Showing papers in "Solid State Phenomena in 2007"

Three-dimensional Visualization in Powder Diffraction

Abstract: A multi-purpose pattern-fitting system, RIETAN-2000, has been extensively utilized to contribute to many structural studies. It offers a sophisticated structure-refinement technique of whole-pattern fitting based on the maximum-entropy method (MEM) in combination with a MEM analysis program PRIMA. We have recently completed a successor system, RIETAN-FP, to RIETAN-2000, adding new features such as standardization of crystal-structure data, an extended March-Dollase preferred-orientation function, and automation of imposing restraints on bond lengths and angles. Further, we have been developing a new three-dimensional visualization system, VESTA, using wxWidgets as a C++ application framework. VESTA excels in visualization, rendering, and manipulation of crystal structures and electron/nuclear densities determined by X-ray/ neutron diffraction and electronic-structure calculations. VESTA also enables us to display wave functions and electrostatic potentials calculated with part of these programs.

Casting Single Crystal Silicon: Novel Defect Profiles from BP Solar's Mono2 TM Wafers

Abstract: A novel crystal growth method has been developed for the production of ingots, bricks and wafers for solar cells. Monocrystallinity is achievable over large volumes with minimal dislocation incorporation. The resulting defect types, densities and interactions are described both microscopically for wafers and macroscopically for the ingot, looking closely at the impact of the defects on minority carrier lifetime. Solar cells of 156 cm2 size have been produced ranging up to 17% in efficiency using industrial screen print processes.

Behavior of Various Organosilicon Molecules in PECVD Processes for Hydrocarbon-Doped Silicon Oxide Films

Abstract: Plasma diagnosis was performed by means of optical emission spectroscopy in the plasma-enhanced chemical vapor deposition process for preparation of hydrocarbon-doped silicon oxide films. The chemical bonding states were characterized by a fourier-transform infrared spectrometer. Based on the results of the diagnosis in organosilane plasma and the chemical bonding states, a reaction model for the formation process of hydrocarbon-doped silicon oxide films was discussed. From the results of optical emission spectroscopy, we found that the oxygen atoms of methoxy groups in TMMOS molecules can be dissociated easily in the plasma and behave as a kind of oxidizing agent. Siloxane bondings in HMDSO, on the other hand, hardly expel oxygen atoms.

Phosphor-Conversion White Light Emitting Diode Using InGaN Near-Ultraviolet Chip

Abstract: We have synthesized a Eu2+-activated Sr3MgSi2O8 blue phosphor and (Sr,Ba)2SiO4 yellow phosphor. We fabricated a phosphor-conversion white light emitting diode(LED) using an InGaN chip that emits 400 nm near-ultraviolet(n-UV) light and phosphors that emit in the blue and yellow region. When the white LED was operated at a forward-bias current of 20 mA at room temperature(RT), the color temperature(Tcp), average color rendering(Ra), operating voltage(Vf) and luminous efficacy(ηL) were estimated to be 5800K, 72.08, 3.4V, and 7.61 lm/W, respectively. The commission International de I’Eclarirage(CIE) chromaticity coordinates obtained from the measured spectra remained almost constant during the forward-baias current increase from 0.5 mA to 60 mA.

Anisotropy of the Vacancy Migration in Ti, Zr and Hf Hexagonal Close-Packed Metals from First Principles

Abstract: The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been investigated by Density Functional Theory (DFT) calculations performed with the SIESTA code. The migration energies are found to be systematically lower by »0.15 eV within the basal plane than out of the basal plane. The electronic origin of this significant contribution to diffusion anisotropy is evidenced by the analysis of the local electronic densities of states and by a comparison with and empirical potential. The average value of the migra- tion energy is in very good agreement with available experimental data in Zr. The activation energies for self-diffusion obtained assuming a vacancy mechanism are in good agreement with experiments in Zr and Hf, although slightly too small, but a significant discrepancy is observed in Ti.

Study of Gettering Mechanisms in Silicon: Competitive Gettering between Phosphorus Diffusion Gettering and Other Gettering Sites

Abstract: The effectiveness of phosphorus diffusion gettering (PDG) and related segregation coefficients for different metal impurities were measured applying thermal treatments in the temperature range 800-950 °C for different times. We used multi-crystalline and mono-crystalline CZ p-type wafers with different boron concentrations and different levels of dislocations and bulk micro-defects (BMD). In all sample types, for Cu and Ni we found complete gettering in the temperature range investigated. In the case of Fe, the segregation coefficient increases with both increase in temperature and extension of time. The increase is qualitatively changing when going above 900 °C. At 950 °C the segregation coefficient increases faster at shorter diffusion time but at extended diffusion time it increases slower as compared to diffusion at 900 °C. At the same temperature and time of phosphorus diffusion the segregation coefficient is found to be independent of the metal impurity concentration in the range of 1012-1015 cm-3 investigated. We have shown that the presence of BMD and dislocations in bulk silicon does not impede the ability of PDG to completely remove Fe, Ni and Cu metal impurities from the bulk. Further analysis suggests that the PDG has the same gettering efficiency for mono-crystalline silicon and multi-crystalline silicon. We conclude that if any bulk precipitation of Fe, Ni and Cu impurities is present in multi-crystalline silicon it cannot seriously compete with PDG. However we found that increasing the boron concentration in the samples reduces the segregation coefficient of Fe, and this reduction is more severe at lower temperatures. Finally, by applying a post anneal ramp down from 900 °C to 700 °C after phosphorus diffusion, we found that the Fe segregation coefficient increases by a factor of 36 for lightly B doped samples, from 53 to 1919, leading to a significant reduction of Fe in the bulk after 2 hours ramp down anneal.

Hard a-Si3N4/MeNx Nanocomposite Coatings with High Thermal Stability and High Oxidation Resistance

Abstract: This article reports on a new class of amorphous a-Si3N4/MeNx nanocomposite coatings with a high (≥50 vol.%) content of Si3N4 phase; here Me=Zr, Ta, Mo and W. These nanocomposites exhibit high (>1000°C) thermal stability against crystallization and high (>1000 °C) oxidation resistance if the metal Me incorporated in the nanocomposite is correctly selected. It was found that the Zr-Si-N film deposited on Si(100) substrate exhibits no increase of the mass (m=0) in thermogravimetric measurements performed in flowing air up to 1300 °C, i.e. up to the temperature that is the thermal limit for Si substrate but not for nanocomposite.

Zirconia based nanomaterials for oxygen sensors : Generation, characterisation and optical properties

Abstract: Microwave driven hydrothermal synthesis and hydrothermal synthesis were used to obtain ZrO2 nanopowders. Their production with varying phase composition, the characterisation and selected optical properties concerning their potential use as luminescence oxygen sensors are reported. It was found that the powders obtained by the microwave driven hydrothermal method and annealed at 750 0C in air show experiment repeatability within an accuracy of 6 %.

Classification of oxidation behavior of disilicides

Abstract: This study focuses on classification of structures of oxide scales formed on disilicides. The oxide scales formed on disilicides can be grouped into 3 types, (a) silica scale, (b) mixed oxide (silica+metal oxide) scale, and (c) double layered (silica/metal oxide) scale. Disilicide that forms an oxide scale of the type (a) generally show excellent oxidation resistance. As such disilicides, there are FeSi 2 , CoSi 2 , MoSi 2 , WSi 2 , etc. In FeSi 2 and CoSi 2 , a protective silica scale must be formed due to the selective oxidation of Si, whereas in MoSi 2 and WSi 2 evaporation of metal oxide plays an important role for the formation of a silica scale. Oxidation of TaSi 2 and NbSi 2 belonged to the type (b), and only CrSi 2 the type (c).

Pt-Supported Nanocrystalline Ceria-Zirconia Doped with La, Pr or Gd: Factors Controlling Syngas Generation in Partial Oxidation/Autothermal Reforming of Methane or Oxygenates

Abstract: Nanocrystalline CeO2-ZrO2 (Ce:Zr 1:1) samples doped with La, Pr or Gd cations (containing up to 30 at.%) were prepared via the Pechini route. Pt (1.4 wt.%) was supported via impregnation with H2PtCl6 solution followed by drying and calcination. The samples’ surface features were studied by SIMS and FTIRS of adsorbed CO. The oxygen mobility was characterized by the dynamic oxygen isotope exchange and H2 TPR. Catalytic activity was studied in the flow installation using diluted feeds (0.7% CH4 +0.5% O2 or 1% C3H6O + 0.5% O2 +0.5% H2O in He). In the selective oxidation of methane (POM), the catalytic activity correlates with Pt dispersion controlled by the oxidized sample’s ability to stabilize Pt2+ cations as precursors of small reactive Pt clusters formed under reaction conditions. This is favoured by a larger doping cation (La) and a developed network of nanodomain boundaries. At comparable Pt dispersion, the highest performance was demonstrated by a La-doped system, which correlates with the highest surface/near-surface oxygen mobility controlled by the strength of Ce-O bonds in the surface layer. In the autothermal reforming of acetone, the activity trends differ from those in POM because of the more prominent role of the oxygen mobility required to prevent surface coking.

Characterization of indium tin oxide (ITO) thin film fabricated by indium-tin-organic sol including ITO nanoparticle

Abstract: In this paper, indium tin oxide (ITO) thin film was fabricated by indium-tin- organic sol including ITO nanoparticle, and optical transmittance was characterized.. ITO nanoparticle was designed to dope 10wt% SnO 2 into In 2 O 3 lattice, and was synthesized under low temperature not exceeding 300°C in order to make ultrafine sized particle. As a result, the synthesized ITO nanoparticle showed ultrafine size about 5 nm and high specific surface area of 117 m 2 /g.. Also, (222) preferred orientation was observed from X-ray diffracted peak. Then, using the particles and indium-tin-organic sol, ITO thin film was fabricated. The indium-tin-organic sol was made using organic solution in which indium and tin acetic reagents were dissolved into an organic solvent. The ITO nanoparticle was included into indium-tin-organic sol in order to deposit ITO sol thin film by spin coating. The ITO mixture sol was spin-coated on a glass substrate of which area was 7x 7 cm, and heat-treated at 500°C. As a result, the optimized ITO sol-gel thin film could be successfully fabricated with good optical transmittance over 83% and electrical resistance less than 7 x 10 3 Q by including the 0.6wt% ITO nanoparticle into the ITO sol..

Internal Dissolution of Buried Oxide in SOI Wafers

Abstract: High temperature anneal of SOI wafers in oxygen-free atmosphere results in internal buried oxide dissolution and top Si layer etching. Dissolution rate is determined by interstitial oxygen diffusion through the top Si layer and evaporation from the top Si surface in the form of SiO. It has been observed that kinetics of the process follows linear-parabolic law. Simple thermodynamic model is proposed, which explains observed dependences on temperature and top Si layer thickness.

Orientation of Interstitials in Clusters in α-Fe: A Comparison between Empirical Potentials

Abstract: We have addressed two issues concerning the relative stabilities of various orienta- tions of interstitial clusters in iron by making a comprehensive comparison between four recent empirical potentials. First, we have investigated the effect of finite temperature on the com- petition between clusters made of a few dumbbells oriented along h111i or h110i. We show by quasi-harmonic calculations that h111i clusters have much larger vibrational formation en- tropies and that they are therefore stabilized with respect to h110i clusters at high temperature. Second, we have compared the formation energies of loops with several hundred atoms with Burgers vector 1 2 h111i or h100i. The 1 2 h111i loops are found to be always more stable, but the energy differences with h100i loops depend strongly on the potential.

Cluster Dynamics Modeling of Materials: Advantages and Limitations

Abstract: The aim of this paper is to give a short review on cluster dynamics modeling in the field of atoms and point defects clustering in materials. It is shown that this method, due to its low computer cost, can handle long term evolution that cannot, in many cases, be obtained by Lattice Kinetic Monte Carlo methods. Indeed, such a possibility is achieved thanks to an important drawback that is the loss of space correlations of the elements of the microstructures. Some examples, in the field of precipitation and irradiation of metallic materials are given. The limitations and difficulties of this method are also discussed. Unsurprisingly, it is shown that it goes in a very satisfactory way when the objects are distributed homogeneously. Conversely, the source term describing the primary damage under irradiation, by nature heterogeneous in space and time, is tricky to introduce especially when displacement cascades are produced.

Heartbeat Monitoring Technique Based on Corona-Poled PVDF Film Sensor for Smart Apparel Application

Abstract: Flexible piezoelectric polymer materials for smart apparel and wearable computer applications are of great interest. Among known ferroelectric and piezoelectric polymers, polyvinylidene fluoride (PVDF) exhibit β-phase under poling and is known to give highest piezo-, pyro-, and ferroelectric properties. Previous reports suggests that, during corona poling of the PVDF film, a high surface electric potential is generated resulting in a high internal electric field within the polymer film causing the polarization of the dipoles along the direction of the applied electric field. The resultant phase change from α- to β-phase and the dipole switching generates displacement of charges or piezoelectricity. And also mechanical variation would change dipole density of PVDF film. In this report, we measured human heartbeat signal from an DAQ interfaced with a custommade voltage-amplifier with specific frequency filtering function using the corona-poled PVDF film of various sizes and thickness as a piezoelectric sensor and analyzed it. We employed elastic textile band to sensor system for comfortable fit on wrist or ankle. And then, we found the feasibility of applying flexible PVDF film sensor to smart apparel application which can sense heartbeat rate, blood pressure, respiration rate, accidental external impact on human body, etc.

Ceria-Zirconia Nanoparticles Doped with La or Gd: Effect of the Doping Cation on the Real Structure

Abstract: The real structure of nanocrystalline CeO2-ZrO2 (Ce:Zr=1:1) systems prepared via the polymerized polyester precursor (Pechini) route and doped with La3+ or Gd3+ cations, up to 30 at.%, was studied by X-ray powder diffraction, EXAFS and Raman spectroscopy and the surface features characterized by XPS and SIMS. Undoped CeO2-ZrO2 system revealed nanoscale heterogeneity, perhaps due to the co-existence of Zr- or Ce-enriched domains. With large La3+ dopant the system remains bi-phasic within the studied ranges of composition, incorporation of the smaller Gd3+ cation stabilizes the single-phase solid solution. For both systems, the increase of dopant content was accompanied by a decline of domain size and an increase of the average lattice parameter of fluorite-like phases. Depletion of the surface layer by smaller Zr4+ cations was observed, while the surface content of a doping cation is either, close to that in the bulk (La) or below it (Gd). Such a spatial distribution of components results in some ordering of cations within the lattice. It is reflected in different modes of rearrangement of oxygen coordination polyhedra with the Gd or La content (distances and coordination numbers by EXAFS), and specificity of XRD patterns not conforming to a simple model with statistical distribution of oxygen vacancies.

Precipitation of Phases in Al-Mg-Si-Cu Alloy with Sc and Zr Additions during Heat Treatment

Abstract: In the present work the sequence of phases in Al-1%Mg-0.6%Si-1%Cu (in wt %) alloys with 0.4%Sc and 0.2%Zr additions during heat treatment has been studied. The investigated alloys were cast into copper mould to ensure high cooling rate during solidification and the retention of Sc and Zr in the supersaturated solid solution. Two−step preliminary heat treatment was applied: the annealing at temperature 300°C or 350°C followed by quenching from 540°C to water. The precipitation of the Q′ and Al3Sc phases has been observed during the first step of annealing. The annealing at 540°C has led to complete dissolution of the Q′ particles and growth of the spherical particles of the Al3Sc phase. The increase of hardness during subsequent ageing at 165°C (typical ageing temperature of the 6xxx alloys) has been caused by the formation of needle-like Q′ phase which coexist with Al3Sc precipitates.

Stable Deposition and Patterning of Metal Layers on the PDMS Substrate and Characterization for the Development of the Flexible and Implantable Micro Electrode

Abstract: PDMS(polydimethylsiloxane) is a flexible and biocompatible material and is widely used in bio- or medical-related fields. Recently, PDMS has been used as a substrate of implantable electrodes but has exhibited limits in stable metal layer deposition and patterning. In this paper, we have developed processes for both the stable metallization of PDMS surface and the selective patterning of conductive elements. The surface treatment via the oxygen plasma ions significantly affects the adhesion of metal layers to the PDMS surface, while the other factors exhibited no significant relations. On the basis of our procedure resulted in the effective production of the stable and fine (line width: 20
) electrode patterns on the PDMS substrate. Finally, we fabricated PDMS-based flexible and implantable micro electrode for the subretinal prosthesis.

Mechanism of Shunting of Nanocrystalline Silicon Solar Cells Deposited on Rough Ag/ZnO Substrates

Abstract: Using a textured substrate is a basic requirement for light trapping in a thin film solar cell. In this contribution, the structure of μc-Si:H n-i-p solar cells developed on a rough Ag/ZnO coated glass substrate is carefully studied, in order to understand the substrate surface morphology dependence of solar cell properties, especially of the yield of working cells. From cross-sectional transmission electron microscopy (TEM) images it is clear that cells developed on substrates with tilted large Ag crystal grains contain pinholes that result in short-circuiting of the entire device. The formation of these pinholes is due to the inability of conformal coverage of the sub-micron sized cavities that are created by these Ag grains. Controlling the Ag deposition temperature is found to be essential to have a well performing μc-Si:H n-i-p cell.

Electrical and thermal properties of platinum thin films prepared by DC magnetron sputtering for micro-heater of microsensor applications after CMP process

Abstract: Chemical mechanical polishing (CMP) of platinum thin films was performed for the improvement of surface morphology. Platinum thin films after CMP process with alumina slurry showed the increase of surface morphology without a remarkable difference of the thermal characteristics of as-annealed platinum thin films. The power consumption of platinum thin films micro-heater also became very low by improvement of surface morphology after CMP process. The similar or improved electrical and thermal characteristics of platinum thin films for micro-heater of sensor applications as well as evaluation possibility of sensing property by the improved surface morphology were obtained after CMP process.

Wafer Cleaning Using Supercritical CO2 in Semiconductor and Nanoelectronic Device Fabrication

Abstract: Introduction As semiconductor devices continue to shrink and become faster, new materials and processes will be required to enable the continuation of this progress. Supercritical CO2 (SCCO2) -based technology has been proposed for various steps in device fabrication, such as cleaning and deposition. SCCO2 diffuses rapidly, has low viscosity, near zero surface tension like a gas, and thus, can penetrate easily into deep trenches and vias. It also enables cleaning without pattern collapse or stiction. SCCO2 has the solvating properties of a liquid and thus can dissolve chemicals, such as alcohols and fluorinated hydrocarbons, forming a homogenous supercritical fluid solution. SCCO2-based processing has been investigated because of its potential to strip photoresist residues (thanks to its compatibility with low-k materials) [1] and because it can restore the k-values of low-k materials for Cu/low-k integration in the back end of the line (BEOL) [2]. The application of SCCO2 to ultra-low-k material processing is a promising future technique. In this paper, we first review various applications of SCCO2 in BEOL and then we demonstrate several applications of supercritical CO2 addressed to the front end of the line (FEOL) in semiconductor and nanoelectoronic device fabrication.

Effect of delta ferrite on corrosion resistance of type 316L stainless steel in acidic chloride solution by micro-droplet cell

Abstract: More than 2% δ-ferrite phase is presented in type 316L stainless steels due to improvement of hot workability or welding process. In order to clarify the mechanism on how the δ-ferrite affects the corrosion resistance, electrochemical tests in a micro-droplet cell were conducted and microstructures were analyzed by TEM-EDS and SEM-EDS. Especially, the micro-droplet cell was modified at our laboratory to observe the polarization behavior of δ-ferrite phase in an acidic chloride solution. The anodic polarization behavior in both active zone and passive zone was affected by δ-ferrite. The degradation of corrosion resistance in the low potential zone with δ-ferrite phase is due to the preferential dissolution of a matrix, γ phase. It is explained with the fact that the formation of δ-ferrite lowers the concentrations of chromium and molybdenum in γ phase. It is also confirmed by potentiostatic test, EDS analyses and phase-equilibrium prediction. In addition, the micro-droplet cell and EDS analysis reveled that the pitting potential in the neutral pH solution was affected by Ca containing inclusion rather than δ-ferrite phase.

Development of Large Area Plasma Reactor Using Multiple Low-Inductance Antenna Modules for Flat Panel Display Processing

Abstract: This article reports characteristics of plasmas sustained with LIA modules and profile control capabilities. Experiments with a meter-scale reactor demonstrated uniform plasma production to attain densities as high as 5x1011 cm-3 at an argon pressure of 1.3 Pa and an RF power of 4 kW. Design issues for large-area plasma sources with a scale-size of 3 m were also presented to exhibit the feasibility of novel large-area plasma sources to meet the requirements of the next-generation meters-scale processing.

Laser-Arc Hybrid Welding

Abstract: Hybrid welding of stainless steels or aluminum alloys was performed using the heat sources of YAG laser and TIG, or YAG laser and MIG, respectively. The effects of welding conditions and melt flows on penetration depth, weld bead geometry and bubble/porosity formation were investigated with X-ray transmission real-time observation method. A great effect of melt flows on penetration depth and weld geometry was consequently confirmed. Concerning porosity suppression in YAG-TIG hybrid welding of stainless steel, no bubble generation was attributed to no porosity formation. On the other hand, it was revealed that the disappearance of bubbles from the concave molten pool surface played an important role of no porosity in YAG laser-MIG hybrid welding of aluminum alloys.

The Luminescence Properties of ZnO:Al Nanopowders Obtained by Sol-gel, Plasma and Vaporization-Condensation Methods

Abstract: ZnO nanocrystals were synthesized and characterized by XRD and SEM methods. The luminescence spectra and decay kinetics were studied under pulsed laser excitation (266 nm, 8 ns). ZnO and ZnO:Al powders were prepared by sol-gel and plasma chemical synthesis. These powders were used as a raw material for the SPVD (Solar Physical Vapour Deposition) process. In this way, the vaporisation-condensation phenomenon (VC) led to the formation of ZnO whiskers and nanopowders. The luminescence properties of the VC nanopowders were studied and compared to those displayed by the raw material. The Al dopant, present in the raw powders as a solid solution and ZnAl2O4 precipitates, was only present as Al ions in the nanopowders after the SPVD. The blue luminescence intensity increased considerably after SPVD. The whiskers type microstructures showed nonlinear blue luminescence dependent on the excitation pulse density.

Study on Thermal Conductivity of Polyetheretherketone/Thermally Conductive Filler Composites

Abstract: Thermal properties of PEEK/silicon carbide(SiC) and PEEK/carbon fiber(CF) were investigated from ambient temperature up to 200°C measured by laser flash method. Thermal conductivity was increased from 0.29W/m-K without filler up to 2.4 W/m-K with at 50 volume % SiC and 3.1W/m-K with 40 volume % carbon fiber. Values from Nielsen theory that predicts thermal conductivity of two-phase system were compared to those obtained from experiment.

A Carbon Nanotube Smart Material for Structural Health Monitoring

Abstract: This study introduces a nano smart material to develop a novel sensor for Structural Health Monitoring (SHM) of mechanical and civil systems. Mechanical, civil, and environmental systems need to become self-sensing and intelligent to preserve their integrity, optimize their performance, and provide continuous safety for the users and operators. Present smart materials and structures have fundamental limitations in their sensitivity, size, cost, ruggedness, and weight. Smart materials developed using nanotechnology have the potential to improve the way we generate and measure motion in devices from the nano to the macro scale in size. Among several possible smart nanoscale materials, Carbon Nanotubes (CNT) have aroused great interest in the research community because of their remarkable mechanical, electrochemical, piezoresistive, and other physical properties. To address the need for new intelligent sensing based on CNT, this study presents piezoresistivity and electrochemical properties and preliminary experiments that can be applied for SHM. This study is anticipated to develop a new multifunctional sensor which can simultaneously monitor strain, stress and corrosion on a structure with a simple electric circuit.

Passivation Studies of Germanium Surfaces

Abstract: Introduction Germanium is gaining interest as a substrate for use in high mobility channel CMOSFETs applications. Therefore effective surface cleaning is an important factor in realizing these high mobility characteristics of the Ge substrate. Effective cleaning requires efficient contaminant removal with minimal substrate loss and good passivation characteristics. Effective passivation prevents further oxidation and contamination of the surface. Effective hydrogen passivation on silicon can be achieved with HF and NH4F solutions [1-3]. However studies have found that hydrogen passivation is not stable on Ge surfaces and oxidizes readily in ambient [4-5]. This paper examines the passivation characteristics of hydrogen halide solutions (HF, HCl, HBr and HI) on the germanium surface using X-Ray Photoelectron Spectroscopy (XPS).

Large-Scale Synthesis of Polymer-Stabilized Silver Nanoparticles

Abstract: The synthesis and characterization of polymer-stabilized silver nanoparticles (Ag NPs) for water-based silver inks are studied. In order to synthesize Ag NPs with spherical shape, the conventional polyol processes require an excess of poly(vinyl pyrrolidone) (PVP) (10 ~ 1000 times than AgNO3) and therefore result in low productivity per reactor volume. In this study, poly(acrylic acid) (PAA) with carboxylic acid group was used instead of PVP. Even at less molar ratio of PAA to AgNO3 (

Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide by Heteropolyacid/Metal Oxide Catalysts

Abstract: CexTi1-xO2 and H3PW12O40/CexTi1-xO2 catalysts were prepared by sol-gel method, and they were applied to the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in a batch reactor. The reaction was carried out in an autoclave reactor at 170oC and 5 MPa. It was found that CexTi1-xO2 exhibited a higher catalytic performance than pure CeO2 and TiO2. The catalytic performance of CexTi1-xO2 was the maximum when x=0.1. It was also revealed that H3PW12O40/CexTi1-xO2 catalysts showed a remarkably enhanced catalytic performance than the corresponding CexTi1-xO2 catalysts. The amount of DMC produced by 15 wt% H3PW12O40/ Ce0.1Ti0.9O2 catalyst was six times higher than that produced by Ce0.1Ti0.9O2 catalyst. It is concluded that both Bronsted acid sites provided by H3PW12O40 and base sites in CexTi1-xO2 played an important role in improving the catalytic performance of H3PW12O40/CexTi1-xO2.