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Direct evidence of the transformation of WOx species in WO3 nanoclusters on WOx–ZrO2 system was achieved by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy on samples obtained by a conventional precipitation method and annealed from 560 to 800 °C. WO3 Nanoclusters with 2-nm crystal size orthorhombic structure were identified on the ZrO2 surface after annealing at 800 °C.
A graphite wafer has been implanted with Mg+ to produce a uniform Mg concentration. Subsequent H+ implantation covered the Mg+-implanted and -unimplanted regions. Ion-beam analysis shows a higher H retention in graphite embedded with Mg than in regions without Mg. A small amount of H diffuses out of the H+-implanted graphite during thermal annealing at temperatures up to 300 °C. However, significant H release from the region implanted with Mg+ and H+ ions occurs at 150 °C; further release is also observed at 300 °C. The results suggest that there are efficient H trapping centers and fast pathways for H diffusion in the Mg+-implanted graphite, which may prove highly desirable for reversible H storage.
Sintered compacts of nano-sized and micron-sized BaTiO3 show sharp ferroelectric transition and high dielectric constant at specific compositions. The sintered compacts with 1 wt% nano-BaTiO3 show a maximum dielectric constant of 1680. At the transition temperature (Tc) there are two maxima at 0.5 and 2 wt%. The variation in the dielectric constant at Tc is also reflected in the behavior of the ferroelectric transition as studied by differential scanning calorimetry. This interesting oscillatory variation of the dielectric constant and dielectric loss with increase in the amount of nanoparticles in the sintered compacts is observed for the first time. The variation of the dielectric properties and the ferroelectric transition of the sintered compacts could be related to subtle changes in the microstructure.
In this paper, details are given for the structural evolution of (Ti33Zr33Hf33)70(Ni50Cu50)20Al10, (Ti25Zr25Hf25Nb25)70(Ni50Cu50)20Al10, and (Ti33Zr33Hf33)70(Ni33Cu33Ag33)20Al10 amorphous alloys, part of wider program of alloy development by equiatomic substitution. All three alloys initially crystallize by forming a nanoscale icosahedral phase. However, at higher temperatures, their decomposition sequences differ significantly. The nanoscale icosahedral phase in the (Ti33Zr33Hf33)70(Ni50Cu50)20Al10 alloy decomposes into a mixture of Zr2Cu-type and icosahedral phases. This icosahedral phase still exists after heating up to 970 K, indicating a high thermal stability of this phase. The nanoscale icosahedral phase in the (Ti33Zr33Hf33)70(Ni33Cu33Ag33)20Al10 alloy also transforms into a mixture of Zr2Cu-type and icosahedral phase during the second exothermic reaction but then transforms into a mixture of Zr2Cu-type and Ti2Ni-type phases. The nanoscale icosahedral phase in the (Ti25Zr25Hf25Nb25)70(Ni50Cu50)20Al10 alloy decomposes into a mixture of Ti2Ni-type and MgZn2-type phases during the second exothermic reaction. It is concluded that the formation of the Zr2Cu-type phase retards the decomposition of the nanoscale icosahedral phase, which increases the thermal stability. In contrast, formation of Ti2Ni-type and MgZn2-type phases accelerates the decomposition of the nanoscale icosahedral phase, which decreases its thermal stability.
Structural ceramics are considered as potential candidate materials for use in hybrid bearings in rocket turbopumps, operated under high stress in cryogenic environment. The friction and wear-related surface failure is considered as one of the critical factors in selecting the materials for cryo-turbopumps of Space Shuttle Main Engine (SSME). To obtain fundamental understanding of the tribological properties of ceramics in cryogenic environment, a very first set of sliding wear tests were carried out on self-mated Al2O3, a model brittle ceramic material, in liquid nitrogen (LN2) under varying load (2–10 N) and high rotational speed of 2550 rpm, using a newly designed cryogenic tribometer. The present research attempts to answer some important questions: (i) What would be the influence of LN2 on frictional and fracture behavior at sliding contacts? (ii) How does the material removal process occur in LN2 environment? Our experimental results reveal that self-mated alumina exhibits low steady-state coefficient of friction ∼0.13–0.18 and suffers from high wear rate (10−5 mm3/Nm) under the selected testing conditions. The novelty of the present work also lies in presenting some interesting results, for the first time, concerning the deformation and fracture of alumina at cryogenic temperature under high speed sliding conditions. Detailed scanning electronic microscope observation of the worn surfaces indicates that severe damage of both ball and flat occurs in cryogenic environment by transgranular and intergranular fracture. The observed wear behavior is explained in terms of thermal heat dissipation and brittle fracture of alumina in LN2.
The use of rutile-type titanium dioxide (TiO2) thin films as advanced gate dielectrics has been hampered by thermodynamic instability during the high deposition or annealing temperature of 800 °C. In this work, we demonstrate that rutile-type TiO2 thin films can be produced on p-type Si (100) at lower substrate temperature by means of bias-assisted cathodic arc deposition. The influence of the substrate bias on the microstructural and dielectric characteristics of the TiO2 thin films is investigated in detail. Our results show that by applying a suitable bias to the Si substrate, as-deposited rutile-type TiO2 thin films can be obtained at 450 °C. The permittivity of the materials increases significantly from 21 up to 76. The interfacial and electrical properties of TiO2/Si (100) are also improved. The effects and mechanism of the bias on the microstructural and dielectric characteristics are described.
Copper segregation in a subsurface layer during annealing of Cu60Zr30Ti10 bulk metallic glass at 773 K under oxygen atmosphere has been investigated by x-ray diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. The formation of metallic copper is strongly dependent on the annealing environment. Various oxides with metallic copper are formed after annealing in oxygen atmosphere, but only crystalline intermetallic phases are found under vacuum annealing. Besides, surface characterization results show that the sample annealed in oxygen and vacuum result in enrichment and depletion of Cu on the surface region, respectively.
Ultrafine, PbZr0.53Ti0.47O3 powder was synthesized by homogeneous precipitation of metal ions in aqueous solution using urea. The results obtained from different characterization methods were compared with those obtained from the conventional precipitation method using ammonia in terms of crystallization, homogeneity, and microstructure. The as-dried precipitate converted to the single-phase crystalline lead zirconate titanate powder when calcined at 550 °C and above. The calcined powder showed smaller particle size, minimum agglomeration, and uniform shape. The growth of the particles was very little at higher temperatures. Powdered samples that precipitated using urea crystallized directly to rhombohedral lead zirconate titanate, without any intermediate pyrochlore phase formation. The NH3-precipitated powder converted to rhombohedral lead zirconate titanate via metastable pyrochlore and it showed phase segregation upon annealing at higher temperatures. The reaction kinetics has been studied by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry.
The spectroscopic properties of Ba2Mg(BO3)2:Ce3+ and Ba2Mg(BO3)2:Eu2+ in vacuum-ultraviolet-vis range were investigated. The 5d crystal-field splitting components and the barycenter of Ce3+, the lowest 5d levels, the emission and the Stokes shifts of Eu2+ and Ce3+ in the host lattices, as well as the host-related absorption are identified and discussed in terms of the crystal structure of the host lattice.
Nano- and micron-sized cellulose crystals were prepared and utilized as reinforcements for polyurethane composites. The cellulose crystals obtained from microcrystalline cellulose (MCC) were incorporated into a polar organic solvent, dimethylformamide (DMF), and ultrasonicated to obtain a stable suspension. The suspension was an effective means for incorporating the cellulose crystals into the polyol-isocyanate mixture, utilized to produce polyurethane composite films. The use of DMF presents an interesting alternative for the use of cellulose crystals as reinforcement of a broad new range of polymers. Moreover, the rheology of the uncured liquid suspensions was investigated, and analysis of the results indicated the formation of a filler structure pervading the liquid suspension. Besides, films were prepared by casting and thermal curing of the stable suspensions. Thermomechanical and mechanical testing of the films were carried out to analyze the performance of the composites. The results indicated that a strong filler-matrix interaction was developed during curing as a result of a chemical reaction occurring between the crystals and the isocyanate component.
The fracture of nanoporous methylsilsesquioxane thin-film glasses in moist air and aqueous solutions was investigated. We demonstrate the effects of controlled volume fractions of nanometer sized pores on the films resistance to fracture. Subcritical cracking accelerated by the presence of moisture, controlled pH, and hydrogen peroxide solutions is reported. Surprising changes in the near threshold growth rate behavior were observed for buffered solutions. We demonstrate that these changes are related to the unexpected diffusion of the aqueous solutions into the highly hydrophobic films. The presence of the solution changes the surface stress of the internal pore surfaces, which changes the stress state of the film. The change in film stress surrounding the crack alters the crack driving force and has profound effects on the resulting crack-growth threshold behavior.
Dimensional and finite element analyses were used to analyze the relationship between the mechanical properties and instrumented indentation response of materials. Results revealed the existence of a functional dependence of (engineering yield strength σE,y + engineering tensile strength σE,b)/Oliver & Pharr hardness on the ratio of reversible elastic work to total work obtained from an indentation test. The relationship links up the Oliver & Pharr hardness with the material strengths, although the Oliver & Pharr hardness may deviate from the true hardness when sinking in or piling up occurs. The functional relationship can further be used to estimate the sum σE,y + σE,b according to the data of an instrumented indentation test. The σE,y + σE,b value better reflects the strength of a material compared to the hardness value alone. The method was shown to be effective when applied to aluminum alloys. The relationship can further be used to estimate the fatigue limits, which are usually obtained from macroscopic fatigue tests in different modes.
The U–6 wt% Nb (U6Nb) alloy in the water-quenched (WQ) state has been in service for a number of years. Its long-term reliability is affected by the changes of the alloy microstructure and mechanical properties during service. In this paper, the water quenched U–6 wt% Nb (WQ-U6Nb) alloy in service for 15 years at ambient temperatures was studied using an analytical transmission electron microscopy (TEM) analysis. We found that the long-term natural aging resulted in a disorder–order phase transformation, leading to the formation of anti-phase boundaries (APBs). The newly found ordered phase was then identified by proposing two phase transform schemes, which were also discussed with regards to the potential subsequence of the microstructural evolution for the alloy in further service. The initial study also provides convincing evidence for the disorder–order transformation, which has been predicted by numerous studies to be a transient thermodynamic event before spinodal decomposition. This suggests that the long-term naturally aged WQ–U6Nb is a good model alloy to study thermodynamic and kinetic phenomena requiring chronic processes.
High-quality lanthanum zirconium oxide (La2Zr2O7 or LZO) films have been deposited and processed on Ni–W substrates using a sol-gel processing approach. It has been demonstrated that crack-free coatings with thicknesses up to 100 nm can be processed in a single step, while thicker coatings (200–225 nm) were processed using a multiple coating and annealing process. Using simulated metalorganic deposition (MOD)-YBa2Cu3O7−δ (YBCO) processing conditions, the barrier properties of the sol-gel LZO coating with a thickness of 120 nm were found to be comparable to that of the standard 3-layer buffer stack deposited using physical vapor deposition. Secondary ion mass spectroscopy depth profile analysis of LZO films annealed in oxygen-18 shows that LZO effectively stops the diffusion of Ni within the first 80–100 nm. Using MOD processes, a CeO2 cap layer and superconducting YBCO layer were deposited on sol-gel LZO/Ni–W. For the first time, using such an all-solution conductor architecture, a critical current (Ic) of 140 A/cm with a corresponding critical current density (Jc) of 1.75 MA/cm2 has been demonstrated. Using a very thin Y2O3 seed layer (∼10 nm) deposited by electron beam evaporation; improved texture quality in the LZO layers has been demonstrated. The performance of the LZO deposited on these samples was evaluated using a sputtered CeO2 cap layer and MOD YBCO layer. Critical currents of up to 255 A/cm (3.2 MA/cm2) with 0.8-μm-thick YBCO films have been demonstrated, comparable to the performance of films grown using physical vapor deposited yttria stabilized zirconia as a barrier layer. Similar experiments using an MOD-CeO2 cap layer and MOD-YBCO layer yielded critical currents of 200 A/cm (2.5 MA/cm2) with 0.8-μm-thick YBCO films.
In this study, we demonstrated that the failure of bulk metallic glasses (BMGs) results from a sudden temperature rise within a shear band. Using a shear transformation zone model, we successfully calculated the temperature within a shear band and found it consistent with the observation from an in situ infrared thermographic system. The instantaneous temperature within a shear band at fracture agrees remarkably well with the glass transition temperature (Tg providing a new criterion to determine the strength of BMGs from their Tg. This agreement also discloses the fact that catastrophic failure of BMG is caused by the sudden drop in viscosity inside the shear band when the instantaneous temperature within a shear band approaches Tg.
We report a detailed study of the grain orientations and grain boundary (GB) networks in YBa2Cu3O7-δ (YBCO) films ∼0.8 μm thick grown by both the in situ pulsed laser deposition (PLD) process and the ex situ metalorganic deposition (MOD) process on rolling-assisted biaxially textured substrates (RABiTS). The PLD and MOD growth processes result in columnar and laminar YBCO grain structures, respectively. In the MOD-processed sample [full-width critical current density Jc(0 T, 77 K) = 3.4 MA/cm2], electron back-scatter diffraction (EBSD) revealed an improvement in both the in-plane and out-of-plane alignment of the YBCO relative to the template that resulted in a significant reduction of the total grain boundary misorientation angles. A YBCO grain structure observed above individual template grains was strongly correlated to larger out-of-plane tilts of the template grains. YBCO GBs meandered extensively about their corresponding template GBs and through the thickness of the film. In contrast, the PLD-processed film [full width Jc(0 T, 77 K) = 0.9 MA/cm2] exhibited nearly perfect epitaxy, replicating the template grain orientations. No GB meandering was observed in the PLD-processed film with EBSD. Direct transport measurement of the intra-grain Jc(0 T, 77 K) values of PLD and MOD-processed films on RABiTS revealed values up to 4.5 and 5.1 MA/cm2, respectively. As the intra-grain Jc values were similar, the significantly higher full-width Jc for the MOD-processed sample is believed to be due to the improved grain alignment and extensive GB meandering.
Ferroelectric domain patterning with an electron beam is demonstrated. Polarization of lead zirconate titanate thin films is shown to be reoriented in both positive and negative directions using piezoresponse force and scanning surface potential microscopy. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. A threshold of 500 μC/cm2 and a saturation of 1500 μC/cm2 were identified. Regardless of beam energy, the polarization is reoriented negatively for beam currents less than 50 pA and positively for beam currents greater than 1 nA.
Mesoporous barium titanate powders having a 100- to 300-nm size were prepared by hydration and condensation of titanium tetra-isopropoxide and barium precursors in the presence of an organic surfactant, tetradecylamine, which was used as a self-assembly micelle. The processing and sintering of these mesoporous barium titanate powders has been investigated. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used to identify the structural characteristics and morphologies of the powders. Mesoporous wormhole-like powders with surface areas around 53 ∼ 108 m2/g could be obtained after removing the micelle organics by calcination at 400 °C for 3 h. Powders derived using barium hydroxide were found to form a larger pore size and a higher surface area. The addition of acetic acid was also effective in increasing the surface area. A formation mechanism for the mesoporous structure is depicted. Heat treatment caused the mesoporous spheres to shrink, and 155- ∼ 330-nm grain sizes were readily obtained after pressureless sintering at 900 ∼ 1000 °C for 1 h in air.
Representative strain plays an important role in indentation analysis; by using the representative strain and stress, the normalized indentation load becomes a function of one variable, which facilitates the reverse analysis of obtaining the material plastic properties. The accuracy of such function is critical to indentation analysis. Traditionally, polynomial functions are used to fit the function, which does not incorporate correct elastic/plastic limits and has no physical basis. In this paper, we have proposed a new limit analysis-based functional formulation based on the theoretical solutions of conical/wedge indentation on elastic and rigid plastic solids. It is found that both limits agree well with numerical results, and the new approach involves no—or at most one—fitting parameter, which can be obtained with much less effort compare with the traditional polynomial approach. Reverse analyses on five different materials have shown that the new and simple limit analysis-based formulation works better than the traditional polynomial fit. The new technique may be used to quickly and effectively measure material plastic properties for any conical indenter if the elastic modulus is known a priori.
Bulk glass formation of the Co–Cr–Mo–C–B–Er alloy system was investigated in this paper. The Co50Cr15Mo14C15B6 (at.%) alloy could be cast into fully glassy rod with a diameter up to 2 mm. By adding 2 at.% Er to this alloy, the critical diameter for glass formation reached 10 mm. The excellent glass formability of the Er-doped alloy was mainly attributed to its relatively large reduced glass transition temperature of 0.61, near-eutectic composition, and the necessity of redistribution of the Er atoms for precipitation of crystalline Co6Mo6C phase in the undercooled liquid on cooling.