscible alloys and metal matrix composites for a variety of engineering applications.     

Normal-metal hot-electron bolometer with Andreev reflection from superconductor boundaries

This paper describes the design and experimental testing of a high-sensitivity hot-electron bolometer based a film of normal metal, exploiting the Andreev reflection from superconductor boundaries, and cooled with the help of a superconductor-insulator-normal metal junction. At the measured thermal conductivity, G≈6×10⁻¹² W/K, and a time constant of τ=0.2 μs, and a temperature of 300 mK, the estimated noise-equivalent power NEP=5×10⁻¹⁸ W/Hz^1/2, assuming that temperature fluctuations are the major source of noise. At a temperature of 100 mK, the thermal conductivity drops to G≈7×10⁻¹⁴ W/K, which yields NEP=2×10⁻¹⁹ W/Hz^1/2 at a time constant of τ=5 μs. The microbolometer has been designed to serve as a detector of millimeter and FIR waves in space-based radio telescopes. Zh. éksp. Teor. Fiz. 115, 1085–1092 (March 1999)     

Filler-matrix thermal boundary resistance of diamond-copper composite with high thermal conductivity

A composite material with a high thermal conductivity is obtained by capillary infiltration of copper into a bed of diamond particles of 400 μm size, the particles having been pre-coated with tungsten. The measured thermal conductivity of the composite decreases from 910 to 480 W m⁻¹ K⁻¹ when the coating thickness is increased from 110 to 470 nm. Calculations of the filler/matrix thermal boundary resistance R and the thermal conductivity of the coating layer λ _i using differential effective medium, Lichtenecker’s and Hashin’s models give similar numerical values of R and λ _i ≈ 1.5 W m⁻¹ K⁻¹. The minimal thickness of the coating h ∼ 100 nm necessary for ensuring production of a composite while maximizing its thermal conductivity, is of the same order as the free path of the heat carriers in diamond (phonons) and in copper (electrons). The heat conductance of the diamond/tungsten carbide coating/copper interface when h is of this thickness is estimated as (0.8–1) × 10⁸ W m⁻² K⁻¹ and is at the upper level of values characteristic for perfect dielectric/metal boundaries.     

Micro- and nanocomposite Ti-Al-N/Ni-Cr-B-Si-Fe-based protective coatings: Structure and properties

A new type of nanocomposite Ti-Al-N/Ni-Cr-B-Si-Fe-based coatings 70–90 μm thick produced by combined magnetron sputtering and a plasma detonation technology is created and studied. Phases Ti₃AlN + Ti₃Al₂N₂ and the phases caused by the interaction of plasma with a thick Al₃Ti + Ni₃Ti coating are detected in the coatings. The TiAlN phase has a grain size of 18–24 nm, and other phases has a grain size of 35–90 nm. The elastic modulus of the Ti-Al-N coating is E = 342 ± 1 GPa and its average hardness is H = 20.8 ± 1.8 GPa. The corrosion rate of this coating is very low, 4.8 μg/year, which is about three orders of magnitude lower than that of stainless steel (substrate). Wear tests performed according to the cylinder-surface scheme demonstrate high wear resistance and high adhesion between the thick and thin coatings.     

A new method for characterization of thermal properties of human enamel and dentine: Influence of microstructure

For better selection of “tooth-like” dental restorative materials, it is of great importance to evaluate the thermal properties of the human tooth. A simple method capable of non-destructively characterizing the thermal properties of the individual layers (dentine and enamel) of human tooth is presented. The traditional method of monotonic heating regime was combined with infrared thermography to measure the thermal diffusivities of enamel and dentine layers without physically separating them, with 4.08 (±0.178) × 10⁷ m²/s measured for enamel and 2.01 (±0.050) × 10⁷ m²/s for dentine. Correspondingly, the thermal conductivity was calculated to be 0.81 W/mK (enamel) and 0.48 W/mK (dentine). To examine the dependence of thermal conductivity on the configuration of dentine microstructure (microtubules), the Maxwell-Eucken and Parallel models of effective thermal conductivity are employed. The effective thermal conductivity of dentine in the direction parallel to tubules was found to be about 1.1 times higher than that perpendicular to the tubules, indicating weak anisotropy. By adopting the Series model, the bulk thermal conductivity of enamel and dentine layers is estimated to be 0.57 W/mK.     

Thermal Conductivity of poly-p-phenylene-2,6-benzobisoxazole Film along the In-Plane Axis in the 10–300 K Temperature Range

The thermal conductivities of compression molded thin films of poly-p-phenylene-2,6-benzobisoxazole (PBO) were measured in directions along an in-plane axis in the 10–300?K temperature range by a steady-state heat flow method, with interest in the use of the material for superconductivity applications. The thermal conductivities of the PBO films increased from 0.3?W/mK to 9.0?W/mK with increasing temperature from 10?K to 300?K and these were much higher than those of polyimide films, epoxy resin and glass fiber reinforced plastics at all temperatures. The 9.0?W/mK at 300?K was 60% of that of stainless steel (SUS304). It was 6?W/mK at 150?K, which was half that of SUS304 and was 3.3?W/mK at 77?K, which was 33% of that of SUS304. The thermal conductivities of the PBO films were lower than those of a cloth of high strength ultrahigh molecular weight polyethylene fiber reinforced plastics in the 30?K–180?K temperature range and were almost equivalent to its values in the 180?K–300?K temperature range. The main contribution to the thermal conduction in the PBO films was from thermal phonon conduction along the molecular chains. Although many kinds of high thermal conductivity polymeric materials have been prepared by a uni-directional drawing process or by adding high thermal conductive additives, the PBO film showed high thermal conductivity without a uni-directional drawing process or high thermal conductive additive.     

Modeling and analysis of birefringence in magneto-optical thin film made by SiO2/ZrO2 doped with ferrite of cobalt

We report the device characteristics of the metal–dielectric high-reflectivity (HR) coated 1.55 μm laterally coupled distributed feedback (DFB) laser with metal surface gratings by using holographic lithography. The HR coating films are composed of Au/Ti/SiO₂. It provides a variety of advantages compared to the uncoated DFB laser on the same processed wafer while there is no degradation on current–voltage characteristics. For 3 μm wide and 300 μm long HR coated DFB laser, it exhibits a maximum output power of ∼17 mW and a threshold current of 14.2 mA at 20°C under continuous-wave mode. It is clear that the threshold current and slope efficiency are improved by 36% and 96%, respectively, due to the reduction of mirror loss. The metal–dielectric HR coating on one facet of DFB laser is found to have significantly increased characteristic temperature (i.e., T ₀∼88 K). Furthermore, the stable single-mode operation with an increased single-mode suppression ratio was achieved.     

320×256 HgCdTe IR FPA with a built-in shortwave cut-off filter

A photovoltaic detector design based on the graded band gap HgCdTe MBE structure with high conductivity layer (HCL) at interface, which provides photodiodes series resistance and a shortwave cut.off filter is developed. The optimal HCL parameters giving high quantum efficiency and minimal noise equivalent temperature difference were determined by calculations and experimentally confirmed. The hybrid 320×256 IR FPA operating in 8–12 μm spectral range was fabricated. The threshold power responsivity and minimal noise equivalent temperature difference values at wavelength maximum were 1.02×10⁻⁷ W/cm², 4.1×10⁸ V/W and 27 mK, respectively.     

Diffusional mechanism of deintercalation in LiFe1 − yMnyPO4 cathode material

The paper presents the investigations on the structural, electrical and electrochemical properties of Mn substituted phospho-olivines LiFe_1 − yMn_yPO₄ and of W, Ti or Al doped LiFePO₄. The microscopic nature of the observed macroscopic, metallic-like conductivity of W, Ti, Al doped phospho-olivine samples is discussed. Some fundamental arguments against the bulk type conductivity are presented.A single phase, diffusional mechanism of deintercalation was found to appear for Mn-substituted LiFe_1 − yMn_yPO₄ samples in the whole range of lithium concentration, in contrast to the pure LiFePO₄, LiMnPO₄ and W, Ti, Al doped phospho-olivines, where a two-phase mechanism of electrochemical lithium extraction/insertion is observed.     

Thermal analysis of InP-based quantum cascade lasers for efficient heat dissipation

We have theoretically investigated the thermal characteristics of double-channel ridge–waveguide InGaAs/InAlAs/InP quantum cascade lasers (QCLs) using a two-dimensional heat dissipation model. The temperature distribution, heat flow, and thermal conductance (G _th) of QCLs were obtained through the thermal simulation. A thick electroplated Au around the laser ridges helps to improve the heat dissipation from devices, being good enough to substitute the buried heterostructure (BH) by InP regrowth for epilayer-up bonded lasers. The effects of the device geometry (i.e., ridge width and cavity length) on the G _th of QCLs were investigated. With 5 μm thick electroplated Au, the G _th is increased with the decrease of ridge width, indicating an improvement from G _th=177 W/K⋅cm² at W=40 μm to G _th=301 W/K⋅cm² at W=9 μm for 2 mm long lasers. For the 9 μm×2 mm epilayer-down bonded laser with 5 μm thick electroplated Au, the use of InP contact layer leads to a further improvement of 13% in G _th, and it was totally raised by 45% corresponding to 436 W/K⋅cm² compared to the epilayer-up bonded laser with InGaAs contact layer. It is found that the epilayer-down bonded 9 μm wide BH laser with InP contact layer leads to the highest G _th=449 W/K⋅cm². The theoretical results were also compared with available obtained experimentally data.     

Nuclear orientation at isolde/cern

A facility for Nuclear Implantation into Cold On-Line Equipment (NICOLE) is being installed at the new on-line isotope separator ISOLDE 3 at CERN. The first on-line run was in the beginning of July 1988. The low temperature equipment has been successfully tested and first off-line experiments on various isotopes have been performed. NMR/ON has been done on vaious isotopes (Co, Xe, Pt, Au) in iron host. First experience with the top-loading dilution refrigertor (Oxford Instruments Limited) shows that it performs very well. The cooling power is 400 μW at 100 mK and 34 μW at 25 mK. The base temperature can be kept continuously well below 5 mK. NMR/ON can be performed at temperatures below 5.5 mK. The base temperature on-line is expected to be lower then 6 mK. The sample can be cooled down from room temperature to 10 mK within two hours, to 6 mK within 3 hours which is not only important for off-line but also for on-line experiments when samples have to be changed to remove long lived daughter activity. The latest results will be reported.     

Laser surface modification of Al–4Cu–1Mg alloy for enhanced thermal conductivity

The feasibility of enhancing thermal conductivity of Al–4Cu–1Mg alloy by depositing 80Cu–20Mo coating using high-power lasers has been examined. Coatings of 667±2.5 μm thickness were formed with metallurgically sound interface. Results showed an 86% increase in the thermal conductivity of Al–4Cu–1Mg alloy due to laser-deposited 80Cu–20Mo alloy coating. This coating approach can potentially be used on low coefficient of thermal expansion metal matrix composites to enhance their thermal conductivity in electronic devices.     

Thermal conductivity of thermoelectric Al‐substituted ZnO thin films

ZnO:Al thin films with a low electrical resistivity were grown by magnetron sputtering on sapphire substrates. The cross‐plane thermal conductivity (κ = 4.5 ± 1.3 W/mK) at room temperature is almost one order of magnitude lower than for bulk materials. The thermoelectric figure of merit ZT at elevated temperatures was estimated from in‐plane power factor and the cross‐plane thermal conductivity at room temperature. It is expected that the thermal conductivity drops with increasing temperature and is lower in‐plane than cross‐plane. Consequently, the thin film ZT is at least three times higher than for bulk samples at intermediate temperatures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the ⁴ F _3/2→⁴ F _11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.     

Structural,morphological and thermoelectric properties of self-decorated copper selenide nanosheets synthesized at room temperature

In this work, an economical, surfactant-free and scalable solution synthesis method at room temperature for self-decorated copper selenide (Cu_2-xSe) nanosheets is reported. Structural and morphological characterizations clearly revealed the formation of single cubic phase Cu_2-xSe nanosheets in nearly stoichiometric ratio. The tentative mechanism for fabrication of self-decorated Cu_2-xSe nanosheets was proposed. Furthermore, nanostructured bulk Cu_2-xSe by hot pressing was explored for thermoelectric performance. High electrical conductivity (1.1 × 10⁵ S/m), moderate Seebeck coefficient (87 μV/K) and low thermal conductivity (1.11 W/mK) at 753 K were obtained. The figure of merit (ZT) ~ 0.56 and power factor (PF) ~ 860 μW/mK² at 753 K showed better performance than some reported Cu_2-xSe nanostructured or bulk counterparts under same temperature. Also, theoretically device ZT ~0.16 and efficiency up to 3% could be achieved. The results indicate that this green and novel synthesis process is an alternative to other reported time or energy consuming processes.     

High-efficiency mid-infrared laser from synchronous optical parametric oscillation and amplification based on a single MgO:PPLN crystal

This paper presents a specially designed optical parametric oscillator (OPO) which achieved high-efficiency mid-infrared laser of 2.83 μm. The cascaded nonlinear interactions of OPO and optical parametric amplifier (OPA) were simultaneously realized in a single MgO:PPLN crystal. The signal oscillation of 1.70 μm was used to pump a secondary parametric process that resulted in amplification of the idler laser of 2.83 μm. When the MgO:PPLN crystal with a grating period of 31.2 μm was pumped by a 1.064 μm laser and operated at 148°C, the quasi-phase-matching of both OPO and OPA could be simultaneously achieved. Average output power of 7.68 W at 2.83 μm was obtained for 25 W of pump at 7 kHz. The power conversion efficiency of 2.83 μm laser was 30.7%, which was evidently higher than common OPOs.     

Thermal conductivity of the diamond-paraffin wax composite

The thermal conductivity of diamond-paraffin wax composites prepared by infiltration of a hydrocarbon binder with the thermal conductivity λ _m = 0.2 W m⁻¹ K⁻¹ into a dense bed of diamond particles (λ _f ∼ 1500 W m⁻¹ K⁻¹) with sizes of 400 and 180 μm has been investigated. The calculations using universally accepted models considering isolated inclusions in a matrix have demonstrated that the best agreement with the measured values of the thermal conductivity of the composite λ = 10–12 W m⁻¹ K⁻¹ is achieved with the use of the differential effective medium model, the Maxwell mean field scheme gives a very underestimated calculated value of λ, and the effective medium theory leads to a very overestimated value. An agreement between the calculation and the experiment can be provided by constructing thermal conductivity functions. The calculation of the thermal conductivity at the percolation threshold has shown that the experimental thermal conductivity of the composites is higher than this critical value. It has been established that, for the composites with closely packed diamond particles (the volume fraction is ∼0.63 for a monodisperse binder), the use of the isolated particle model (Hasselman-Johnson and differential effective medium models) for calculating the thermal conductivity is not quite correct, because the model does not take into account the percolation component of the thermal conductivity. In particular, this holds true for the calculation of the heat conductance of diamond-matrix interfaces in diamond-metal composites with a high thermal conductivity.     

Thermal properties of AlN-based atom chips

We have studied the thermal properties of atom chips consisting of high thermal conductivity aluminum nitride (AlN) substrates on which gold microwires are directly deposited. We have measured the heating of wires of several widths and with different thermal couplings to the copper mount holding the chip. The results are in good agreement with a theoretical model where the copper mount is treated as a heat sink and the thermal interface resistance between the wire and the substrate is vanishing. We give analytical formulas describing the different transient heating regimes and the steady state. We identify criteria to optimize the design of a chip as well as the maximal currents I_c that can be fed in the wires. For a 600 μm thick-chip glued on a copper block with Epotek H77, we find I_c = 16 I_c = 16~A for a 3 μm high, 200 μm wide-wire.     

Stoichiometry of BaTiO3 nanoparticles

Barium titanate nanoparticles with various nominal Ba/Ti ratios were prepared through direct synthesis from solution (DSS) and further annealed at different temperatures. Their deviation from stoichiometry was studied through XRD analysis, and a large deviation from stoichiometry has been observed. The grain size we studied ranges from 50 nm to 1 μm. For the as-prepared particles, the grain size is about 50 nm, and the maximum excess of Ti is over 15%. For the samples annealed at 800 °C, the grain size is increased to 100 nm, and the maximum excesses of Ba and Ti are 8 and 9%, respectively. The defects formed during synthesis and surface effect of nanoparticles are both estimated for their contributions to the deviation from stoichiometry, and the defects have been found to be mainly responsible for the large deviation observed.     

2 μm single-frequency Tm:YAG laser generated from a diode-pumped L-shaped twisted mode cavity

A 2 μm single-frequency Tm:YAG laser was developed by using a diode pumped L-shaped twisted-mode-cavity. By suppressing the spatial hole-burning, the 2 μm single-frequency laser was obtained, with the output power of 1.46 W and the slope efficiency of 19.2%.