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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Ion beam-assisted pulsed laser deposition of (Ba,Sr)(Ti,Zr)O{sub 3} films on Pt-Si substrates

Ion beam-assisted pulsed laser deposition with an Ar-oxygen ion mixture was used to prepare Ba{sub 0.6}Sr{sub 0.4}Ti{sub 0.7}Zr{sub 0.3}O{sub 3} (BSTZ) thin films on Pt-coated Si substrates. The ion beam with an anode voltage of 600 V was effective to reduce the thermal budget, i.e., to achieve similar crystallinity with approximately 100 deg. C lower deposition temperature compared to the cases without ionization. It was revealed that the dielectric properties (relative dielectric constant {epsilon}{sub r} and its electric field tunability), out-of-plane lattice parameter of (001)-oriented grains (a{sub 001}), and the existence of (110)-oriented grains are correlated with one another. Elongation of a{sub 001} was suppressed, resulting in large {epsilon}{sub r} values comparable with that of a ceramic bulk of the same composition, in the BSTZ films that contain (110)-oriented grains. Less volume of amorphous BSTZ region is supposed to be playing an important role for the bulklike properties of these BSTZ films.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Lead pyrovanadate single crystal as a new SRS material

Lead pyrovanadate Pb{sub 2}V{sub 2}O{sub 7} single crystals of optical quality suitable for laser experiments are obtained. Vibrational modes are identified based on the analysis of the polarised Raman spectra of the single crystals. The main parameters (width at half maximum, peak and integral intensities) of the spectral lines most promising for SRS conversion in this material are estimated. These parameters are compared with the corresponding parameters of the most frequently used lines of known Raman materials: yttrium and gadolinium vanadates, potassium and lead tungstates, and lead molybdate. (active media)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

First-principles study of dislocations in hcp metals through the investigation of the (1121) twin boundary

Herein, we use first principles calculations to study the energy of the (1121) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal close-packed metals. The (1121) twin boundary is unique in that it is composed of--and can form by the glide of--basal dislocations nucleating at every c lattice parameter. The effect of the number of atoms between boundaries on the boundary energy, and the resulting lattice strains of the relaxed structures are quantified. It is shown that the energies obtained converge within 32-64 atoms/supercell. The structures with a higher second-order elastic constant term, c{sub 44}, also have higher boundary energies. It is further shown that the critical resolved shear stresses of the basal dislocations at 0 K, which make up the (1121) twin, are so low as to be below the threshold of the first principles calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Synthesis and Characterization of Lead Calcium Titanate Nanocomposite

Lead calcium titanate nanoparticles (PCT-NPs) were synthesized by the modified sol-gel method. The mixed metal alkoxide precursor was prepared by moderate refluxing using lead acetate, calcium acetate, titanium iso-propoxide and 2-methoxyethanol. The properties of PCT-NPs were studied as a function of calcination temperatures. Results from the powder X-ray diffraction (XRD) and thermal analysis (TGA/DTA), indicated that the single-phase perovskite PCT-NPs were obtained after heat treatment at temperature of 700 deg. C. The average particle size of synthesized PCT-NPs was found to be around 30 nm from transition electron microscopy (TEM).

36 MATERIALS SCIENCE↗
DOE OSTI2011

Evolution of the surface structures on SrTiO{sub 3}(110) tuned by Ti or Sr concentration

The surface structure of the SrTiO{sub 3}(110) polar surface is studied by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Monophased reconstructions in (5 x 1), (4 x 1), (2 x 8), and (6 x 8) are obtained, respectively, and the evolution between these phases can be tuned reversibly by adjusting the Ar{sup +} sputtering dose or the amount of Sr/Ti evaporation. Upon annealing, the surface reaches the thermodynamic equilibrium that is determined by the metal concentration. The different electronic structures and absorption behaviors of the surface with different reconstructions are investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2011

First-principles study of the critical thickness in asymmetric ferroelectric tunnel junctions

The absent critical thickness of fully relaxed asymmetric ferroelectric tunnel junctions is investigated by first-principles calculations. The results show that PbTiO{sub 3} thin film between Pt and SrRuO{sub 3} electrodes can still retain a significant and stable polarization down to thicknesses as small as 0.8 nm, quite unlike the case of symmetric ferroelectric tunnel junctions. We trace this surprising result to the generation of a large electric field by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even complete elimination, for the critical thickness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Nanoindentation Studies Of Hard Nanocomposite Ti-B-N Thin Films

Titanium boron nitride (Ti-B-N) films were deposited by reactive magnetron sputtering using single Titanium diboride (TiB{sub 2}) target in different Ar-N{sub 2} gas mixtures. The influence of N{sub 2}:Ar ratio on the microstructure and mechanical properties of the deposited films have been investigated. Atomic force microscopy analysis indicated the grain size decreases with incorporation of nitrogen in the films. Nanoindentation studies have shown the hardness decreases with nitrogen incorporation.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Magnetoelectric effect in Fe-embedded BaTiO{sub 3} single crystal

In this work, we experimentally demonstrate the magnetoelectric effect in a multilayered structure of Fe-BaTiO{sub 3}-Fe, with 70 A BaTiO{sub 3} (BT) sandwiched between 2 layers of implanted Fe, which was further treated by swift heavy ion (Ag{sup +15}) induced ion beam mixing/annealing. Due to this specific experimental procedure, condition of atomic orbital overlap between the Fe and Ti atoms could be favored in Fe-implanted BT crystal, showing a magnetoelectric effect arising from interfacial bonding at Fe/BT interface, as proposed by Duan et al.[Phys. Rev. Lett. 97, 047201 (2006)]. Results are successfully interpreted in terms of magnetostriction behavior of polycrystalline Fe.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Electrical characteristics of Au and Ag Schottky contacts on Nb-1.0 wt %-doped SrTiO{sub 3}

The (Au,Ag)/Nb-1.0 wt %-doped SrTiO{sub 3} (NSTO), i.e., Au/NSTO, Ag/NSTO, Ag/Au/NSTO, and Au/Ag/NSTO junctions were fabricated by magnetic controlled sputtering and annealing process. Backward diodelike behaviors were observed in all as-prepared samples, and forward rectifying characteristics were gradually developed in the four junctions after annealed in oxygen at 400 deg. C, 550 deg. C, 700 deg. C, and 850 deg. C, respectively. Compared with the Ag/NSTO junctions, the Au/NSTO junctions showed higher temperature evolving procedure of forward rectifying behavior. Moreover, ideal Schottky behaviors appeared in the samples annealed at 850 deg. C, which were attributed to the formation of interface states. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analysis showed that the interface states mainly originated from the migration of metal into NSTO matrix and the changes in chemical composition at the metal/NSTO interfaces. The results of current-voltage measurements reveal that annealing at higher temperature alters the interface barrier and thereby ameliorates the stability of leakage current remarkably.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Finite Element Simulation of Machining of Ti6Al4V Alloy

Titanium and its alloys are an important class of materials, especially for aerospace applications, due to their excellent combination of strength and fracture toughness as well as low density. However, these materials are generally regarded as difficult to machine because of their low thermal conductivity and high chemical reactivity with cutting tool materials. Moreover, the low thermal conductivity of Titanium inhibits dissipation of heat within the workpiece causing an higher temperature at the cutting edge and generating for higher cutting speed a rapid chipping at the cutting edge which leads to catastrophic failure. In addition, chip morphology significantly influences the thermo-mechanical behaviour at the workpiece/tool interface, which also affects the tool life.In this paper a finite element analysis of machining of TiAl6V4 is presented. In particular, cutting force, chip morphology and segmentation are taken into account due to their predominant roles to determine machinability and tool wear during the machining of these alloys. Results in terms of residual stresses are also presented. Moreover, the numerical results are compared with experimental ones.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Constraint Optimization Problem For The Cutting Of A Cobalt Chrome Refractory Material

This paper shows a complete approach to solve a given problem, from the experimentation to the optimization of different cutting parameters. In response to an industrial problem of slotting FSX 414, a Cobalt-based refractory material, we have implemented a design of experiment to determine the most influent parameters on the tool life, the surface roughness and the cutting forces. After theses trials, an optimization approach has been implemented to find the lowest manufacturing cost while respecting the roughness constraints and cutting force limitation constraints. The optimization approach is based on the Response Surface Method (RSM) using the Sequential Quadratic programming algorithm (SQP) for a constrained problem. To avoid a local optimum and to obtain an accurate solution at low cost, an efficient strategy, which allows improving the RSM accuracy in the vicinity of the global optimum, is presented. With these models and these trials, we could apply and compare our optimization methods in order to get the lowest cost for the best quality, i.e. a satisfying surface roughness and limited cutting forces.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Transport Properties Of Fe{sub 3-x}Ti{sub x}O{sub 4}(x = 0.0 and 0.0206) Epitaxial Thin Films

Fe{sub 3-x}Ti{sub x}O{sub 4}(x = 0 and 0.0206) epitaxial thin films with thickness of 150 nm were prepared by pulsed-laser deposition technique on SrTiO{sub 3}(100), {alpha}-Al{sub 2}O{sub 3}(0001), Si (111) and Float Glass substrates. X-ray diffraction (XRD) pattern infers that parent and Ti doped magnetites are grown in single phase with (111) orientation. The Verwey transition temperature for Fe{sub 3}O{sub 4} thin films are 121 K (Float Glass), 123 K (SrTiO{sub 3}), 123.5 K ({alpha}-Al{sub 2}O{sub 3}) and 128 K (Si). It is shown that the Ti doping at B site results in the disappearance of Verwey transition in {l_brace}Ti{sub x}{sup 4+}Fe{sub 1-x}{sup 3+}Fe{sup 2+}{r_brace}O{sub 4}{sup 2-}.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial integration of ferromagnetic correlated oxide LaCoO{sub 3} with Si (100)

We have grown epitaxial strained LaCoO{sub 3} on (100)-oriented silicon by molecular beam epitaxy using a relaxed epitaxial SrTiO{sub 3} buffer layer. Superconducting quantum interference device magnetization measurements show that, unlike the bulk material, the ground state of the strained LaCoO{sub 3} on silicon is ferromagnetic with a T{sub C} of 85 K. First principles calculations suggest that a ferromagnetic ground state can be stabilized in LaCoO{sub 3} by a sufficiently large biaxial tensile strain with the transition accompanied by a partial untilting of the CoO{sub 6} octahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Oxygen incorporation effects in annealed epitaxial La{sub (1-x)}Sr{sub x}MnO{sub 3} thin films

This paper presents our results regarding oxygen incorporation effects in epitaxial La{sub (1-x)}Sr{sub x}MnO{sub 3} thin films, deposited on SrTiO{sub 3} (001) single crystal substrates, by annealing in different gas mixtures of argon and oxygen. A particular emphasis is placed on the correlation of structural properties with the magnetic properties of the films, Curie temperature, and coercive field. In this sense, we demonstrate that the evolution of the diffuse part of the {omega}-scans performed on the films are due to oxygen excess in the film lattice, which creates cationic vacancies within the films. Also, we show that two regimes of oxygen incorporation in the films exist, one in which the films evolve toward a single phase and oxygen stoichiometry is recovered, and a second one dominated by oxygen over-doping effects. In order to support our study, XPS measurements were performed, from which we have evaluated the Mn{sup 3+}/Mn{sup 4+} ionic ratio.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Structural and magnetic properties of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} thin films integrated onto Si(100) substrates with SrTiO{sub 3} as buffer layer

La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (LSMO) thin films with a thickness d of 10, 20, 60, and 100 nm were grown on 20-nm-thick SrTiO{sub 3}-buffered (100) silicon substrates by a reactive molecular beam epitaxy. For all samples, x-ray diffraction (XRD) revealed an excellent epitaxy with in-plane cubic [100] and [010] axes of LSMO. The XRD measured values of the out-of-plane lattice parameter suggest that the strain state does not vary significantly from sample to sample. A super conducting quantum interference device reveals that the room temperature magnetization at saturation increases with d and nearly reaches the bulk value for d = 100 nm; the Curie temperature ranges in the 320-350 K interval, to compare to 360 K in the bulk. Ferromagnetic resonance (FMR) in cavity (at 9.5 GHz) and microstrip FMR used to investigate the dynamic magnetic properties, revealed a fourfold anisotropy showing its easy axes along the [110] and [110] directions. In the thickest samples (d > 20 nm), the FMR spectra present two distinct resonant modes. This splitting is presumably due to the simultaneous presence of two different magnetic phases.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Aurivillius phases of PbBi{sub 4}Ti{sub 4}O{sub 15} doped with Mn{sup 3+} synthesized by molten salt technique: Structure, dielectric, and magnetic properties

Doping of manganese (Mn{sup 3+}/Mn{sup 4+}) into the Aurivillius phase Pb{sub 1-x}Bi{sub 4+x}Ti{sub 4-x}Mn{sub x}O{sub 15} was carried out using the molten salt technique for various Mn concentrations (x=0, 0.2, 0.4, 0.6, 0.8, and 1). Single phase samples could be obtained in the composition range with x up to 0.6 as confirmed by X-ray and neutron diffraction analysis. Dielectric measurements show a peak at 801, 803, 813 and 850 K for samples with x=0, 0.2, 0.4, and 0.6, respectively, related to the ferroelectric transition temperature (T{sub c}). The main contribution of the in-plane polarization for x{<=}0.2 which was calculated from the atomic positions obtained by the structure analysis is the dipole moment in the Ti(1)O{sub 6} layer; however, for x{>=}0.4 the polarization originates from the dipole moment in the Ti(2)O{sub 6} layer. Mn doping in the Pb{sub 1-x}Bi{sub 4+x}Ti{sub 4-x}Mn{sub x}O{sub 15} does not show any long range magnetic ordering. -- Graphical abstract: The dipole moment of TiO{sub 6} dependence of x in Pb{sub 1-x}Bi{sub 4+x}Ti{sub 4-x}Mn{sub x}O{sub 15} (0{<=}x{<=}0.6): (a) along a-axis and (b) on the ab plane. This figure indicated that polarization in the ab-plane contribute to the total polarization. Display Omitted Highlights: {yields} We have prepared Pb{sub 1-x}Bi{sub 4+x}Ti{sub 4-x}Mn{sub x}O{sub 15}, 0{<=}x{<=}1 by the molten-salt method. {yields} The oxides adopted A2{sub 1}am space group. {yields} Dipole moments of the inner Ti(1)O{sub 6} are the main polarization contributor for x{<=}0.2. {yields} Dipole moments of the outer Ti(2)O{sub 6} are the main polarization contributor for x>0.2. {yields} Ferromagnetic interactions show the contribution of mixed valence of Mn{sup 3+}/Mn{sup 4+}.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Anomaly in the conductivity relaxation parameters at the phase transition of ferroelectric materials: A time domain study

The influence of the ferroelectric to paraelectric transition on the relaxation parameters of conductive processes in ferroelectric materials is studied in the time domain. Three well-known ferroelectric systems were chosen with transition temperatures in different regions, these are, high-temperature PbNb{sub 2}O{sub 6}-based ceramics; nanostructured Pb(Zr{sub 0.6}Ti{sub 0.4})O{sub 3} ceramics; and submicron BaTiO{sub 3}. The thermal evolution of relaxation parameters shows clear anomalies in their typical behavior when conductivity processes arise in the temperature range where the ferroelectric transition takes place. The method here described allows obtaining information about the correlation between charge transport and the motion of the off-center ions at the phase transition.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Dielectric and ferroelectric response of compositionally graded bilayer and trilayer composites of BaTiO{sub 3} and 0.975BaTiO{sub 3}-0.025Ba(Cu{sub 1/3}Nb{sub 2/3})O{sub 3}

In this paper, we report the dielectric and ferroelectric response of compositionally graded bilayer and trilayer composites consisting of BaTiO{sub 3} (BT) and 0.975BaTiO{sub 3}-0.025Ba(Cu{sub 1/3}Nb{sub 2/3})O{sub 3} (BTBCN). Two types of graded bilayer samples were synthesized, one with same thickness of BT and BTBCN while other with different layer thicknesses. The graded trilayer sample consisted of BT layer sandwiched between two BTBCN layers of equal thickness. Scanning electron microscopy and transmission electron microscopy images showed a sharp interface with needle-shape domains across the interface. The domain size on BT side was found to be larger than that on BTBCN side. The temperature dependence of dielectric response for all composite systems was found to exhibit shifting in characteristic Curie peak compared to constituent material which was associated to coupling between layers. Moreover, the differences in grain size, tetragonality, domain mobility of each layer was found to perturb the electrical response of composite. The polarization mismatch between uncoupled BT and BTBCN established internal electric field in composite specimen and defined new polarization states in each layer by perturbing free energy functional of the composite specimen. Dynamic hysteresis behaviors and power-law scaling relations of all specimens were determined from polarization-electric field hysteresis loop measurements as a function of frequency. All systems were found to exhibit similar dynamic scaling relationships. Hysteresis area <A>, P{sub r}, and E{sub C} decreased with increasing frequency due to delayed response but increased with increasing applied electric field due to enhancement of driving force. Trilayer system was found to exhibit strong internal-bias field and double hysteresis behavior. The coupling effect resulting due to polarization mismatch between layers had substantial influence on the dynamic hysteresis behavior and power-law scaling relations.

36 MATERIALS SCIENCE↗