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How Tough is Human Cortical Bone? In-Situ Measurements on Realistically Short Cracks

Description: Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behavior of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behavior of human cortical bone which accurately assesses its toughness at appropriate size-scales. Here we use in-situ mechanical testing in the scanning electron microscope and x-ray computed tomography to examine how physiologically-pertinent short (<600 mu m) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 mu m of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction due to major crack deflections/twists principally at cement sheathes. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimeters in length that bridging mechanisms can develop leading to the (larger-crack) toughnesses generally quoted for bone.
Date: May 10, 2008
Creator: Ritchie, Robert O.; Koester, K. J.; Ager III, J. W. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

Performance of Ultra Hard Carbon Wear Coatings on Microgears Fabricated by Liga

Description: Stiction and friction are of concern for the reliable, long-term application of Ni-alloy micromachines. We have found that the application of a 30-70 nm hard carbon coating produces a significant reduction in the friction coefficient and wear rate of electroformed Ni substrates in reciprocating sliding contact under simulated MEMS operating conditions. To evaluate the performance of coated components, a series of 70-pm-thick microgears ranging in diameter from 0.2 to 2.2 mm were fabricated from electroformed Ni via standard LIGA processes and fixtured on posts in preparation for the coating procedure. A pulsed vacuum- arc deposition process was used to deposit a carbon coating on the gears with the plasma incident at a shallow angle to the gears' top surface. A sample bias of -2 keV was used in order to produce a coating with relatively low stress and good adhesion while maintaining high hardness. This coating process is known to be somewhat comformal to the component surfaces. The coating uniformity, particularly in the high-aspect-ratio areas between the gear teeth, was evaluated with micro-Raman spectroscopy. It is shown that the coating can be applied uniformly on the top gear surface. Between the gear teeth the coating was the same thickness as on top of the gear down to a point 50 ~m below the top surface. Below that point (i.e. between 50 and 70 Lm), the coating thickness is somewhat thinner, but is still present. These results demonstrate that it is possible to a deposit hard carbon coating on microgears to reduce friction and wear in micromachines.
Date: December 18, 1998
Creator: Ager III, J.W.; Brown, I.G.; Christenson, T.R.; Dugger, M.T.; Follstaedt, D.M.; Knapp, J.A. et al.
Partner: UNT Libraries Government Documents Department

Raman spectroscopy and time-resolved photoluminescence of BN and BxCyNz nanotubes

Description: We report Raman and time-resolved photoluminescence spectroscopic studies of multiwalled BN and B{sub x}C{sub y}N{sub z} nanotubes. The Raman spectroscopy shows that the as-grown B{sub x}C{sub y}N{sub z} charge recombination, respectively. Comparison of the photoluminescence of BN nanotubes to that decay process is characterized by two time constants that are attributed to intra- and inter-BN sheet nanotubes as predicted by theory. nanotubes are radially phase separated into BN shells and carbon shells. The photoluminescence of hexagonal BN is consistent with the existence of a spatially indirect band gap in multi-walled BN.
Date: January 21, 2004
Creator: Wu, J.; Han, Wei-Qiang; Walukiewicz, W.; Ager III, J.W.; Shan, W.; Haller,E.E. et al.
Partner: UNT Libraries Government Documents Department

Pressure-dependent photoluminescence study of ZnO nanowires

Description: The pressure dependence of the photoluminescence (PL) transition associated with the fundamental band gap of ZnO nanowires has been studied at pressures up to 15 GPa. ZnO nanowires are found to have a higher structural phase transition pressure around 12 GPa as compared to 9.0 GPa for bulk ZnO. The pressure-induced energy shift of the near band-edge luminescence emission yields a linear pressure coefficient of 29.6 meV/GPa with a small sublinear term of -0.43 meV/GPa{sup 2}. An effective hydrostatic deformation potential -3.97 eV for the direct band gap of the ZnO nanowires is derived from the result.
Date: September 13, 2004
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Zhang, Y.; Mao, S.S. et al.
Partner: UNT Libraries Government Documents Department

Band anticrossing in dilute nitrides

Description: Alloying III-V compounds with small amounts of nitrogen leads to dramatic reduction of the fundamental band-gap energy in the resulting dilute nitride alloys. The effect originates from an anti-crossing interaction between the extended conduction-band states and localized N states. The interaction splits the conduction band into two nonparabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy. The changes in the conduction band structure result in significant increase in electron effective mass and decrease in the electron mobility, and lead to a large enhance of the maximum doping level in GaInNAs doped with group VI donors. In addition, a striking asymmetry in the electrical activation of group IV and group VI donors can be attributed to mutual passivation process through formation of the nearest neighbor group-IV donor nitrogen pairs.
Date: December 23, 2003
Creator: Shan, W.; Yu, K.M.; Walukiewicz, W.; Wu, J.; Ager III, J.W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Nitrogen-Induced Modification of the Electronic Structure of Group III-N-V Alloys: Preprint

Description: Incorporation of nitrogen in conventional III-V compound semiconductors to form III-N-V alloys leads to a splitting of the conduction band into two nonparabolic sub-bands. The splitting can be described in terms of an anticrossing interaction between a narrow band of localized nitrogen states and the extended conduction-band states of the semiconductor matrix. The downward shift of the lower sub-band edge is responsible for the N-induced reduction of the fundamental band-gap energy. The modification of the conduction-band structure profoundly affects the optical and electrical properties of the III-N-V alloys.
Date: April 1, 1999
Creator: Walukiewicz, W.; Shan, W.; Ager III, J. W.; Chamberlin, D. R.; Haller, E. E. (Lawrence Berkeley National Laboratory); Geisz, J. F. et al.
Partner: UNT Libraries Government Documents Department

Effect of pressure on the luminescence emissions in CuGaSe2

Description: We present the results of a pressure-dependent photoluminescence (PL) study on CuGaSe{sub 2} films grown on GaAs substrate by metalorganic vapor phase epitaxy. The low-temperature PL spectra of the CuGaSe{sub 2} samples measured at atmospheric pressure are dominated by one near band-edge exciton luminescence line and two strong and relatively broad emissions associated with donor acceptor pairs (DAP). All the observed luminescence emission lines shift toward higher energy with increasing pressure at almost the same rate. The nearly identical pressure coefficients of the two DAP emissions as compared to that of the exciton emission confirm the suggestion that the recombination processes associated with the DAPs involve one shallow donor and two different acceptor species with different binding energies and related to two different native defects.
Date: July 13, 2004
Creator: Shan, W.; Walukiewicz, W.; Wu, J.; Yu, K.M.; Ager III, J.W.; Siebentritt, S. et al.
Partner: UNT Libraries Government Documents Department

Valence band hybridization in N-rich GaN1-xAsx alloys

Description: We have used photo-modulated transmission and optical absorption spectroscopies to measure the composition dependence of interband optical transitions in N-rich GaN{sub 1-x}As{sub x} alloys with x up to 0.06. The direct bandgap gradually decreases as x increases. In the dilute x limit, the observed band gap approaches 2.8 eV; this limiting value is attributed to a transition between the As localized level, which has been previously observed in As-doped GaN at 0.6 eV above the valence band maximum in As-doped GaN, and the conduction band minimum. The structure of the valence band of GaN{sub 1-x}As{sub x} is explained by the hybridization of the localized As states with the extended valence band states of GaN matrix. The hybridization is directly confirmed by soft x-ray emission experiments. To describe the electronic structure of the GaN{sub 1-x}As{sub x} alloys in the entire composition range a linear interpolation is used to combine the effects of valence band hybridization in N-rich alloys with conduction band anticrossing in As-rich alloys.
Date: May 4, 2004
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Denlinger, J.D.; Shan, W.; Ager III, J.W. et al.
Partner: UNT Libraries Government Documents Department

Universal bandgap bowing in group III nitride alloys

Description: The energy gaps of MBE-grown wurtzite-structure In{sub 1-x}Al{sub x}N alloys with x {le} 0.25 have been measured by absorption and photoluminescence experiments. The results are consistent with the recent discovery of a narrow bandgap of {approx}0.8 eV for InN. A bowing parameter of 3 eV was determined from the composition dependence of these bandgaps. Combined with previously reported data of InGaN and AlGaN, these results show a universal relationship between the bandgap variations of group III nitride alloys and their compositions.
Date: August 6, 2002
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Ager III, J.W.; Li, S.X.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department

Effect of oxygen on the electronic band structure in ZnO{sub x}Se{sub 1-x} alloys

Description: The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnO{sub x}Se{sub 1-x} epitaxial films (0 {<=} x {<=} 1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.
Date: March 14, 2003
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Wu, J.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department

Indium nitride: A narrow gap semiconductor

Description: The optical properties of wurtzite InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. All these three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free electron concentration of the sample. The peak energy exhibits a very weak hydrostatic pressure dependence and a small, anomalous blueshift with increasing temperature. The bandgap energies of In-rich InGaN alloys were found to be consistent with the narrow gap of InN. The bandgap bowing parameter was determined to be 1.43 eV in InGaN.
Date: August 14, 2002
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Ager III, J.W.; Haller, E.E.; Lu, H. et al.
Partner: UNT Libraries Government Documents Department

Hole transport and photoluminescence in Mg-doped InN

Description: Hole conductivity and photoluminescence were studied in Mg-doped InN films grown by molecular beam epitaxy. Because surface electron accumulation interferes with carrier type determination by electrical measurements, the nature of the majority carriers in the bulk of the films was determined using thermopower measurements. Mg concentrations in a&quot;window&quot; from ca. 3 x 1017 to 1 x 1019 cm-3 produce hole-conducting, p-type films as evidenced by a positive Seebeck coecient. This conclusion is supported by electrolyte-based capacitance voltage measurements and by changes in the overall mobility observed by Hall effect, both of which are consistent with a change from surface accumulation on an n-type film to surface inversion on a p-type film. The observed Seebeck coefficients are understood in terms of a parallel conduction model with contributions from surface and bulk regions. In partially compensated films with Mg concentrations below the window region, two peaks are observed in photoluminescence at 672 meV and at 603 meV. They are attributed to band-to-band and band-to-acceptor transitions, respectively, and an acceptor binding energy of ~;;70 meV is deduced. In hole-conducting films with Mg concentrations in the window region, no photoluminescence is observed; this is attributed to electron trapping by deep states which are empty for Fermi levels close to the valence band edge.
Date: March 24, 2010
Creator: Miller, N.; Ager III, J. W.; Smith III, H. M.; Mayer, M. A.; Yu, K. M.; Haller, E. E. et al.
Partner: UNT Libraries Government Documents Department

Theory of Nanocluster Size Distributions from Ion Beam Synthesis

Description: Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict the existence of a steady state shape for the cluster size distribution that depends only on a characteristic length determined by the ratio of the effective diffusion coefficient to the ion flux. The average cluster size in the steady state regime is determined by the implanted species/matrix interface energy.
Date: June 13, 2008
Creator: Yuan, C.W.; Yi, D.O.; Sharp, I.D.; Shin, S.J.; Liao, C.Y.; Guzman, J. et al.
Partner: UNT Libraries Government Documents Department