Residual stress relaxation and microstructure in ZnO thin films

Stability under normal environmental conditions over a long period of time is crucial for sustainable thin-film device performance. Pure ZnO films with thicknesses in the 140 - 450 nm range were deposited on amorphous glass microscope slides and (100)-oriented single crystal silicon wafers by radio frequency magnetron sputtering. The depositions were performed at a starting temperature of 200 oC. ZnO films had a columnar microstructure strongly textured along the direction. XRD peak-shift analysis revealed that the films were under residual, compressive, in-plane stress of -5.46 GPa for the glass substrate and -6.69 GPa for the Si substrate. These residual stresses could be completely relaxed by thermal annealing in air. When left under normal environmental condition over an extended period of time the films failed under buckling leading to extensive cracking of the films. The XRD and SEM results indicated different mechanisms of stress relaxation that were favored in the ZnO thin films depending on the energy provided. Although thermal annealing eliminated residual stresses, serious micro-structural damage upon annealing was observed. Thermal annealing also led to preferential growth of some ZnO crystals in the films. This kind of behavior is believed to be indicative of stress-induced directional diffusion of ZnO. It appears that for the extended stability of the films, the stresses have to be eliminated during deposition

Chemical synthesis of LSGM powders for solid oxide fuel cell (SOFC) electrolyte

Synthesis of LSGM (La0.9Sr0.1Ga0.8Mg0.2O3-delta), LSFM (La0.9Sr0.1Fe0.8Mg0.2O3-delta), and LSCM (La0.9Sr0.1Cr0.8Mg0.2O3-delta) powders were achieved via organic precursor method. Different organic "carrier" molecules were used for powder synthesis. Citric acid, tartaric acid, Pechini precursors, polyvinyl alcohol, and ethylene diaminetetraacetic acid were selected as organic carriers for their ability to stabilize the metal ions. Each organic carrier material exhibited a different degree of effectiveness in the synthesis of the mixed oxide powders. One of the main factors affecting the phase purity appears to be the interaction of the functional groups with the constituent cations. The effectiveness of the organic carrier with varying number and type of functional groups is evaluated and discussed in terms of the phase distribution in the powders after the calcination step

The role of Si impurities in the transient dopant segregation and precipitation in yttrium-doped alumina

Y-doped alumina was sintered at 1500 degrees C for 10 h under ultra-clean experimental conditions without experiencing any abnormal grain growth. The yttrium was fairly homogeneously distributed at the grain boundaries, with a mean value of (Gamma) over bar (Y) = 5.5 at nm(-2). The Y-Al-O precipitates in the clean, Y2O3-doped alumina specimen were the YAP (YAlO3) phase, whereas only the YAG (Y3Al5O12) phase was present in the Y2O3-doped alumina samples contaminated with SiO2. The excess concentrations of Y and Si atoms at the grain boundaries that, at the same time, provoke the formation of structurally complex YAG precipitates and abnormal grain growth were both estimated to be at 4-5 at nm(-2). The compositions of the triple point pocket phases found in the region of the exaggeratedly grown alumina grains indicate the presence of alumino-silicate bulk liquids at the sintering temperature

Ultrafine conducting fibers: metallization of poly(acrylonitrile-co-glycidyl methacrylate) nanofibers

Electrospun poly(glycidylmethacrylate) (PGMA) and poly(acrylonitrile-co-glycidyl methacrylate) (P(AN-GMA)) nanofibers were coated with monodisperse silver nanoparticles by using an electroless plating technique at ambient conditions. Oxirane groups on the surface of nanofibers were replaced with reducing agent, hydrazine. Surface modified nanofibers were allowed to react with ammonia solution of AgNO3. A redox reaction takes place and metallic silver nucleate on fibers surface. Parameters affecting the particle size were determined

TEM and STEM investigations of SrO-doped Sr(Ti,Nb)O3-δ thermoelectrics

Sr(Ti1-xNbx)O3-δ solid solutions are promising materials for n-type high-temperature thermoelectrics1. In our study 10 mol% of SrO excess was added to stoichiometric composition with x=0.2 in order to introduce Ruddlesden-Popper (RP) type-planar faults2,3 into the material, thus minimizing thermal conductivity. TEM and STEM were used to study possible ordering and/or distribution of Nb on Ti sites in the perovskite structure. All results were obtained in a Jeol ARM-200F with a CFEG and Cs probe corrector. HAADF imaging was performed at angles from 70 to 175 mrad, while ABF imaging from 11 to 23 mrad. EDXS spectra were acquired using JEOL Centurio Dry SD100GV SDD Detector. RP planar faults, as viewed along [001] zone axis, are shown in HRTEM micrograph in figure 1. The commonly observed number of perovskite unit cells between the planar faults is >2, which corresponds to various homologous compounds with the formula Srn+1(Ti,Nb)nO3n+1. However, solid solution Sr(Ti,Nb)O3-type grains with no RP faults can also be observed (bottom inset in Fig. 1). A HR HAADF STEM image of ordered RP faults (Fig. 2) shows that while the measured intensities of individual Sr atomic columns along a single fault do not scatter significantly, the (Ti,Nb)O atom columns exhibit quite large differences in measured intensities, thus indicating significant variation in Nb and Ti content within a single atom column. Quantitative analysis of measured intensities is in progress. The comparison between simultaneously acquired HAADF and ABF images of a single RP fault is shown in figure 3. While pure oxygen atomic columns cannot be resolved in the HAADF image, they can be readily observed using ABF imaging. The positions of oxygen atom columns along the planar faults are in full agreement with the structural model of a RP planar fault. Additional information on Nb distribution within perovskite matrix/RP faults was obtained by EDXS. While low magnification EDXS mappings show enrichment of Sr at RP faults accompanied by a corresponding decrease in Ti and Nb content, atom-resolved EDXS mappings confirm that individual mixed (Ti,Nb)O atom columns contain different Nb content (annotated atom column). Additionally, the spot EDXS line analysis (net counts) again shows much larger scatter in accumulated net counts for Ti as compared with Sr. The results being presented clearly show that no Nb is incorporated into the SrO RP faults and that the Nb is inhomogeneously incorporated within (Ti,Nb)O atom columns

X-ray single phase LSGM at 1350 °C

Synthesis of X-ray-phase-pure (La1−xSrxGa1−yMgyO3−δ, LSGM, where x = 0.1, y = 0.15 and 0.17) powders were achieved at temperatures as low as 1350 °C via organic precursor method using tartaric acid as the carrier material. LSGM materials were characterized for their phase purity, crystallization and electrical properties. Pellets sintered at 1350 °C for 6 h were single phase and dense (>99%). Electron microscopy analysis of X-ray single-phase pellets revealed MgO precipitates with sizes ranging from 50–300 nm. Phase formation and distribution in this complicated multi-cation-oxide system as a function of temperature were reported and discussed. Amorphous LSGM first crystallizes at 625 °C. However, elimination of undesired phases require higher temperatures. Impedance measurements as a function of temperature up to 545 °C revealed that the X-ray phase pure pellets may have extrapolated ionic conductivity values as high as 0.14–0.16 S/cm at 800 °C. Possible implications of limited MgO solubility on the ionic conductivity are presented

Processing and microstructure of standard and modified macro-defect-free cements

Evolution of macro-defect-free microstructure and microchemistry was studied by TEM/EDS and SEM/EDS. MDF samples were prepared through an alternative, Banbury mixing route. Experiments with varying mixing time at constant mixing rate showed that an intimate distribution of constituents in the composite was assured at the early stages in the shear mixing step. With increasing mixing time, mechanically induced chemical reactions triggered an interplay of network formation and simultaneous network degradation.Analytical electron microscopy studies at the TEM level indicated that there was a progression of microchemistry of the interphase zone with mixing time. At early stages of processing, the interphase was richer in Al ions than the contiguous cement grain. With increasing mixing time, the interphase chemistry approached equilibrium and the composition of the adjacent cement grain.Experiments with varying mixing rates showed that the time constants associated with the window of processibility were strongly affected by the mixing activity. A quiescently prepared calcium aluminate polyvinyl alcohol couple did not form an interphase layer between the polymer and the cement grains. It was deduced that the microstructure and microchemistry of MDF cements were a direct result of mechano-chemical interactions between the polymer and the hydrating calcium aluminate cements.Different amounts of a titanate chelate cross coupling agent were added to MDF paste during shear mixing. A titanate chelate modifier was shown to cross link polyvinyl alcohol at room temperature. XPS studies established that a C\rm\sb{PVA}-O-Ti-O-C\rm\sb{PVA} type bonding was responsible for the cross linking.These cross linking reactions further reduced the time constants of processing with increased amounts of the cross coupling agent. Analytical electron microscopy studies by TEM and STEM revealed that the modifier did not alter the MDF microchemistry significantly.Water durability tests showed that upon prolonged immersion in still water, composites were leached and degraded. The cross coupling agent could not eliminate the moisture sensitivity problem of HAC, MDF cement.U of I OnlyETDs are only available to UIUC Users without author permissio