bluuebunny/crossref_metadata_2025_split huggingface dataset:
Influence of Doping Concentration and Ambient Temperature on the Cross-Plane Seebeck Coefficient of InGaAs/InAlAs superlattices
Authors: Zhang Yan , Vashaee Daryoosh , Singh Rajeev , Shakouri Ali , Zeng Gehong , Chiu Yi-Jen 
Published: April 2011

ABSTRACTWe have developed thin film heaters/sensors that can be integrated on top of superlattice microcoolers to measure the Seebeck coefficient perpendicular to the layer. In this paper, we discuss the Seebeck coefficients of InGaAs/InAlAs superlattices grown with Molecular Beam Epitaxy (MBE) that have different doping concentrations, varying between 2e18, 4e18, and 8e18 to 3e19 cm−3. It was interesting to find out that – contrary to the behavior in bulk material – the Seebeck coefficient did not decrease monotonically with doping concentration. A preliminary theory of thermoelectric transport in superlattices in the regime of miniband formation has been developed to fit the experimental results. The miniband formation could enhance the thermoelectric power factor (Seebeck coefficient square times electrical conductivity) and thereby improve the Figure of merit, ZT. With this improvement, InGaAs/InAlAs superlattice microcooler become a promising candidate for on-chip temperature control.
Thermoelectric Properties of Bi-substituted Ca3Co4O9 Single Crystal
Authors: Mikami M. , Chong K. , Funahashi R. 
Published: April 2011

ABSTRACTWe have grown single crystals of Bi-substituted Ca3Co4O9 by a solution method. The cationic ratio (Ca, Bi)/Co of the grown crystals measured by an energy dispersive X-ray spectrometer tended to exceed that of the starting ratio (Ca, Bi)/Co=3/4. For instance, the average cationic composition of the grown crystals was Ca:Bi:Co=3.3:0.3:4, while that of the starting material was Ca:Bi:Co=2.7:0.3:4. So, the crystallographic structure of the obtained crystals may correspond to the Ca2Co2O5 phase rather than the Ca3Co4O9 phase. Thermoelectric properties in the direction of ab-axis were measured at various temperatures. Seebeck coefficient (S) of Ca3.3Bi0.3Co4O9+δ is positive and increases with increasing temperature from 130 to 200 μV/K in a temperature region of 300–973 K. The electrical resistivity (ρ) of the sample is about 1.5 mΩcm at whole temperature region of 300–973 K. This value is lower than that of non-substituted Ca3Co4O9. The thermoelectric power factor (S2/ρ) is improved by the Bi-substitution, resulting from the reduction of resistivity.
Phonons and Thermal Conductivity in Skutterudite Thermoelectrics
Authors: Kendziora C. A. , Nolas G. S. 
Published: April 2011

ABSTRACTWe study Raman phonon vibrations and relate them to thermal conductivity for empty and filled skutterudites designed for thermoelectric applications, where low thermal conduction is critical. Polarized Raman scattering spectra of crystallite and polycrystalline samples are compared with theoretical predictions and analyzed in comparison to the thermal conduction properties. Our emphasis is on the CoSb3 skutterudite and its filled derivatives, including materials with Ge, Sn, and La in the cages. We observe a strong correlation between aspects of the Raman spectrum and low thermal conductivity. This presents Raman spectroscopy as a characteristic screening tool for potential thermoelectrics and is a crucial step toward predicting lattice thermal conductivities.
Theoretical Thermal Conductivity of Periodic Two-Dimensional Nanocomposites
Authors: Yang Ronggui , Chen Gang 
Published: April 2011

ABSTRACTA phonon Boltzmann transport model is established to study the lattice thermal conductivity of nanocomposites with nanowires embedded in a host semiconductor material. Special attention has been paid to cell-cell interaction using periodic boundary conditions. The simulation shows that the temperature profiles in nanocomposites are very different from those in conventional composites, due to ballistic phonon transport at nanoscale. The thermal conductivity of periodic 2-D nanocomposites is a strong function of the size of the embedded wires and the volumetric fraction of the constituent materials. At constant volumetric fraction the smaller the wire diameter, the smaller is the thermal conductivity of periodic two-dimensional nanocomposites. For fixed silicon wire dimension, the lower the atomic percentage of germanium, the lower the thermal conductivity of the nanocomposites. The results of this study can be used to direct the development of high efficiency thermoelectric materials.
Synthesis and High Temperature Thermoelectric Properties of Alkaline-Earth Metal Hexaborides MB6 (M=Ca, Sr, Ba)
Authors: Takeda Masatoshi , Kurita Yosuke , Yokoyama Keisuke , Miura Takahiro , Suzuki Tsuneo , Suematsu Hisayuki , Jiang Weihua , Yatsui Kiyoshi 
Published: April 2011

ABSTRACTPolycrystalline alkaline-earth hexaborides (MB6: M =Ca, Sr, Ba) were synthesized and their thermoelectric and transport properties were examined to discuss their possibility as high temperature thermoelectric materials. Hall measurements showed that carrier concentration of the BaB6 was the highest among the three hexaborides and that of CaB6 was the lowest. Substitution of part of the alkaline earth metals with one of the others changed the carrier concentration of the hexaboride. As the carrier concentration increased, Seebeck coefficient increased and electrical conductivity decreased. These results suggest that the thermoelectric properties of the divalent hexaborides depend largely on the carrier concentration, and optimum carrier concentration which gives maximum power factor was estimated to be approximately 2x1026 m−3. Consequently, such a substitution enables us to control Seebeck coefficient and electrical conductivity of the hexaborides, and will also be effective to reduce the lattice heat conduction due to the alloying effect. A thermoelectric device was fabricated using SrB6 and boron carbide thin films as n-type and p-type elements, respectively. To the best of our knowledge, this is the first demonstration of a thermoelectric device composed of only boron-rich solids.
Evidence of a Order-Disorder Transition in the Crystalline Phase of Cd6Yb, 1/1 cubic approximant of icosahedral Cd 5.7 Yb
Authors: Sorloaica N. , Pope A. L. , Winkler D. W. , Tritt Terry M. , Keppens V. , Mandrus D. , Sales B. 
Published: April 2011

ABSTRACTThe electrical and thermal transport properties (electrical resistivity, thermopower, heat capacity and thermal conductivity) of the crystalline phase of the binary Cd6Yb system has been measured over a temperature range from 10 K to 300 K. Evidence for a phase transition in Cd6Yb is observed in the electrical transport properties with distinct changes in the temperature dependence of the resistivity and thermopower around T ≈ 110K. An anomaly in the heat capacity and a thermal conductivity is also observed at this same temperature. Hysteretic behavior is not evident in the temperature dependence of any of the electrical and thermal transport properties. In addition, the elastic properties using resonant ultrasound (RUS) techniques have been investigated over a similar temperature range. A large “resonance dip” is observed in the RUS data at T ≈ 110K, which is indicative of some type of structural change in the crystalline material at this temperature. These data will be presented and discussed in context of the undergoes reversible order-disorder transition in the 1/1 cubic approximant at about 110 K, which makes the system very interesting compared to the quasicrystal phase Cd5.7Yb
Performance of Integrated Thin-film Thermoelectrics in cooling “hot-spots” on Microprocessors – Experimental setup and Results
Authors: Cancheevaram Jai , Alley Randy , Siivola Edward , Venkatasubramanian Rama 
Published: April 2011

ABSTRACTThe improving performance of microprocessors is increasing the Thermal Design Power [TDP] and cause localized power densities commonly referred to as “Hot-spots”, which reach in excess of 200 W/cm2. The focus of this work is to present Thin-film Superlattice Thermoelectric materials [TFST] as a solution for microprocessor die hot spot cooling. TFST have measured ZT values of ∼2 at 300K, response times of the order of 15μs and potential to pump heat flux of several hundred Watts/cm2. TFST devices appear suitable for managing the junction temperatures at such hot spots such that the recommended maximum operational temperature, Tjmax, is not exceeded. This promotes stable processor performance and increased reliability. To test this theory, hot spots were identified on the microprocessor die by infrared (IR) imaging during operation and a suitable TFST module was mounted onto the processor. Thermal transients at the hot spot show that active heat pumping and spreading by TFST significantly reduces the constraints on the thermal design. This investigation suggests that in the future, with the transition of Superlattice material performance into higher module ZT values; these hot spots may be effectively removed.
High Temperature Measurement System Design for Thermoelectric Materials In Power Generation Application
Authors: Loo Sim , Short Jarrod , Hsu Kuei Fang , Kanatzidis Mercouri , Hogan Tim 
Published: April 2011

ABSTRACTRecent interest in thermoelectric materials for power generation applications has initiated the development of a measurement system in our laboratory for characterization of materials in the 80K to 800K temperature range. This system has been specifically designed for measuring thermoelectric power and electrical conductivity as needed for determining the power factor of the measured samples. This is a single sample measurement system based on a continuous flow cryostat. Significant effort has gone into the computer controlled data acquisition and PID controlled temperature stabilization. Investigation of the influence of temperature stability on the measured data will be presented along with important aspects of the system design, development, and testing. Data collected on reference materials and new thermoelectric materials of interest will be presented.
Modelling and Optimal Design of Multilayer Cantilever Microactuators
Authors: Han L. H. , Lu T. J. 
Published: April 2011

ABSTRACTThermal expansion is an important actuation mechanism for microelectromechanical system (MEMS). We investigate multi-layered cantilever microactuators actuated by resistive heating with an electric current. The actuator consists of films and sandwiched microheaters. An electrothermal-elastic model for characterizing microactuators is developed. The general analytical expressions relating tip deflection with an applied electric field, temperature change, the film thickness and the applied load at the tip are obtained. The large deflection is considered in the model. The width effect of the cantilever is also investigated using 3D finite element analysis. Based on the analytical solutions and a mixed variable programming (MVP) algorithm, the optimal design of the actuators is presented. The algorithms optimize simultaneously with respect to the number of films and the material choice from a list of candidate materials. The minimum weight and maximum deflection are chosen as the design objectives.
Bulge Test on Free Standing Gold Thin Films
Authors: Li Yawen , Cima Michael J. 
Published: April 2011

ABSTRACTBulge test was performed on gold and gold/nitride composite films with in plane sizes ranging from 20 to 200μm. The films were prepared using the microfabrication process of a drug delivery MEMS device, and the bulge test setup was constructed using some of the packaging components of the device. Incremental pressure at 5psi interval was applied to the films, whose deflection was measured using interferometry. The extracted gold film properties from bulge test on either pure gold films or composite films yielded comparable values. The lower modulus (126∼168 GPa) comparing with the bulk gold could be due to the less dense microstructure of the evaporated gold film.
Indentation and Finite Element Modelling Investigations of the Indentation Size Effect in Aluminium Coatings on Borosilicate Glass Substrates
Authors: Spary I. , Jennett N. M. , Bushby A. J. 
Published: April 2011

ABSTRACTNanoindentation is one of the few available methods and the most commercially widespread technique for investigating the elastic and plastic properties of small volumes such as thin films. Quantitative methods for obtaining the indentation (plane strain) modulus and hardness of a coating have been published and finite element models (FEM) of the elastic-plastic response of indentation have been developed. Comparison of the FEM output with actual indentation data has shown that, as the indentation size reduces, the apparent yield stress of the material increases. We have shown that the increase in yield stress is predictable and falls on a master curve (MRS Symp. Proc., Vol 788, p123, 2003). Predictions have been tested and agree for a range of metals (Cu, Al, W, Ir). This points to there being a fundamental length scale for dislocation-based deformation and raises the question as to whether the yield stress of thin films may be altered by reducing thickness. This study therefore investigates the indentation response of Al coatings on Borosilicate glass as a function of coating thickness and indentation depth. FEM of the indentation contact will be compared with indentation data and AFM measurements of the surface profile to investigate the relative contributions of the indentation size effect and the effect of hardening due to the additional constraint of substrate proximity to the plastic zone.
Indentation Curve Analysis for Pile-up, Sink-in and Tip-Blunting Effects in Sharp Indentations
Authors: Choi Yeol , Lee Baik-Woo , Lee Ho-Seung , Kwon Dongil 
Published: April 2011

ABSTRACTHardness and elastic modulus can be derived from instrumented sharp indentation curves by considering the effects of materials pile-up and sink-in and tip blunting. In particular, this study quantifies pile-up or sink-in effects in determining contact area based on indentation-curve analysis. Two approaches, finite-element simulation and theoretical modeling, were used to describe the detailed contact morphologies. The ratio of contact depth to maximum indentation depth was proposed as a key indentation parameter and was found to be a material constant independent of indentation load. In addition, this parameter can be determined strictly in terms of indentation-curve parameters, such as loading and unloading slopes at maximum depth and indentation energy ratio. This curve-analysis method was verified by finite-element simulations and nanoindentation experiments.
Determining Stress-strain Curves for Thin Films by Experimental/Computational Nanoindentation
Authors: Lee Baik-Woo , Choi Yeol , Lee Yun-Hee , Kim Ju-Young , Kwon Dongil 
Published: April 2011

ABSTRACTThe nanoindentation technique has great promise in evaluating mechanical properties such as nanohardness and elastic modulus at micrometer or nanometer scales, since sample preparation and testing procedures are very easy. However, the nanohardness and elastic modulus cannot be directly related to basic material flow properties. Here a novel and simple experimental/computational method is proposed to extract stress-strain curves based on finite-element modeling (FEM) of nanoindentation. This method was verified for bulk Al by comparing the stress-strain curves extracted with those obtained from tensile testing, and was applied to Al thin films (0.5 μm and 1 μm) deposited on a Si substrate.
Thermomechanical Behavior of Tantalum Thin Films: The Effects of Oxygen and the β–α Phase Transformation
Authors: Knepper Robert , Stevens Blake , Baker Shefford P. 
Published: April 2011

ABSTRACTβ-Ta films were prepared in an ultra-high vacuum sputter deposition system, and the stresses that arose during thermal cycles to 750° C were measured using an in situ substrate curvature measurement system. The oxygen content in the environment was controlled during both deposition and thermomechanical testing. In films with no added oxygen, phase transformation from β to α takes place in distinct “jumps.” Addition of controlled amounts of oxygen to the annealing atmosphere led to increases in compressive stress, inhibited the phase transformation, and altered the magnitudes of the “jumps” in stress. The stresses in Ta films were found to be sensitive to oxygen content as well as thermal history.
Thin Film Material Parameters Derived from Full Field Nanometric Displacement Measurements in Non-uniform MEMS Geometries
Authors: Cárdenas-García Jaime F. , Cho Sungwoo , Chasiotis Ioannis 
Published: April 2011

ABSTRACTMEMS-scale polycrystalline silicon 2 μm thin film specimens fabricated via the Multi User MEMS Processes (MUMPs) have been employed to obtain the non-uniform nanometric displacement fields in the vicinity of prefabricated circular and elliptical micron-sized perforations. For the hole diameter-to-specimen width ratios considered in this work, and for all practical purposes, the displacement solution for a hole in an infinite plate is applicable. This method requires the ability to reliably and repeatably acquire nanometer level displacements on freestanding thin films. These tensile tests were conducted by a custom microtensile tester with the aid of Atomic Force Microscopy (AFM) and a special gripper that makes use of electrostatically assisted UV adhesion to handle and load miniature MEMS specimens. This non-conventional procedure for material parameter determination relies on Digital Image Correlation (DIC) to compare two AFM images, one before and one after specimen loading, and thus compute the nanometer level displacement fields (<50 nm global displacements) in 15×15-μm2 (or smaller) areas, with better than 2–3 nm resolution in displacements. By posing and solving a non-linear least squares inverse problem where, for known applied loads and measured displacement fields in an infinite plate with either a circular or an elliptical hole, it is possible to recover the elastic modulus (E). The main advantage of this approach is the full utilization of high-resolution displacement measurements over a specific specimen area, using only measurements acquired at one load level. Further statistical measurements of material properties may be obtained at varying load levels.
Monotonic Testing and Tension-Tension Fatigue Testing of Free-standing Al Microtensile Beams
Authors: Barbosa Nicholas , El-Deiry Paul , Vinci Richard P. 
Published: April 2011

ABSTRACTFree-standing Al thin films were loaded statically and dynamically through the use of a custom-built microtensile system. The system is capable of performing monotonic loading/unloading up to 50 μm/s (10-1/s) and tension-tension fatigue experiments at 100 Hz on 600 μm long, 100 μm wide, and 1 μm thick free-standing Al microtensile beams. Monotonic loading/unloading and stress relaxation experiments have been performed. The microtensile beams show plasticity as well as a relaxation dependence on strain rate and stress level. Displacement controlled tension-tension fatigue experiments have also been performed. A trend of decreasing cycles to failure with increasing displacement amplitude and increasing mean displacement has been noted but requires further experimental exploration.
Effect of Monazite Coating on Tensile Properties of Nextel 720 Fibers, Tows and Minicomposites
Authors: Pai Devdas M. , Yarmolenko Sergey , Sankar Jagannathan , Kailasshankar Balasubramanian , Murphy Christopher , Freeman Edwardo , Zawada Larry P. 
Published: April 2011

ABSTRACTThe tensile strengths of Nextel™ 720 fibers as well as tows and minicomposites made of these fibers were measured to study the effects of monazite coating at room temperature (RT) and 1200°C (HT). Tests were conducted on as-received materials (called unsoaked) as well as materials that had been thermally soaked at HT for 100 hours. Weibull analysis of fiber strength data shows close agreement between the Weibull modulus obtained from the statistical fracture distribution and the modulus obtained from a study of the gage length dependence of strength. The monazite coating does not change the RT strength of unsoaked single fibers, but has a beneficial effect in terms of strength retention under HT testing. This beneficial effect of the coating disappears for soaked samples. The results for tows and minicomposites also indicate that for the unsoaked condition, tensile failure appears to be driven by fiber surface flaws and the presence of the monazite coatings clearly improves both RT and HT strength of fibers, tows and minicomposites. However, the beneficial effects of the coating disappear after long term exposure to HT.
Fracture Behavior of Micro-Sized Specimens Prepared from a TiAl Thin Foil
Authors: Takashima K. , Halford T. P. , Rudinal D. , Higo Y. , Bowen P. 
Published: April 2011

ABSTRACTFracture tests have been carried out on micro-sized specimens prepared from a fully lamellar γ-TiAl based alloy thin foil. Micro cantilever beam type specimens with dimensions = 50 × 10 × 20 μm were prepared from one lamellar colony of the thin foil by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 10 μm were also introduced into the micro-sized specimens by focused ion beam machining. Notch directions were introduced into samples in order to select the trans- and inter-lamellar directions, respectively. Fracture tests were carried out using a mechanical testing machine for micro-sized specimens. Fracture tests for the micro-sized specimens were performed successfully, showing the fracture behaviour to be dependent upon the notch orientation. The fracture toughness of specimens with a notch direction perpendicular to the lamellar direction was 4.7 – 6.9 MPam1/2, while that with a notch direction in the inter-lamellar direction was 1.4 – 2.7 MPm1/2. This indicates that the orientation of the lamellar microstructure greatly affects the fracture properties of micro-sized components prepared from fully lamellar γ-TiAl based alloy thin foils. It is required to consider the results obtained in this investigation when designing actual micro scale structures using TiAl thin foils.
Strain control in SrRuO3 thin films by using a lattice constant tunable buffer
Authors: Terai K. , Ohnishi T. , Lippmaa M. , Koinuma H. , Kawasaki M. 
Published: April 2011

ABSTRACTHeteroepitaxial oxide thin films are usually grown on single crystal substrate which offer a similar lattice constant as the target material. In general, there are no substrates that are suitable for film fabrication and have a good lattice matching. In our previous report, we succeeded in fabricating a lattice constant tunable buffer by using a Ba1-xSrxTiO3 / BaTiO3 bilayer structure on SrTiO3. The in-plane lattice constant of the buffer layer can be tuned from 3.91 to 3.99 Å and the lattice constant is determined only by the Ba/Sr ratio in the Ba1-xSrxTiO3 layer. The buffer is suitable for growing both strain-free films and strained films. In this report we demonstrate the use of the lattice constant tunable buffer for strain control in SrRuO3 magnetic thin films. SrRuO3 has a magnetic anisotropy which changes under lattice strain. We show that the easy axis of magnetization is parallel to the film surface under tensile strain on a Ba0.5Sr0.5TiO3 buffer. The easy axis is perpendicular to the surface in compressively strained films. The tensile strain in a film also results in an increase of the ferromagnetic ordering temperature from a bulk value of 160 K to 164 K.