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Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defects in the binary C15 NbCrQ and NbCoz, and C 14 NbFe2 systems on both sides of stoichiometry were studied by both bulk density and X-ray Iattiee parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000"C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr2 Laves phase alloys after quenching from 1400"C. However, there are essentially no thermal vacancies in NbFe2 alloys after quenching from 1300oC. Anti-site hardening was found on both sides of stoichiometry for all the tie Laves phase systems studied, while the thermal vacancies in NbCr2 alloys quenched from 1400'C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds and the thermal vacancy softening is unique to the Laves phase. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.
Date: January 11, 1999
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

Photoluminescence Investigations of InGaAsN Alloys Lattice-Matched to GaAs

Description: InGaAsN is a semiconductor alloy system with the property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In this paper, we have measured the conduction-band mass measurements by three different techniques for 2% nitrogen in InGaAsN lattice matched to GaAs. Additionally, we also report pressure dependent measurements of the conduction-band mass between ambient and 40 kbar. Based on our results, we suggest that the observed changes in masses are a result of {Lambda}-X mixing.
Date: June 1, 1999
Creator: Jones, E.D.; Modine, N.R.; Allerman, A.A.; Fritz, I.J.; Kurtz, S.R.; Wright, A.F. et al.
Partner: UNT Libraries Government Documents Department

A generic approach to improved semi-solid forming of metals

Description: Lack of technology for the production of large inexpensive feedstock, with uniform spherical primary phase throughout as required for semi-solid forming, has restricted realization of the full potential for the semi-solid forming process. Furthermore, narrow process windows and alloy chemistry restrictions increase process costs and limit performance attributes possible with existing semi-solid metal systems. Successful semi-solid forming trials utilizing Chesapeake Composites Corporation's DSC trademark Metals for feedstock indicate that this represents a generic approach to providing a permanent highly uniform, spherical solid phase, without electromagnetic or mechanical shearing. This approach also provides for further growth of semi-solid forming by providing for: low cost large diameter billet stock, reduced semi-solid forming costs, extension of semi-solid forming to new alloy systems, and semi-solid formed components with substantially enhanced physical and mechanical proper ties.
Date: June 5, 2002
Creator: Klier, E. M.
Partner: UNT Libraries Government Documents Department

Phase stability in Be-Nb and Be-Nb-Zr intermetallics

Description: Sputter deposition of Be-Nb alloys at low temperature (30{degrees}C) produces an amorphous phase for compositions >5 at.% Nb. A metastable crystalline phase which can be considered a highly faulted form of the Be{sub 12}Nb occurs at higher deposition temperatures or by low-temperature annealing of the amorphous phase. Because of structural similarities, this metastable phase is a precursor to the formation of either Be{sub 12}Nb or Be{sub 17}Nb{sub 2} upon high temperature annealing. There was no evidence of the Be{sub 5}Nb phase which has been postulated on some phase diagrams. The Be{sub 12}Nb phase can accomodate considerable Zr in the structure and the Be{sub 13}Zr can accomodate Nb into its structure. The Be{sub 13}Zr becomes the predominant phase when the Zr/Nb composition ratio > 1. High temperature annealing of the ternary results in dual-phase regions of Be{sub 12}(Nb,Zr) + Be{sub 17}(NbZr){sub 2} or Be{sub 13}(Zr,Nb) + Be{sub 17}(Zr,Nb){sub 2}, but the coexistence of Be{sub 12}(Nb,Zr) + Be{sub 13}(Zr,Nb) has not been observed. 10 refs., 5 figs.
Date: September 1, 1991
Creator: Brimhall, J.L.; Charlot, L.A. & Bruemmer, S.M.
Partner: UNT Libraries Government Documents Department

Electron concentration and phase stability in NbCr2-based Laves phase alloys

Description: Phase stability in NbCr{sub 2}-based transition-metal Laves phases was studied, based on the data reported for binary X-Cr, Nb-X, and ternary Nb-Cr-X phase diagrams. It was shown that when the atomic size ratios are kept identical, the average electron concentration factor, e/a, is the dominating factor in controlling the phase stability of NbCr{sub 2}-based transition-metal Laves phases. The e/a ratios for different Laves polytypes were determined as followed: with e/a < 5.76, the C15 structure is stabilized; at an e/a range of 5.88--7.53, the C14 structure is stabilized; with e/a > 7.65, the C15 structure is stabilized again. A further increase in the electron concentration factor (e/a > 8) leads to the disordering of the alloy. The electron concentration effect on the phase stability of Mg-based Laves phases and transition-metal A{sub 3}B intermetallic compounds is also reviewed and compared with the present observations in transition-metal Laves phases. In order to verify the e/a/phase stability relationship experimentally, additions of Cu (with e/a = 11) were selected to replace Cr in the NbCr{sub 2} Laves phase. Experimental results for the ternary Nb-Cr-Cu system are reported and discussed in terms of the correlation between the e/a ratio and phase stability in NbCr{sub 2}-based Laves phases. A new phase was found, which has an average composition of Nb-47Cr-3Cu. Within the solubility limit, the electron concentration and phase stability relationship is obeyed in the Nb-Cr-Cu system.
Date: May 12, 1997
Creator: Zhu, J. H.; Liaw, P. K. & Liu, C. T.
Partner: UNT Libraries Government Documents Department

High efficiency, radiation-hard solar cells

Description: The direct gap of the In{sub 1-x}Ga{sub x}N alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells of the type used for space-based surveillance satellites. To evaluate the suitability of In{sub 1-x}Ga{sub x}N as a material for space applications, high quality thin films were grown with molecular beam epitaxy and extensive damage testing with electron, proton, and alpha particle radiation was performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In{sub 1-x}Ga{sub x}N retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. This indicates that the In{sub 1-x}Ga{sub x}N is well-suited for the future development of ultra radiation-hard optoelectronics. Critical issues affecting development of solar cells using this material system were addressed. The presence of an electron-rich surface layer in InN and In{sub 1-x}Ga{sub x}N (0 &lt; x &lt; 0.63) was investigated; it was shown that this is a less significant effect at large x. Evidence of p-type activity below the surface in Mg-doped InN was obtained; this is a significant step toward achieving photovoltaic action and, ultimately, a solar cell using this material.
Date: October 22, 2004
Creator: Ager, J. W., III & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Competing mechanisms for ordering tendencies in BCC CuAuZn{sub 2} and FCC AuFe alloys

Description: We have briefly discussed the ASRO (atomic short-range order) in AuFe and CuAuZn{sub 2}. General points are that (1) we have implemented a first-principles theory of ASRO in N-component alloys which allows determination of the electronic origins of said ASRO; (2) such calculations can provide much information on the high- and (sometimes) low-temperature alloys; and (3) this approach has identified the origin for the novel special-point ASRO in AuFe. Displacement effects, i.e., non-rigid lattice effects, as well as the other contributions beyond band-energy, are being incorporated into the multicomponent alloy calculations. Such improvements will allow us to investigate other alloys, where charge effects may play a role, to ``design`, for example, higher temperature intermetallics through alloying.
Date: July 1, 1995
Creator: Johnson, D.D.; Althoff, J.D.; Staunton, J.B.; Ling, M.F. & Pinski, F.J.
Partner: UNT Libraries Government Documents Department

Mutual Passivation in Dilulte GaNxAs1-x Alloys

Description: The dilute GaN{sub x}As{sub 1-x} alloys (with x up to 0.05) have exhibited many unusual properties as compared to the conventional binary and ternary semiconductor alloys. We report on a new effect in the GaN{sub x}As{sub 1-x} alloy system in which electrically active substitutional group IV donors and isoelectronic N atoms passivate each other's activity. This mutual passivation occurs in dilute GaN{sub x}As{sub 1-x} doped with group IV donors through the formation of nearest neighbor IV{sub Ga-}N{sub As} pairs when the samples are annealed under conditions such that the diffusion length of the donors is greater than or equal to the average distance between donor and N atoms. The passivation of the shallow donors and the N{sub As} atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental band gap. This mutual passivation effect is demonstrated in both Si and Ge doped GaN{sub x}As{sub 1-x} alloys. Analytical calculations of the passivation process based on Ga vacancies mediated diffusion show good agreement with the experimental results.
Date: March 21, 2005
Creator: Yu, K.M.; Walukiewicz, W.; Wu, J.; Mars, D.E.; Scarpulla, M.A.; Dubon, O.D. et al.
Partner: UNT Libraries Government Documents Department

Exceptional Electron Transport Properties of In-rich InGaN

Description: Recent years have seen an explosion of interest in the narrow band gap end of the InGaN alloy system, particularly in InN. The existence of surface electron accumulation and a tendency for n-type conductivity have been well-established and are explained by an extremely large electron affinity and the location of the Fermi level stabilization energy (E{sub FS}) high in the conduction band [1]. These characteristics pose significant challenges to the integration of In-rich InGaN into devices and demonstrate the need for a better understanding of the relationship between native defects and electronic transport in the alloy system. It has been previously shown that high-energy particle irradiation can predictably control the electronic properties of In-rich InGaN [1]. With increasing irradiation dose, the electron concentration (n) increases and the electron mobility ({mu}) decreases until the Fermi level reaches E{sub FS}, which is the saturation point. The value of n at saturation decreases with decreasing In fraction, due to the raising of the conduction band edge with respect to E{sub FS}.
Date: October 22, 2006
Creator: Jones, R.E.; van Genuchten, H.C.M.; Yu, K.M.; Walukiewicz, W.; Li, S.X.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department


Description: The processing-structure-property relationship is investigated for electrodeposited foils of the gold-copper alloy system. A model is presented that relates the deposition process parameters to the nanocrystalline grain size. An activation energy of 1.52 eV {center_dot} atom{sup -1} for growth is determined for a long pulse (&gt;10 msec) mode, and is 0.16 eV {center_dot} atom{sup -1} for short pulses (&lt;5 msec). The affect of nanocrystalline grain size on the mechanical properties is assessed using indentation measurements. A Hall-Petch type variation of the Vickers microhardness with nanocrystalline grain size (&gt;6 nm) is observed for Au-Cu samples with 1-12 wt.% Cu as tested in cross-section. The hardness increases three-fold from a rule-of-mixtures value &lt;1 GPa to a maximum of 2.9 GPa.
Date: February 25, 2005
Creator: Jankowski, A F; Saw, C K; Harper, J F; Vallier, R F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

Method for the prediction of the hydriding thermodynamics of ternary PD-based alloys.

Description: A method has been developed to calculate the hydriding thermodynamics of ternary Pd-X-Y systems, where X and Y are substitutional alloying elements, by using the properties of the binary Pd-X and Pd-Y systems. Experimental data was collected on the Pd-Rh-Co system to test the validity of this method. Hydrogen pressure-composition isotherms of several binary Pd-Rh and Pd-Co alloys and Pd-Rh-Co ternary alloys were measured to determine the thermodynamics of hydrogen absorption, hydride formation and decomposition, and hydrogen capacity. Good agreement between the calculated and measured values for the ternary Pd-Rh-Co system, in the dilute alloying regime (&lt; 10 at.% total alloying additions), was obtained using our method. Examining literature results on other ternary Pd-X-Y systems checked the universality of this method. Again, the method succeeds in predicting the hydriding thermodynamics for both lattice contracted and lattice expanded alloy systems, Pd-Ni-Rh and Pd-Ag-Y respectively.
Date: January 1, 2001
Creator: Teter, D. F. (David F.) & Mauro, M. E. (Michael Ernest)
Partner: UNT Libraries Government Documents Department

Theoretical Diffusion Profiles in Single Phase Ternary Systems

Description: In order to illustrate the physical meaning of the four independent interdiffusion coefficients which describe a single-phase ternary system, a range of concentration profiles and diffusion paths for the generalized diffusion couple A-10B/A-10C were plotted using three different models of diffusion coefficient behaviour. The shapes of the diffusion paths predicted by these various models were compared with that of an experimentally determined diffusion path from the cobalt solid solution of the substitutional alloy system cobalt-chromium-aluminum. The results indicate that even the most sophisticated dilute solution model fails to predict completely the behaviour of the real system.
Date: February 1, 1980
Creator: Roper, G.W. & Whittle, D.P.
Partner: UNT Libraries Government Documents Department

Fundamental Curie temperature limit in ferromagnetic Ga1-xMnxAs

Description: We provide unambiguous experimental evidence that the upper limit of {approx}110 K commonly observed for the Curie temperature TC of Ga{sub 1-x}Mn{sub x}As is caused by the Fermi-level-induced hole saturation. This conclusion is based on parallel studies of the location of Mn in the lattice, the effectiveness of acceptor center, and ferromagnetism on a series of Ga{sub 1-x-y}Mn{sub x}Be{sub y}As layers, in which the concentration of magnetic moments and of free holes can be independently controlled by the Mn and Be contents. Ion channeling and magnetization measurements show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T{sub C} with increasing Be concentration. At the same time the free hole concentration remains relatively constant at {approx}5 x 10{sup 20}cm{sup -3}. These results indicate that the concentrations of free holes as well as of ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors. Based on these results, we propose to use heavy n-type counter-doping of Ga{sub 1-x}Mn{sub x}As (by, e.g., Te) to suppress the formation of Mn interstitials at high x, and thus improve the T{sub C} of the alloy system.
Date: September 24, 2002
Creator: Yu, K. M.; Walukiewicz, W.; Wojtowicz, T.; Lim, W. L.; Liu, X.; Bindley, U. et al.
Partner: UNT Libraries Government Documents Department

Non-linear dynamics and pattern selection at the crystal-melt interface. Final report, September 1, 1984--February 28, 1998

Description: Pattern formation at the solidifying crystal-melt interface was investigated with a model binary alloy system of the molecular crystal succinonitrile doped with the laser dye Coumarin 152. Videomicroscopy and computerized digital image analysis provided detailed data for quantitative comparison with theoretical predictions. Application of a single localized laser pulse or a row of UV spots to the interface during solidification allowed for direct measurements of pattern stability. These thermal perturbations also resulted in dramatic modifications of the observed solidification patterns, suggesting that the controlled thermal perturbation approach may allow the microstructure of metallic alloys to be controlled during solidification.
Date: July 1, 1998
Creator: Cummins, H. Z.
Partner: UNT Libraries Government Documents Department

The determination of the solidification paths and the liquidus surface in the quasicrystalline region of the Al-Cu-Ru systems

Description: Objective is to determine if conventional slow-growth methods will be successful for growing single quasicrystals in this system. Reaction schemes, isopleths, and a ternary liquidus diagram were constructed using DTA, XRD, microscopy, and energy dispersive spectroscopy. Results suggest a peak in the liquidus near the predicted compositions of Al{sub 65}Cu{sub 23}Ru{sub 12} or Al{sub 65}Cu{sub 20}Ru{sub 15} in the face-centered icosahedral phase field. Further study is needed to verify the existence of a congruently melting phase. There appears to be composition for which slow-growth methods will produce a single-quasicrystal. Directional solidification studies were done on 65Al25Cu10Ru.
Date: December 6, 1995
Creator: Haines, S.
Partner: UNT Libraries Government Documents Department

The influence of melting process and parameters on the structure and homogeneity of titanium-tantalum alloys

Description: Alloys of titanium with refractory metals are attractive materials for applications requiring high temperature strength and corrosion resistance. However, the widely different characteristics of the component elements have made it difficult to produce sound, compositionally homogeneous ingots using traditional melting techniques. This is particularly critical because the compositional ranges spanned by the micro- and macrosegregation in theses systems can easily encompass a number of microconstituents which are detrimental to mechanical properties. This paper presents results of a study of plasma (PAM) and vacuum-arc (VAR) melting of a 60 wt% tantalum, 40 wt% titanium binary alloy. The structural and compositional homogeneity of both PAM consolidated + PAM remelted, and PAM consolidated + VAR remelted ingots were characterized and compared using optical and electron microscopy and x-ray fluorescence microanalysis. Additionally, the effect of melting parameter, including melt rate and magnetic stirring, was studied. Results indicate that PAM remelting achieves more complete dissolution of lie starting electrode, due to greater local superheat, than does VAR remelting. PAM remelting also produces a finer as-solidified grain structure, due to the smaller molten pool and lower local solidification times. Conversely, VAR remelting produces an ingot with a more uniform macrostructure, due to the more stable movement of the solidification interface and more uniform material feed rate. Based on these results, a three-step process of PAM consolidation, followed by a PAM intermediate melt and a VAR final melt, has been selected for further development of the alloy and processing sequence.
Date: December 31, 1995
Creator: Dunn, P.S.; Korzewka, D.; Garcia, F.; Damkroger, B.K.; Van Den Avyle, J.A. & Tissot, R.G.
Partner: UNT Libraries Government Documents Department

Intermetallic insertion anodes for lithium batteries.

Description: Intermetallic alloys have been investigated for many years as anode materials for lithium batteries and, more recently, as alternative electrodes to carbon, because of several intrinsic advantages including high capacity and safety. Some of the most studied alloys utilize tin as the active component because of its high theoretical capacity (996 mAh/g) and its slightly higher operating voltage (&lt;400 mV) compared to metallic lithium. To date, the use of binary lithium alloys as anodes has been limited to the select number of main group elements (e.g. Sn) that can be lithiated at an appropriate voltage with acceptable kinetics. A major disadvantage of binary Li{sub x}M alloy systems is that major phase changes occur during the electrochemical cycling of lithium. Severe volume expansion and contraction of the metal matrix, which limit the cycle life of the lithium cell, normally accompany these phase changes. The. most successful approach to overcoming this limitation has been the use of intermetallic alloys MM{prime} consisting of two (or more) metals, at least one of which is an ''active'' alloying element (M) and the other an ''inactive'' (M{prime}) element. During the reaction with lithium, such a system breaks up into regions of Li{sub x}M and inactive M{prime}. In our work, we have extended the concept of intermetallic electrodes to include topotactic reactions in which the intermetallic compound provides a host structure for lithium.
Date: July 19, 1999
Creator: Johnson, C. S.; Kahaian, A. J.; Kepler, K. D.; Scott, M.; Thackerary, M. M. & Vaughey, J.
Partner: UNT Libraries Government Documents Department

[Equilibrium composition of interphase boundaries]. Final report

Description: The principal thrust of this research was to develop improved understanding of the factors which control interfacial segregation at interphase boundaries in metallic alloy systems. The work consisted of both modeling as well as experimental components. Summaries of the research results in each of these areas is given.
Date: October 1, 1998
Creator: Wynblatt, P.
Partner: UNT Libraries Government Documents Department

Solidification at the High and Low Rate Extreme

Description: The microstructures formed upon solidification are strongly influenced by the imposed growth rates on an alloy system. Depending on the characteristics of the solidification process, a wide range of growth rates is accessible. The prevailing solidification mechanisms, and thus the final microstructure of the alloy, are governed by these imposed growth rates. At the high rate extreme, for instance, one can have access to novel microstructures that are unattainable at low growth rates. While the low growth rates can be utilized for the study of the intrinsic growth behavior of a certain phase growing from the melt. Although the length scales associated with certain processes, such as capillarity, and the diffusion of heat and solute, are different at low and high rate extremes, the phenomena that govern the selection of a certain microstructural length scale or a growth mode are the same. Consequently, one can analyze the solidification phenomena at both high and low rates by using the same governing principles. In this study, we examined the microstructural control at both low and high extremes. For the high rate extreme, the formation of crystalline products and factors that control the microstructure during rapid solidification by free-jet melt spinning are examined in Fe-Si-B system. Particular attention was given to the behavior of the melt pool at different quench-wheel speeds. Since the solidification process takes place within the melt-pool that forms on the rotating quench-wheel, we examined the influence of melt-pool dynamics on nucleation and growth of crystalline solidification products and glass formation. High-speed imaging of the melt-pool, analysis of ribbon microstructure, and measurement of ribbon geometry and surface character all indicate upper and lower limits for melt-spinning rates for which nucleation can be avoided, and fully amorphous ribbons can be achieved. Comparison of the relevant time scales reveals that surface-controlled melt-pool ...
Date: December 19, 2004
Creator: Meco, Halim
Partner: UNT Libraries Government Documents Department

Mutual passivation of group IV donors and isovalent nitrogen in diluted GaN{sub x}As{sub 1-x} alloys

Description: We demonstrate the mutual passivation of electrically active group IV donors and isovalent N atoms in the GaN{sub x}As{sub 1-x} alloy system. This phenomenon occurs through the formation of a donor-nitrogen bond in the nearest neighbor IV{sub Ga}-N{sub As} pairs. In Si doped GaInN{sub 0.017}As{sub 0.983} the electron concentration starts to decrease rapidly at an annealing temperature of 700 C from {approx} 3 x 10{sup 19}cm{sup -3} in the as-grown state to less than 10{sup 16}cm{sup -3} after an annealing at 900 C for 10 s. At the same time annealing of this sample at 950 C increases the gap by about 35 meV, corresponding to a reduction of the concentration of the active N atoms by an amount very close to the total Si concentration. We also show that the formation of Si{sub Ga}-N{sub As} pairs is controlled by the diffusion of Si via Ga vacancies to the nearest N{sub As} site. The general nature of this mutual passivation effect is confirmed by our study of Ge doped GaN{sub x}As{sub 1-x} layers formed by N and Ge co-implantation in GaAs followed by pulsed laser melting.
Date: July 23, 2003
Creator: Yu, K.M.; Wu, J.; Walukiewicz, W.; Shan, W.; Beeman, J.; Mars, D.E. et al.
Partner: UNT Libraries Government Documents Department

A genetic approach to improved semi-solid forming of metals. Final report for period September 4, 1999 - March 4, 2000

Description: Lack of technology for the production of large inexpensive feedstock, with uniform spherical primary phase throughout as required for semi-solid forming, has restricted realization of the full potential for the semi-solid forming process. Furthermore narrow process windows and alloy chemistry restrictions increase process costs and limit performance attributes possible with existing semi-metal systems. Successful semi-solid forming trials utilizing Chesapeake Composites Corporation's DSC{sup TM} Metals for feedstock indicate that this represents a generic approach to providing a permanent highly uniform, spherical solid phase, without electromagnetic or mechanical shearing. This approach also provides for further growth of semi-solid forming by providing for: low cost large diameter billet stock, reduced semi-solid forming costs, extension of semi-solid forming to new alloy systems, and semi-solid formed components with substantially enhanced physical and mechanical properties.
Date: May 4, 2000
Creator: Klier, Eric M.
Partner: UNT Libraries Government Documents Department