Search

A challenge in exploiting the electrical properties of graphene is the fabrication of large area thin films in a scalable and inexpensive manner. The lack of a reliable and simple graphite exfoliation procedure has hindered the possibility of making films starting from pure graphene. Recently it has suggested that organic solvents with surface energies close to the graphene, are able to exfoliate flakes by ultrasonication [1], generating a stable suspension of graphene sheets. Of these, exfoliation with 1-Methyl-2-pyrrolidinone (NMP) yielded the highest concentration of 1–5 graphene layers [1]. Here we present a scalable manner to obtain films of pure graphene on virtually any substrate with reproducible nanometric control over the number of layers and their optical and electronic characteristics.

A study was conducted to compare the minimum reaction temperatures for the hydrothermal synthesis of pure lead titanate crystallites with precursors I and II. The synthesis conditions (i.e., temperature, pH, and input reagent concentrations) were calculated using a thermodynamic model. A comparison between the theoretically modeled and experimental conditions was made. Although the models predicted reactions as low as 25/spl deg/C, the minimum temperatures for a 72 h reaction time with precursors I and II were 120 and 140/spl deg/C, respectively. Furthermore, by increasing the Pb/Ti molar stoichiometric ratio from 1.0 to 2.0 (with precursor LT) the reaction temperature for phase-pure lead titanate could further be reduced below 90/spl deg/C; while with precursor I phase-pure lead titanate was difficult to obtain and was not obtained below 140/spl deg/C. It can be concluded that precursor composition and stoichiometry influence the reaction temperature.

A variety of organic bases has been identified as possible substitutes for inorganic alkali hydroxide and/or alkali halide mineralizers for the hydrothermal synthesis of ferroelectric perovskite ceramic materials. Thermodynamic modeling using newly developed software for automatic generation of stability and yield diagrams of the desired product phase was used to predict optimum conditions for precipitation of the phase-pure lead zirconate titanate with composition near the morphotropic phase boundary in the Pb-Zr-Ti-tetramethylammonium hydroxide (TMAOH) system. The predictions have been experimentally corroborated at 160/spl deg/C using coprecipitated Zr/Ti gel and lead acetate as a source of zirconium/titanium and lead, respectively. Thermoanalytical techniques show that the TMAOH is completely removed from the final powder product by washing with deionized water.

Exposing quartz crystals to anhydrous HF gas at 400/spl deg/C results in a high density of helical etch channels, a white crusty surface, and other unusual features. The treatment results in a negligible thickness change. There is no similarity between the characteristics of these helical etch channels and the conventional etch channels produced by liquid etchants, nor is there a correlation between the incidence of helical etch channels and the type of quartz, or the surface finish. Exposure of quartz blanks to HF/H/sub 2/O vapors emanating from a 49% aqueous HF solution at /spl sim/20/spl deg/C resulted in conventional etch channels, faceting, and measurable frequency changes, indicating that the presence or absence of water is responsible for the differences in etching behaviors.

Piezoelectric (Pb/sub 0.5/Bi/sub 0.5/)(Ti/sub 0.5/(Fe/sub 1-x/Mn/sub x/)/sub 0.5/)O/sub 3/ (x=0.00-0.02) ceramic powder was prepared by alcohol dehydration of an aqueous multicomponent ammonium citrate solution. The tetragonal phase with the desired stoichiometry formed after heat treatment at as low as 800 degrees C. A high tetragonality (c/a ratio of 1.140+or-0.001) was achieved after heat treatment at 1000 degrees C for 1 h. Flexible 0.3 composites prepared from the 1000 degrees C powder (x=0.01) and Eccogel polymer exhibited piezoelectric charge coefficients (d/sub 33/) of 60 pC/N and a voltage coefficients (g/sub 33/) of 205*10/sup -3/ Vm/N. >