bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement and Modelling of the Radiation Damage of Silicon by MeV Ag Ions
Authors: Lindner Jörg K.N. , Eder Johann , Stritzker Bernd
Published: April 2011

Depth profiles of the radiation damage produced by 4 MeV Ag ions in Si(111) at temperatures of 210-450 K are studied by optical reflectivity depth profiling and TEM for doses between 1012 and 1015 Ag/cm2. For high implantation temperatures, the depth of maximum damage is shown to be dose dependent. Point defect diffusion is shown to result in long tails of defect depth profiles. High-temperature amorphization is observed to proceed via the formation and bridge-like coalescence of isolated amorphous volumina. The damage at the depth of the maximum in the nuclear stopping power is described as a function of dose and temperature by the Hecking model. The model parameters and a comparison with those obtained for lighter ions reflect the particular properties of heavy ion collision cascades.
Amorphisation of Perovskite: the Effect of Composition and Pre-Existing Cation Vacancies
Authors: Smith Katherine L. , Lumpkin Gregory R. , Blackford Mark G. , Vance Eric R.
Published: April 2011

Eight perovskites of different compositions and pre-existing vacancy contents were irradiated with 1.5 MeV Kr+ ions using the HVEM-Tandem User Facility at Argonne National Laboratory. The critical dose of 1.5 MeV Kr+ ions for amorphisation (Dc) of NaNbO3, SrTiO3 and two natural perovskites varies with composition and may decrease with Na content. The Dc values of 5 members of the solid solution series from SrTiO3 to La0.67TiO3 do not exhibit a simple relationship with vacancy content: In particular, Dc(Sr0.7La0.2TiO3) is extraordinarily high (>170 × 1014 ions cm−2) compared to those of the other (Sr, La)-perovskites in the series, which have Dc, values between 2.4 and 11×1014 ions cm−2.
High-Resolution Electron Microscopy Studies of the Precipitation of Copper Under Neutron Irradiation in An Fe-1.3Wt%Cu Alloy
Authors: Nicol A. C. , Jenkins M. L. , Kirk M. A.
Published: April 2011

We have studied by electron microscopy the copper-rich precipitates in an Fe-1.3wt%Cu model alloy irradiated with neutrons to doses of 8.61 × 10−3dpa and 6.3×10−2 dpa at a temperature of-270°C. In the lower dose material a majority (ca. 60%) of the precipitates visible in high-resolution electron microscopy were twinned 9R precipitates of size ∼ 2−4 nm, whilst ca. 40 % were untwinned. In the higher dose material, a majority (ca. 75%) of visible precipitates were untwinned although many still seemed to have a 9R structure. The average angle α between the herring-bone fringes in the twin variants was measured as 1250, not the 1290 characteristic of precipitates in thermally-aged and electron-irradiated material immediately after the bcc-→9R martensitic transformation. We argue that these results imply that the bcc-→9R transformation of small (< 4 nm) precipitates under neutron irradiation takes place at the irradiation temperature of 270°C rather than after subsequent cooling. Preliminary measurements showed that precipitate sizes did not depend strongly on dose, with a mean diameter of 3.4 ± 0.7 nm for the lower dose material, and 3.0 ± 0.5 rim for the higher dose material. This result agrees with the previous assumption that the lack of coarsening in precipitates formed under neutron irradiation is a consequence of the partial dissolution of larger precipitates by high-energy cascades.
Influence of Initial Grain Boundary Composition on the Evolution of Radiation-Induced Segregation Profiles
Authors: Busby J.T. , Was G.S. , Bruemmer S.M. , Edwards D. J. , Kenik E.A.
Published: April 2011

Radiation-induced segregation (RIS) has been identified as a potential contributor to irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels in reactor core components. The occurrence of grain boundary segregation prior to irradiation influences both the shape and magnitude of RIS profile development during subsequent irradiation. In an effort to better understand the impact of this pre-irradiation enrichment on RIS profile development, the evolution of grain boundary Cr segregation profiles with irradiation dose has been characterized. Commercial purity and high-purity austenitic stainless steels with different initial levels of grain boundary Cr have been irradiated with neutrons (at 275°C) or protons (at 360-400°C) to doses up to ∼5 dpa. Grain boundary composition profiles were measured before and after irradiation using scanning transmission electron microscopy with energy dispersive xray spectroscopy (STEM-EDS). The initial enrichment of Cr is shown to delay radiation-induced Cr depletion and produce a “W-shaped” profile at low irradiation doses. Further irradiation causes the central peak of the W to decrease, eventually resulting in the classical “V-shaped” depletion profile. Possible mechanisms for the pre-irradiation enrichment and its evolution into a “W-shaped” profile will be discussed.
Li-Defect Reactions During Low Dose Ion Implantation of 8LI into ZNSE Single Crystals
Authors: Restle M. , Dalmer M. , Wahl U. , Hofsäss H.
Published: April 2011

We present lattice site location and diffusion studies of ion implanted 8Li in ZnSe single crystals at sample temperatures between 180 K and 550 K using the emission channeling technique. Below 200 K, Li is immobile in ZnSe and occupies tetrahedral interstitial sites. Above 250 K, interstitial Li becomes mobile and for an accumulated dose above 1×1012 cm-2 the majority of the implanted Li atoms occupy substitutional sites, presumably Zn sites. However, for room temperature implantation at doses below 1×1012 cm-2, the majority of implanted Li still occupies interstitial sites. This behavior is explained by recombination processes between Zn interstitials and vacancies, thus reducing the vacancy concentration and maintaining a high fraction of interstitial Li. Substitutional Li is stable up to about 500 K and diffuses out for temperatures above. We calculate 0.5 eV for the migration energy of interstitial Li and 1.38 eV for the binding energy of substitutional Li.
Post-Irradiation Annealing of Dislocation Microstructure and Radiation-Induced Segregation in Proton-Irradiated Stainless Steels
Authors: Busby J.T. , Was G.S. , Kenik E.A.
Published: April 2011

Both microstructural effects (radiation hardening) and microcompositional effects (radiationinduced segregation (RIS)) have been identified as potential contributors to irradiation-assisted stress corrosion cracking (IASCC). However, the importance of each in IASCC is unclear. In an effort to isolate their roles, post-irradiation annealing of proton-irradiated 304L stainless steel samples has been studied. Model simulations of post-irradiation annealing at intermediate temperatures (400-650°C) indicate that microstructural features such as dislocation loops are removed faster than RIS. Simulations also predict that there exist time-temperature combinations that will significantly reduce the dislocation loop population while leaving the grain boundary segregation essentially unaffected. Ultra-high purity (UHP) 304L stainless steel samples have been irradiated with 3.2 MeV protons at 360°C to 1.0 dpa and then annealed in-situ as thinned TEM disks in the temperature range of 500°C- 625°C for times between 20 and 60 minutes. RIS and dislocation loops were characterized before and after annealing in the same areas of each specimen. Post-irradiation anneals at 500TC for 45 minutes or 60 minutes resulted in no appreciable change in dislocation loop population or RIS. Annealing at 600°C for 20 minutes., or 40 minutes and at 625°C for 40 minutes, resulted in decreasing dislocation loop densities and increasing loop size with increasing annealing time or temperature, while the amount of RIS did not change significantly from the pre-annealed condition.
Fabrication of a Simple Materials System for Study of Hg in a Stainless Steel
Authors: Allen Charles W. , Mccormick Anthony W. , Kestel Bernard J. , Baldo Peter M. , Zaluzec Nestor J. , Rehn Lynn E.
Published: April 2011

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory, is expected to employ a Hg target encased in a stainless steel. Little is known about the metallurgical behavior of this materials engineering system, which will occur in a service environment involving elevated temperatures and intense radiation. Under normal equilibrium conditions, however, Hg is known to be insoluble in and non-reactive with solid Fe and Cr but to form one or more intermetallics with Ni. Hg has been implanted into alloy 304L. After implantations at 400 and 500 °C to a fluence of 3×1016 cm−2 sub-micron sized precipitates of Hg are formed, as judged, for example, from their solidification behavior on cooling during TEM observation. The formation of such a system of microtargets and possible studies employing them as in situ TEM specimens are discussed, which can provide useful empirical information in conjunction with SNS target development.
Fracture Toughness and Internal Friction of GlidCop®Al-25 Alloy
Authors: Tähtinen S. , Pyykkönen M. , Smuk S. , Hänninen H. , Jagodzinski Y. , Tarasenko O.
Published: April 2011

Fracture toughness was found to decrease rapidly with increasing temperature in dispersionstrengthened GlidCop®Al-25 copper alloy both in the as-supplied condition and neutron irradiated to a dose of 0.3 dpa. Internal friction study revealed two-component peak. Grain-boundary sliding was recognized to be responsible for the low-temperature component of the peak, which disappears after irradiation and restores after the heating above 900 K. This points out that the changes in the particle — grain boundary interaction, apparently, due to the defects at the interfaces produced by irradiation are responsible for the drop of fracture toughness in A125 alloy.
Comparison Between 590 Mev Proton Irradiation and Neutron Irradiation on the F82H Ferritic/Martensitic Steel
Authors: Schäublin R. , Victoria M.
Published: April 2011

The microstructure of the low activation F82H ferritic/martensitic steel, which is a candidate for future fusion reactor first wall, has been studied in order to determine the differences between 590 MeV proton irradiations and fission neutron irradiations. A range of doses and temperatures are investigated. The present work focuses on the microstructure of irradiation induced defect clusters for cases of proton irradiations to doses up to 1.7 dpa, and for neutron irradiation to a single dose of 2.5 dpa. The irradiation temperatures were 40°C and 250°C in the case of the proton irradiation and 250°C in the case of the neutron irradiation. In the case of the proton irradiation no defects are observed in the transmission electron microscope in the case of the 0.45 dpa dose, while they start to be clearly visible for doses higher than about 1.0 dpa. In the case of the neutron irradiation a high density of defects and small loops is evidenced. These preliminary results are presented here.
Time and Space Instabilities in Binary Alloys at Phase Transitions Under Irradiation
Authors: Mikhailovskiy V. V. , Russell K. C. , Sugakov V. I.
Published: April 2011

A manifestation of antisite defects created by nuclear irradiation in binary ordered alloys is investigated. Calculations show that the concentration of such defects can be large at typical values of intensity of irradiation and temperature. The appearance of structural defects can cause the time and space instabilities in crystal during irradiation. Time instabilities are connected with the acceleration of antisite defect relaxation due to the heat that is released during this relaxation. The instability leads to appearance of self-oscillations of defect density and temperature of crystal. Space instability arises due to interaction between defects created by irradiation. It leads to spatial periodical modulation of antisite defect density.
Glissile and Sessile Vacancy and Self-Interstitial Clusters in BCC and FCC Metals
Authors: Osetsky Yu.N. , Gao F. , Bacon D.J.
Published: April 2011

The production bias model of radiation damage has focused attention on the production of glissile clusters (GCs) in the displacement cascades. The formation of glissile SIA clusters in molecular dynamics simulations of cascades has been reported frequently whereas information about sessile clusters (SCs) is rather sparse. Comprehensive models of microstructure evolution in metals under irradiation conditions should include all possible defects (and clusters) and interactions between them. Detailed information of this nature can be obtained by means of atomic-scale computer simulation. In the present paper we review some recent results of atomistic simulation of sessile and glissile defect clusters in metals, dealing with their mechanisms of formation and their properties. In particular, we discuss the formation of SCs and GCs in the cascade region and also their creation in bulk metal as a result of cluster growth.
Monte Carlo Investigation of Cascade Damage Effects in Metals Under Low Temperature Irradiation
Authors: Barashev A.V. , Bacon D.J. , Golubov S.I.
Published: April 2011

The Monte Carlo (MC) method is used to study cascade damage effects in damage accumulation in pure metals at temperatures below stage III, when vacancies and their clusters are immobile. The irradiation is modelled by sequential introduction of collections of defects representing the primary damage state of cascades placed randomly in the simulation volume. The cascades generated in molecular dynamics simulations for recoil energies from 2 to 20keV are used. Concentrations of point defects as well as defect cluster densities are monitored as a function of dose up to 0.02dpa. The results are compared with those obtained in the mean-field approximation. Factors responsible for the difference in damage accumulation under homogeneous and cascade irradiation conditions are revealed. The effects of temperature and recoil energy are studied. Problems in MC modelling of one-dimensional diffusion of interstitial clusters connected with the limited volume of the simulation box are revealed.
Rigidity Constraints in Amorphization of Multiply-Polytopic Multiply-Connected Ceramic Structures
Authors: Hobbs Linn W. , Jesurum C. Esther , Bergert Bonnie
Published: April 2011

The ease with which ceramic structures amorphize under displacive radiation is considered from the point of view of connectivity and the consequent topological constraints which impose rigidity on the original atomic arrangement. These constraints define a measure of topological freedom to rearrange which has been shown previously to correlate with measured energy deposition density during ion-induced amorphization. That earlier assessment of topological freedoms is extended here to include more complex connectivities and is shown to be representable in an analytical form which reflects the progressive destruction of topological constraints.
A Raman Scattering, Ion Channelling and Photoluminescence Study of Argon Ion Radiation Damage in Cu(Ga,In)Se2 - Dose Dependence and Dose Rate Effects
Authors: Lippold G. , Weinert K. , Yakushev M.V. , Pilkington R.D. , Otte K. , Grill W.
Published: April 2011

The ternary chalcopyrite semiconductor CuInSe2 and related ternary compounds are promising materials for the production of high-efficiency thin film solar cells. In this paper we study the dose dependence of ion radiation damage produced by 30 keV and 80 keV Ar ions in single crystals and polycrystalline films of Cu(In,Ga)Se 2 over a wide dose range from 1012 to 1017 cm-2, using Raman spectroscopy and ion channeling measurements. For the first time, we also report on the dose rate dependence with a variation of the beam current density in the range 0.44 to 44 µcm-2. Even for low damage levels no significant dependence of the defect concentration or damage mechanism on the dose rate could be observed. From phonon correlation length considerations we estimate defect densities. They are in agreement with ion channeling data obtained in the 1015 to 1016 dose range, where the breakdown of the lattice structure occurs. In this dose range, the defect density is close to the concentration of implanted atoms. We conclude, that this high impurity concentration is responsible for the amorphization.
Solid Freeform Fabrication using Alumina-Poly(Vinyl Alcohol) Gel-Casting Suspensions
Authors: Morissette S. L. , Lewis J. A. , Cesarano J. , Dimos D. B.
Published: April 2011

Solid freeform fabrication of aqueous alumina (Al2O3)-poly(vinyl alcohol) (PVA) gelcasting suspensions was carried out using a computer-controlled extrusion apparatus fitted with a two-nozzle delivery system. The impact of casting parameters (e.g., tip diameter, mixing rate, etc.) and suspension composition on line resolution/uniformity, printability, and as-cast component properties was probed using laser profilometry and optical microscopy. Line uniformity depended on mixing rate (Rmix) and tip diameter (td) for the chamber and paddle geometry utilized. These data were used to establish processing maps that identify optimal print conditions for varying suspension composition.
Direct Ink-Jet Deposition of Ceramic Green Bodies: I - Formulation of Build Materials
Authors: Seerden K. A. M. , Reis N. , Derby B. , Grant P. S. , Halloran J. W. , Evans J. R. G.
Published: April 2011

A conventional rapid prototyping technology has been used to build prototypes using direct deposition through a drop-on-demand ink-jet printing system. Al2O3- filled waxes have been developed with viscosity values close to those of the materials used in commercial printing systems. Commercial dispersants based on stearic acid and sterylamine have been studied and stable ceramic suspensions with ceramic volume fractions in the range 20–40 volume% produced with suitable viscosity. A suspension of 20% Al2O3in an alkane wax has been successfully printed.
Direct Inkjet Deposition of Ceramic Green Bodies: II – Jet Behaviour and Deposit Formation
Authors: Reis N. , Seerden K. A. M. , Derby B. , Halloran J. W. , Evans J. R. G.
Published: April 2011

In order to successfully build three-dimensional shapes by hot-melt inkjet deposition it is essential to control the building block characteristics, i.e., the deposit geometry, dimensions and fine feature resolution. The deposit formation is mainly dependent on the material systems and their jetting behaviour. It is therefore crucial to understand how the jet formation is affected by the inks 'rheological properties and how to manipulate the jet-head driving parameters to achieve optimum deposition conditions. This paper reports our investigations with a model jet firing station, about the influence of driving parameters of hot-melt drop-on-demand print-heads (e.g., pulse shape and frequency) on the jet and deposit formation characteristics, for both unfilled and powder loaded vehicles.
Three Dimensional Microstructures from Metal Carbonyls
Authors: Lind Jan-Erik , Nyrhila Olli , Kotila Juha , Syvanen Tatu
Published: April 2011

3D-LCVD of nickel and iron carbonyls was studied in order to grow 3-D metal forms under static or scanning Nd:YAG-laser beam. In addition to growth, emphasis was also placed on the prevention of the simultaneous decomposition of carbon monoxide, which interferes with the metal growth process. This was essential, because the fairly high precursor gas pressures of the metal carbonyls are very tempting for the 3D-LCVD. Parameters to be optimized included precursor pressure, laser power, laser scan speed and spot size. In order to optimize the growth parameters, the microstructures of the resulting forms were studied using SEM. Comparison between static and scanning growth is presented with the building philosophy in mind, e.g. whether to build structures layer by layer, from modules or in conjunction with another process to compensate for their shortcomings. The substrates used included steel, graphite and porous bronze.

The results indicated different microstructures for iron and nickel, which were dependent on the total/precursor pressure. In the scanning experiments, nickel produced very thin films of high reflectivity, whereas iron produced a structure which could be described as a crystalline spider's web. The static experiments produced solid rods in the case of nickel, whereas with iron, the rods were hollow, even with same spot sizes. Moreover, an evident change in the microstructure of the nickel forms as a function of pressure was observed. The 3-D growth rate of the static experiments seemed very promising for the forthcoming scanning experiments.
Solid Freeform Fabrication of Silicon Nitride Ceramics
Authors: Gasdaska C. J. , Clancy R. , Jamalabad V. , Dalfonzo D.
Published: April 2011

Silicon nitride ceramics have been prepared using the fused deposition (FD) process in a Stratasys 1650 modeler. Two types of silicon nitride have been prepared: GS44 and AS800. AS800 is processed and used at higher temperatures than GS44. The strength of machined surfaces of either type of silicon nitride prepared using FD is comparable to conventionally processed material. Using standard build conditions strengths for as-built and as-sintered
surfaces are approximately 50% lower. The additive nature of solid freeform processes also allows multi-material combinations to be deposited which result in enhanced performance. For example, combinations of silicon nitride based materials with different thermal expansion coefficients have been prepared which demonstrate strength increases > 20%. In addition, components containing complicated internal cavities may also be fabricated.
Thermoelectric Properties of the Cubic Family of Compounds AgPbBiQ3(Q = S, Se, Te). Very Low Thermal Conductivity Materials
Authors: Sportouch S. , Basteat M. , Brazis P. , Ireland J. , Kannewurf C. R. , Uher C. , Kanatzidis M. G.
Published: April 2011

The AgPbBiQ3class of compounds and their solid solution members are related to the NaCl structure type, where Ag, Pb and Bi atoms are statistically disordered on the Na site and Q atoms occupy the Cl site. These compounds were synthesized by combining the elements in the appropriate ratio and heating under static vacuum at 900° C for 3 days. They are narrow gap semiconductors with band gaps in the range of 0.6 to 0.28 eV. The charge-transport properties were measured on ingots as a function of temperature. The compounds AgPbBiTe3, AgPbBiSe3, AgPbBiTe2.75Se0.25and AgPbBiTe2Se, undoped, possess an electrical conductivity in the range of 70 S/cm to 400 S/cm. These materials exhibit negative thermopower ranging from -40 μV/K to -160 μV/K at room temperature and thermal conductivity less than 1.30 W/mK.
High Temperature Transport Probe for Thermopower and Resistmty Measurements
Authors: Littleton R. T. , Jeffries Jason , Kaeser Michael A. , Long Michael , Tritt Terry M.
Published: April 2011

We have recently developed a device to measure resistivity and Seebeck values of a material as a function of temperature over a range of 80K < T < 700K. These measurements overlap in temperature with our existing apparatus (4K < T < 320K). These measurements are necessary for the investigation of potential thermoelectric materials at elevated temperatures, where power generation applications are important. The probe design allows for various types of samples to be evaluated from needle like samples ( L ≈ 2mm, D ≈ 50μm ) to larger pressed pellet samples ( ≈ 2mm × 2mm × 10mm). Separate high temperature cartridge heaters are used to stabilize both the absolute temperature, T, as well as the temperature difference, ΔT. Thermocouples measuring T+ΔT/2 and T-ΔT/2 are also employed to measure sample voltages for resistance and thermoelectric power. Design details and measurement specifics will be discussed. Data taken on standards a well as some research samples will be presented.
Fabrication of Thermoelectric Modules Using Thermoelectric Pastes and an Additive Technology
Authors: Xi Xiaomei , Matijasevic Goran , Ha Linh , Baxter Dan
Published: April 2011

Polymer based thermoelectric pastes have been developed by dispersing metal and metal alloy powders into a polymer binder system. Thermoelectric materials and elements are formed by depositing the pastes into a mold or onto a substrate and subsequently sintering them to form a conductive network with thermoelectric properties. A Seebeck coefficient of 65 μV/°K, and a resistivity of 0.05 Ω-cm have been achieved in the early stage of material development. A novel processing method has also been developed for the fabrication of small size thermoelectric elements and thermoelectric modules with a multitude of thermoelectric couples using these pastes. Thermoelectric elements as small as 250 μm in diameter and 25 μm in thickness have been achieved. Thermoelectric modules with 275 couples/in2have been fabricated using the thermoelectric pastes and the fabrication technique. Further improvement in the material properties and reduction in the sizes of the thermoelectric elements are underway.
Electrical Properties of Orthorhombic Iron Disilicide
Authors: Tani Jun-Ichi , Kido Hiroyasu
Published: April 2011

In order to clarify the electrical properties of Cr-doped β-FeSi2(Fe1-xCrxSi2) and Mn-doped β--FeSi2(Fel-yMnySi2), the Hall effect and electrical resistivity of Fe1-xCrxSi2(0.01 ≦ x ≦ 0.05) and Fel-yMnySi2(0.01 ≦ y ≦ 0.10) have been measured in the temperature range between 80 and 300 K. fhe solid solutions Fel-xCrxSi2and Fel-yMnySi2are p-type over the measured temperature range. The observed features of RH of these solid solutions are explained by using the two-band model with the existence of two acceptor levels. The hole concentration of Fe1-xCrxSi2at 300 K ranges from 8.9×1018cm−3for x=0.01 to 1.1×1020cm−3for x=0.05, which is one order of magnitude higher than that of Fel-yMnySi2.
Thermoelectric Bismuth Wire Array Composites
Authors: Huber T. E. , Graf M. J. , Foss C. A
Published: April 2011

The resistance R of a high density array of 200 nm diameter wires in anodic alumina is studied experimentally. R decreases from 300 K down to 0.3 K. An anomalous drop in R was found at T<50 K. Possible explanations are considered, including one based on the onedimensional quantum size effect. Also, the large magnetoresistance is suggestive of weaklocalization effects. The implication for the thermoelectric properties is discussed.
Electronic Structure of Complex Bismuth Chalcogenides and Other Narrow-Gap Thermoelectric Materials
Authors: Mahanti S. D. , Larson P. , Chung Duck-Young , Sportouch S. , Kanatzidis M. G.
Published: April 2011

There is considerable current effort to discover new thermoelectric materials with a high figure of merit Z. Some of these new materials are narrow-gap semiconductors with rather complex crystal structures. In this paper we discuss the results of electronic structure calculations in two classes of such systems. The first class consists of BaBiTe3, a structural and chemical derivative of the well-studied Bi2Te3. Similarities and differences in the band structures of these two systems are discussed. The second class consists of half-Heusler or “stuffed”-NaCl compounds MNiX, where M is Y, La, Lu, Yb, and X is a pnictogen; As, Sb, Bi. To understand the physical reason behind the energy gap formation, we compare the electronic structure of YNiSb with that of an isoelectronic system ZrNiSn, another isostructural compound of thermoelectric interest. These calculations were carried out within density functional theory (in generalized gradient approximation) using self-consistent full-potential LAPW method. Energy gaps and effective masses associated with the conduction band minimum and valence band maximum have been calculated and these quantities have been used to estimate transport properties. Large room temperature thermopower values in Bi2Te3 and BaBiTe3 can be understood in terms of multiple conduction and valence band extrema whereas similar large values in ZrNiSn and other half-Heusler compounds can be ascribed to large electron and hole effective mass.
Materials With Open Crystal Structure as Prospective Novel Thermoelectrics
Authors: Uher Ctirad , Yang Jihui , Hu Siqing
Published: April 2011

A useful approach to identify materials with high thermoelectric figure of merit is to search for solids that offer great flexibility to modify and tailor the structure so as to achieve the optimal transport behavior. Among the most promising novel thermoelectric materials are solids with “open crystal structure”. They may be typified by structures with unfilled cages, crystals with an empty atomic sublattice, and by a network of polyhedral cages enclosing guest species. In this paper we present our latest results concerning transport properties in the above classes of solids. Specifically, we focus on the filled skutterudites, half-Heusler alloys, and clathrates.
Materials Development for Thermomagnetic Cooling
Authors: Freibert F. , Migliori A. , Darling T. W. , Trugman S. A.
Published: April 2011

We review thermoelectric effects in a magnetic field at a phenomenological level. The measurement problem is discussed in detail, and results are presented based on the time-dependent voltages developed by thermoelectric materials in zero field and in large magnetic fields. These time dependent voltages provide a direct and unambiguously accurate measurement of both the Peltier and Ettingshausen figures of merit, and can be used to obtain all the relevant transport including thermal conductivity. Discussion of the limiting performance of both Peltier and Ettingshausen coolers are discussed. Data are shown for Ettingshausen materials with part-per-million levels of Sn doping. Thought these materials exhibit high Ettingshausen figures of merit, Sn doping does not appear to improve performance.
Thermoelectric Properties of Cr3S4-Type Selenides
Authors: Snyder G. Jeffrey , Caillat T. , Fleurial J. -P.
Published: April 2011

Several compounds with the Cr3S4 structure type have been studied for their thermoelectric properties. All exhibit low lattice thermal conductivity of about 15 mW/cmK, independent of temperature. Many of the compounds, such as Co3Se4, Ni3Se4, Fe3Se4, Ti3Se4, FeNi2Se4, and FeCo2Se4, are metals with relatively low electrical resistivity and Seebeck coefficient. The Cr containing compounds, such as Cr3Se4, NiCr2Se4, CoCr2Se4, and FeCr2Se4, have the largest Seebeck coefficients and highest resistivity. thermoelectric applications is FeCr2Se4.
The Synthesis of Metastable Skutierudites Using Superlattice Reactants
Authors: Sellinschegg Heike , Williams Joshua R. , Casperson Julie , Nolas George , Johnson David C.
Published: April 2011

The synthesis by controlled crystallization of elementally modulated reactants of a series of kinetically stable, crystalline skutterudites (M′1−xM4Sb12 where M′ = vacancy, RE, Hf, Al, Sn, Pb, Bi, In, Ga, Al and Y; M = Fe, Co) is discussed. Low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150°C. Nucleation of the skutterudite structure occurs exothermically on annealing at temperatures near 200°C regardless of the ternary metal. The metastable ternary compounds and the new binary compound were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 300°C for the binary compound FeSb3 to 550°C for Eu′1−x;Fe4Sb12 to higher temperatures for the filled cobalt end members. The occupation of the lattice site for the ternary cation was found to depend on the composition of the initial reactant. Samples up to a half gram in size have been synthesized and hot-pressed into pellets for measurement of both electrical properties and thermal conductivity. Preliminary evidence is presented for the formation of skutterudite superlattices, again synthesized by the controlled crystallization of elementally modulated reactants.
Thermoelectric Properties of Two Ternary Tellurides
Authors: Sharp Jeff W. , Sales Brian C. , Mandrus David G. , Chakoumakos Bryan C.
Published: April 2011

We present initial assessments of the thermoelectric properties of two ternary tellurides with known crystal structures, Tl2GeTeM5 and Tl2SnTe5. Tl2SnTe5 appears to have a p-type figure of merit about the same as that of Bi2Te3, the best thermoelectric material among binary compounds. A good figure of merit is possible because the lattice thermal conductivity is very low. Based on neutron diffraction data, we have calculated atomic displacement parameters and thermal expansion coefficients. The atomic displacement parameters give some understanding of the low lattice thermal conductivity.
Enhanced Thermionic Emission Cooling in High Barrier Superlattice Heterostructures
Authors: Shakouri Ali , LaBounty Chris , Abraham Patrick , Piprek Joachim , Bowers John E.
Published: April 2011

Thermionic emission current in heterostructures can be used to enhance thermoelectric properties beyond what can be achieved with conventional bulk materials. The Bandgap discontinuity at the junction between two materials is used to selectively emit hot electrons over a barrier layer from cathode to anode. This evaporative cooling can be optimized at various temperatures by adjusting the barrier height and thickness. Theoretical and experimental results for nonisothermal thermionic emission in heterostructures are presented. Single stage InGaAsP-based heterostructure integrated thermionic (HIT) coolers are fabricated and characterized. Cooling on the order of a degree over one micron thick barriers has been observed. Nonisothermal transport in highly doped tall barrier superlattices is also investigated. An order of magnitude improvement in cooling efficiency is predicted for InAlAs/InP superlattices.
Analysis of Antimony-Tin-Based Skutterudites
Authors: Schujman S. B. , Slack G. A. , Nguyen H. C. , Nolas G. S. , Young R. A. , Mohammed F. , Tritt T.
Published: April 2011

Since the proposal of skutterudites as possible Phonon-Glass, Electron-Crystal materials, a lot of work has been done trying to fill the structural voids with foreign “rattling” atoms. In order to keep the electronic count per unit cell constant (and thus, the semiconducting properties of most of the compounds under study) partial replacement of either the cation or the anion in the original formula by an appropriate neighbor in the periodic table is an option. In the case of antimonides, replacing part of the Sb with Ge or Sn in order to compensate the extra charge introduced by void fillers has proved useful for compounds based on rare-earth filled IrSb3. In the case of RhSb3, we found that large quantities of Sn can be incorporated into the skutterudite structure of RhSb3 without either filling the voids or producing charge carriers.

We have analyzed the stability of several cross-sections of the Rh-Sb-Sn ternary system and have found a wide range of compositions with the basic skutterudite structure as we vary the Sn content. In all the cases, the tin goes substitutionally into the antimony sites. The voids remain empty. Density measurements suggest the existence of metal vacancies, confirmed by Rietveld refinement of the powder X-ray diffraction patterns. The possibility of Sn-induced mixed-valence of Rh on the anion sites is being investigated.
Microstructure and Thermoelectric Properties of Thin Foils of Bismuth Telluride Alloys
Authors: Koukharenko E. , Frety N. , Shepelevich V. G. , Tedenac J. C.
Published: April 2011

Ultrarapid quenching processes have been extensively studied during the past few years. particularly for glasses. In the case of polycrystalline materials, the problem is slightly different. In this case, the microstructures obtained by this process are very different from those obtained by normal cooling and they induce new physical properties.
Transport Properties Of Doped CsBi4Te6 Thermoelectric Materials
Authors: Brazis Paul W. , Rocci Melissa , Chung Duck-Young , Kanatzidis Mercouri G. , Kannewurf Carl R.
Published: April 2011

In previous investigations we have introduced a variety of new chalcogenide-based materials with promising properties for thermoelectric applications. The chalcogenide CsBi4Te6 was previously reported to have a high ZT product with a maximum value at 260K. In order to improve this value, a series of doped CsBi4Te6 samples has been synthesized. Current doping studies have been very encouraging, with one sample found to have a maximum power factor of 51.5 μW/cm·K2 at 184 K. This paper reports on material characterization studies through the usual transport measurements to determine optimum doping concentration for various dopants.
Theoretical Modeling of Thermoelectricity in Bi Nanowires
Authors: Sun X. , Zhang Z. , Dresselhaus G. , Dresselhaus M. S. , Ying J. Y. , Chen G.
Published: April 2011

Bismuth as a semimetal is not a good thermoelectric material in bulk form because of the approximate cancellation between the electron and hole contributions. However, quantum confinement can be introduced by making Bi nanowires to move the lowest conduction subband edge up and the highest valence subband edge down to get a one-dimensional (1D) semiconductor at some critical wire diameter dc. A theoretical model based on the basic band structure of bulk Bi is developed to predict the dependence of these quantities on wire diameter and on the crystalline orientation of the bismuth nanowires. Numerical modeling is performed for trigonal, binary and bisectrix crystal orientations. By carefully tailoring the Bi wire diameter and carrier concentration, substantial enhancement in the thermoelectric figure of merit is expected for small nanowire diameters.
Understanding and Tailoring the Mechanical Properties of Liga Fabricated Materials
Authors: Buchheit T. E , Christenson T. R. , Schmale D. T. , Lavan D. A.
Published: April 2011

LIGA fabricated materials and components exhibit several processing issues affecting their metallurgical and mechanical properties, potentially limiting their usefulness for MEMS applications. For example, LIGA processing by metal electrodeposition is very sensitive to deposition conditions which causes significant processing lot variations of mechanical and metallurgical properties. Furthermore, the process produces a material with a highly textured lenticular microstructural morphology suggesting an anisotropic material response. Understanding and controlling out-of-plane anisotropy is desirable for LIGA components designed for out-of-plane flexures. Previous work by the current authors [6] focussed on results from a miniature servo-hydraulic mechanical test frame constructed for characterizing LIGA materials. Those results demonstrated microstructural and mechanical properties dependencies with plating bath current density in LIGA fabricated nickel (LIGA Ni). This presentation builds on that work and fosters a methodology for controlling the properties of LIGA fabricated materials through processing. New results include measurement of mechanical properties of LIGA fabricated copper (LIGA Cu), out-of-planeand localized mechanical property measurements using compression testing and nanoindentation of LIGA Ni and LIGA Cu.
Shape Memory and Magnetostrictive Materials for Mems
Authors: Wuttig Manfred , Quandt Eckhard , Ludwig Alfred , Winzek Bernhard
Published: April 2011

Ferroic materials are ideally suited for actuators in MEMS applications. Ferro- elastic materials, i.e. shape memory alloys (SMAs), produce a significant, o. o. 104 microstrain, phase transformation induced thermoelastic eigenstrain. Rare earth based ferromagnetic materials possess large, o.o. 103 microstrain, saturation magnetostriction which can be developed at fields of the order of mT readily achieved in MEMS geometries. This paper discusses the essentials of the thermoelastic stress evolution in SMA/Substrate bimorphs and of the development of rare earth based ferromagnetic multilayers. SMA actuationunder the constraint of a substrate and the optimization of the multilayers are emphazized.
Dc Magnetron Reactive Sputtering of Low Stress Ain Piezoelectric Thin Films for Mems Application
Authors: Hsieh Peter , Reif Rafael , Cunningham Brian
Published: April 2011

Many MEMS devices require piezoelectric excitation and readout to actuate and sense motion of mechanical structures. Aluminum nitride is advantageous for MEMS fabrication because it is compatible with silicon integrated circuit foundry impurity contamination requirements, can be deposited at low temperatures, provides a high piezoelectric coefficient, and is easily patterned using conventional photolithographic techniques. In this work, AIN thin films were deposited on silicon substrates for use in a MEMS silicon membrane ultrasonic resonator. The ultrasonic resonator is configured as a gravimetric sensing device for chemical detection. Issues of concern with regard to device performance and yield include the maximization of the electromechanical coupling constant (k2), film stress control, and film uniformity; these issues were addressed through a central composite design set of experiments to resolve the film property responses as a function of the deposition parameters. Film characterization was conducted with x-ray diffraction, spectroscopic ellipsometry, and surface profilometry. Optimization of film deposition parameters improved sensor performance and enabled further device miniaturization with the use of thinner films.
Meso-Scale Pressure Transducers Utilizing Low Temperature Co-Fired Ceramic Tapes
Authors: Lynch H. , Park J. , Espinoza-Valejos P. A. , Santiago-Aviles J. J. , Sola-Laguna L.
Published: April 2011

Pressure transducers with promising characteristics at high pressure and temperatures have been developed using low temperature co-fired ceramic (LTCC) tape technology. All parts for the transducer were machined from DuPont 951 series LTCC tapes utilizing either a numerically controlled milling machine, or an isotropic etching technique involving the removal of the glassy binder of a partially sintered LTCC tape. Device dimensions are in the meso (intermediate) scale range, with the smallest device size of 8mm in diameter, and the cavity of 2 mm. Utilizing the anisotropy induced during the casting process a chemical exfoliation technique was developed. This technique allowed us to separate the original tape in three layers, the middle one being highly elastic, isotropic and homogeneous. The middle layer can be chemically thinned to achieve membrane like behavior, with thickness of the order of 50 µm. The pressure is measured as a function of the membrane deformation where two piezo-resistors are screen printed. Two piezo-resistors were used to achieve temperature compensation. Using shrinkage matched paste, nominal thick film technology was used in the screen printing of the piezoresistors. The rest of the transducer was fabricated using several layers of LTCC tapes, which were laminated and fired. Devices of different sizes were fabricated and compared. Devices were characterized by comparing its dynamic response with a micro-fabricated silicon pressure transducer.
Fabrication of Mems Devices by Powder-Filling into DXRL-Formed Molds
Authors: Garino Terry J. , Christenson Todd , Venturini Eugene
Published: April 2011

We have developed a variety of processes for fabricating components for micro devices based on deep x-ray lithography (DXRL). Although the techniques are applicable to many materials, we have demonstrated them using hard (Nd2Fe14B) and soft (Ni-Zn ferrite) magnetic materials because of the importance of these materials in magnetic micro-actuators and other devices and because of the difficulty fabricating them by other means. The simplest technique involves pressing a mixture of magnetic powder and a binder into a DXRL-formed mold. In the second technique, powder is pressed into the mold and then sintered to densify. The other two processes involve pressing at high temperature either powder or a dense bulk material into a ceramic mold that was previously made using a DXRL mold. These techniques allow arbitrary 2-dimensional shapes to be made 10 to 1000 μm thick with in-plane dimensions as small as 50μm and dimensional tolerances in the micron range. Bonded isotropic Nd2Fe14B micro-magnets made by these processes had an energy product of 7 MGOe.
Micrometer-scale machining of metals and polymers enabled by focused ion beam sputtering
Authors: Adams D. P. , Benavides G. L. , vasile M. J.
Published: April 2011

This work combines focused ion beam sputtering and ultra-precision machining for microfabrication of metal alloys and polymers. Specifically, micro-end mills are made by Ga ion beam sputtering of a cylindrical tool shank. Using an ion energy of 20keV, the focused beam defines the tool cutting edges that have submicrometer radii of curvature. We demonstrate 25μm diameter micromilling tools having 2, 4 and 5 cutting edges. These tools fabricate fine channels, 26–28 microns wide, in 6061 aluminum, brass, and polymethyl methacrylate. Micro-tools are structurally robust and operate for more than 5 hours without fracture.
Dependence Of Silicon Fracture Strength And Surface Morphology On Deep Reactive Ion Etching Parameters
Authors: Chen Kuo-Shen , Ayon Arturo A. , Lohner Kevin A. , Kepets Mark A. , Melconian Terran K. , Spearing S. Mark
Published: April 2011

The development of a high power-density micro-gas turbine engine is currently underway at MIT. The initial goal is to produce the components by deep reactive ion etching (DRIE) single crystal silicon. The capability of the silicon structures to withstand the very high stress levels within the engine limits the performance of the device. This capability is determined by the material strength and by the achievable fillet radii at the root of turbine blades and other etched features rotating at high speeds. These factors are strongly dependent on the DRIE parameters. Etching conditions that yield large fillet radii and good surface quality are desirable from a mechanical standpoint. In order to identify optimal DRIE conditions, a mechanical testing program has been implemented. The designed experiment involves a matrix of 55 silicon wafers with radiused hub flexure specimens etched under different DRIE conditions. The resulting fracture strengths were determined through mechanical testing, while SEM analysis was used to characterize the corresponding fillet radii. The test results will provide the basis for process optimization of micro-turbomachinery fabrication and play an important role in the overall engine redesign.
Recent Developments in Experimental and Theoretical Studies of The Mechanical Behavior of Polycrystalline Silicon for Microelectromechanical Systems
Authors: Ballarini R.
Published: April 2011

A review of recent developments in experimental and theoretical studies of the mechanicalbehavior of polycrystalline silicon structures whose dimensions are comparable to those of typicalmicro electromechanical systems devices is presented. It is concluded that although great strides have been made towards understanding this behavior, there is a need to develop robust standardized experimental techniques, and in turn, material data bases that are required to verify and calibrate predictive models for calculating device reliability.
Stress-Caused Deflections of Asymmetrical Coated Single-Crystal Silicon Elements
Authors: Kuechler M. , Griesbach K. , Bertz A. , Gessner T. , Faust W. , Dudek R.
Published: April 2011

Long and narrow beams of single crystal silicon were made by surrounding them with trenches of different width followed by dry release etch. The beams were coated nonconformally during fabrication. Depending on the asymmetry of the films coated onto the beam sidewalls they tend to bend laterally. This deflection has been analyzed experimentally and by FEM-Simulations.