Mixed-phase Materials Research Articles

Microstructure of hydrogenated silicon thin films prepared from silane diluted with hydrogen

We report results obtained from FTIR and TEM measurements carried out on silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from silane diluted with hydrogen. The hydrogen content, the microstructure factor, the mass density and the volume per Si–H vibrating dipoles were determined as a function of the hydrogen dilution. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). With increasing dilution the transition from amorphous to microcrystalline phase appears faster and the average mass density of the films decreases. The μc-Si:H films are mixed-phase void-rich materials with changing triphasic volume fractions of crystalline and amorphous phases and voids. Different bonding configurations of vibrating Si–H dipoles were observed in the a-Si:H and μc-Si:H. The bonding of hydrogen to silicon in the void- and vacancy-dominated mechanisms of network formation is discussed.

A comparison of mixed phase titania photocatalysts prepared by physical and chemical methods: The importance of the solid–solid interface

Mixed phase titania photocatalysts, such as Degussa P25, typically show higher photocatalytic activity than pure phase titania, as reported by many researchers. Our previous experimental results indicate that the solid–solid interface is a key factor in enhancing the photocatalytic reactivity of mixed phase titania and may be the location of catalytic “hot spots”. In this study, titania photocatalysts consisting of varying amount of anatase and rutile phases are prepared by reactive dc magnetron sputtering and by a modified sol–gel method. These titania materials are characterized with a variety of techniques and are tested in the photocatalytic degradation of acetaldehyde. Mixed phase titania thin films prepared by magnetron sputtering have a high density of anatase–rutile interfaces and display the highest photocatalytic activity among the catalysts tested. Studies by electron paramagnetic resonance spectroscopy show a unique distribution of charge trapping sites which are characteristic of the sputtered films. The role of anatase–rutile interface to separate charge and improve the photoactivity of mixed phase materials is discussed.

Synthesis and Reactivity of Al-MMM-2: A New Microporous/Mesoporous Catalyst for the Alkylation of Toluene

Porous, mixed-phase aluminosilicate materials containing a microporous MFI phase and a mesoporous Al-MCM-48 phase were synthesized using a one-pot synthesis method. Initially, the gemini surfactant “18-12-18” was used to form Al-MCM-48, and then the MFI template tetrapropylammonium (TPA+) was added to the reaction mixture at a later time. Subsequent crystallization at 150 °C led to the formation of a series of mixed-phase materials. These new materials, called “Al-MMM-2” (microporous/mesoporous materials), were characterized using powder X-ray diffraction and 29Si MAS-NMR. Consistent with previous results, the amount of the microporous phase formed was dependent on the crystallization time; thus, a range of materials could be formed from a single reaction mixture. In the alkylation of toluene with benzyl alcohol, Al-MMM-2 materials showed greater structural stability, catalytic activity, and product selectivity during repeated reaction cycles, compared to either pure Al-MCM-48 or pure MFI.

Preparation of Mixed-Phase Titanium Dioxide Nanocomposites via Solvothermal Processing

Mixed-phase titanium dioxide nanocrystals with varying phase composition were prepared by a low-temperature solvothermal process. We have re-examined the effect of hydrochloric acid on the formation of rutile phase and found that the proportion of the rutile phase in synthesized mixed-phase materials did not increase monotonically with increasing acidity. Rather, there was an optimum HCl/Ti molar ratio for rutile formation when titanium tetra-isopropoxide was used as the titanium precursor. At high HCl/Ti ratios, Cl- and H2O may act as charge-shielding agents, inhibiting the rutile formation during the solvothermal process. A low H2O/Ti molar ratio was necessary for preparing anatase−rutile composites, because the formation of photocatalytically inactive brookite phase was favored at relatively high H2O/Ti molar ratios. In addition, we found the solvothermal processing to be a possible approach to control interparticle connection. Mixed-phase TiO2 nanocrystals synthesized at relatively high H2O/Ti molar r...

Studies on the cluster sizes in the mixed-phase thin films

Based on the phase separation scenario, by simulating the resistivity of La0.33Pr0.34Ca0.33MnO3 film with the known coexisting-phase sizes, the authors have obtained the theoretical ferromagnetic (FM) cluster sizes of La2∕3Ca1∕3MnO3 film, showing around 0.6μm near Tc. Subsequently the insulator-metal transition occurs when the cluster grows up to 0.7μm, and the abrupt drop in resistivity occurs when the size is around 0.9μm. Furthermore, the FM cluster sizes grow with the magnetic field. The obtained results indicate that the model and method can be used to predict the critical size of the clusters at the phase transition for the mixed-phase materials.

Fabricating highly active mixed phase TiO 2 photocatalysts by reactive DC magnetron sputter deposition

Recent work points out the importance of the solid–solid interface in explaining the high photoactivity of mixed phase TiO 2 catalysts. The goal of this research is to probe the structural and functional relationships of the solid–solid interface created by the reactive DC magnetron sputtering of titanium dioxide. We show that sputter deposition provides excellent control of the phase and interface formation. We explored the effects of the process parameters of pressure, oxygen partial pressure, target power, substrate bias (RF), deposition incidence angle, and post annealing treatment on the structural and functional characteristics of the catalysts. We have successfully made pure and mixed phase TiO 2 films. These films were characterized with AFM, SEM, TEM, and XRD to determine surface morphology, phase distribution and phase content. The performance as photocatalytic surfaces was measured and compared (normalized for surface area) to mixed phase TiO 2 fabricated by other methods, including flame hydrolysis powders, and sol–gel deposited TiO 2 films. The sputtered mixed phase materials were far superior to the commercial standard (Degussa P25) and sol–gel TiO 2 as measured by the gas phase oxidation of the air pollutant acetaldehyde under UV illumination. These results demonstrate that reactive DC magnetron sputtering is a powerful tool for investigating the role of the solid–solid interface in influencing photocatalytic activity. In addition, our work illustrates the feasibility of reactive DC magnetron sputtering as a practical commercial technique for manufacturing highly active nanostructured TiO 2 photocatalysts.

Tungsten bronze-based nuclear waste form ceramics. Part 3: The system Cs 0.3M xW 1− xO 3 for the immobilization of radio cesium

Previous studies in this series have indicated that Cs- and Sr-loaded Mo-doped hexagonal tungsten bronze (MoW-HTB) oxides, either in the form of fine grained powders, or as composite granules, can be converted to leach resistant ceramics at modest temperatures in the range 600–1200 °C. In the present study it has been shown that such waste form ceramics can also be readily prepared through very simple conventional routes involving the blending of cesium nitrate with tungstic acid and other oxide components followed by heating in air. The phase chemistry resulting from the blending of these oxides has been explored. In the Cs 0.3M x W 1− x O 3 compositional system where x = Ti, Zr, Nb and Ta the solid solution limit has been found to be where x = 0.2. For all values of x between 0 and 0.2 mixed phase materials of HTB and WO 3 were obtained and Cs was found associated with HTB phases that are both rich and depleted in M element. At temperatures above about 1000 °C, phase pure HTB compounds in the space group P63/ mcm were obtained. Even when x greatly exceeds 0.2, the additional oxide content did not interfere with the formation of the HTB phase. Durability of the Cs 0.3M x W 1− x O 3 compositions as gauged by the fractional Cs loss in de-mineralized water was lowest when M = Ti and Nb, and greatest when M = Zr. From these results the durability appears intimately linked with the unit cell a-dimension which in turn varies with M cation radius.

The Influence of Thermophoresis Effects During Deposition of Hydrogenated Amorphous Silicon Thin Films with Nanocrystalline Silicon Inclusions

AbstractSilicon cluster formation is known to occur within silane plasmas when a capacitively-coupled deposition reactor is operated at high gas chamber pressures. These clusters are sensitive to the thermophoretic forces that will, depending on the sign of the thermal gradient, direct them toward or away from the silicon film's growing surface. We have developed a dual-chamber deposition system that produces nanocrystalline silicon particles (roughly 3-5 nm in diameter) in a flow-through reactor, and injects these particles into a separate capacitively-coupled plasma chamber where the amorphous film is produced. The structural, optical and electronic properties of these mixed-phase materials are investigated as a function of the controllable thermal gradient applied across the silane plasma during deposition.

Percolative magnetotransport and enhanced intergranular magnetoresistance in a correlated resistor network

Based on phase separation and intergranular tunneling, a correlated resistor network model is proposed to simulate the percolative magnetotransport in some spontaneously phase-separated perovskite oxides and their artificial granular composites. For the inclusion of spin-dependent transport mechanism in a network with correlated structure, a directed tunneling percolation scenario is established for the magnetotransport in these oxides, instead of the previous two-component metal-to-insulator percolative description in a random resistor network. Within this framework, the concentration dependence of intergranular magnetoresistance in these mixed-phase materials is well explained and the mechanism to the largest magnetoresistive effects is revealed.

Liquid-Feed Flame Spray Pyrolysis of Nanopowders in the Alumina−Titania System

We report here the synthesis of (TiO2)x(Al2O3)1-x nanopowders with molar ratios that span the TiO2−Al2O3 composition range. The nanopowders were synthesized using liquid-feed flame spray pyrolysis (LF−FSP) of mixtures of N(CH2CH2O)3TiOiPr (titanatrane isopropoxide) and N(CH2CH2O)3Al (alumatrane) dissolved in the appropriate molar ratios in alcohols used as solvent and fuel. Ethanol, methanol, and butanol were used in an attempt to regulate the flame temperatures, particle sizes, and size distributions, as well as to influence the phase(s) produced in selected mixed-phase materials produced at x = 0.5. The as-produced powders were characterized in terms of phase, size, specific surface area, composition, and morphology by BET, XRD, SEM, TEM, and TGA−DTA. At compositions close to Al2O3, the δ phase is the primary phase observed. As the compositions approach x = 0.5, a combination of rutile, α-Al2O3, and Al2TiO5 is observed. Pure titania powders consist of 90:10 mixtures of anatase and rutile; however, dopin...

Determination of the Lamellar Phase Content in MCM-41 Using X-ray Diffraction, Nitrogen Adsorption, and Thermogravimetry

Self-consistent approaches for quantification of the phase composition of ordered mesoporous materials (OMMs) are proposed on the basis of powder X-ray diffraction (XRD), and, for the first time, on the basis of gas adsorption and thermogravimetry (TGA). A series of hexagonal MCM-41 samples containing different amounts of the lamellar phase was synthesized and characterized using XRD, nitrogen adsorption, and high-resolution TGA. To quantify the phase composition of these samples, pure hexagonal and lamellar materials with appropriate structural properties were prepared, and used to calibrate the quantitative XRD analysis and to develop a new methodology for the phase-composition determination on the basis of gas adsorption and TGA. The phase-composition determination from gas adsorption data was based on the mutual independence of adsorption on different components of mixtures. The relative amounts of components of the mixed phases were obtained by fitting the adsorption isotherms measured on the calcined mixed phase materials with those measured on calcined pure hexagonal and lamellar phases. The resulting fits were remarkably good in the entire pressure range on both adsorption and desorption branches of the isotherms. The TGA quantification of the lamellar phase content was possible because of pronounced differences in the weight loss temperature ranges for hexagonal and lamellar components of the mixed phases. The results of the three methods proposed herein were in a good agreement with one another, and thus these methods were validated for independent use. Moreover, from their very nature, these methods promise to be applicable for determination of the phase composition of other mixed OMM phases.

Ferromagnetic resonance and spin wave resonance in multiphase materials: theoretical considerations

The theory of ferromagnetic resonance (FMR) and spin wave resonance (SWR) is presented for the general case of multiphase ferromagnets. This could be a magnetic multilayer structure or a material with mixed phases of distinct ferromagnetic materials. We discuss the application of the theory to various systems, with a description of amorphous and nanocrystalline materials which have not received much attention with respect to FMR and SWR. In this respect, we treat these materials in a multiphase manner for the first time, where previous FMR measurements on these types of material have been analysed and interpreted as single phase ferromagnets. Although the general theory is applicable to an N phase material, we show the detailed analysis for two phase systems, giving examples of both magnetic multilayers and mixed double phase ferromagnets of the type for mixed nanocrystalline and amorphous magnetic systems. The theory would also be applicable to systems where a magnetic phase is surrounded by a non-magnetic phase, such as co-deposited systems. We also consider, for the first time, the possibility of the existence of standing spin wave modes in mixed phase and granular materials. The adaptability of the general theory to any form of magnetocrystalline anisotropy and interphase interaction is also discussed.

Development of highly conductive n-type μc-Si:H films at low power for device applications

Highly conductive phosphorus-doped n-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by the usual (13.56 MHz) radio-frequency glow discharge of silane (SiH4), phosphine (PH3), and hydrogen (H2) in an ultrahigh-vacuum deposition system. The highest conductivity of the films obtained in this study is 100 S cm−1 after optimizing the hydrogen dilution ratio, chamber pressure, substrate temperature, and doping concentration of phosphorus. The formation of microcrystallinity in the material has been studied by transmission electron microscopy, x-ray-diffraction studies, and Raman spectroscopy. The volume fraction of microcrystallinity in these amorphous-microcrystalline mixed-phase materials has been estimated from Raman spectra. Sizes of the crystallites and volume fraction of microcrystallinity vary with hydrogen dilution, chamber pressure, and substrate temperature. The variations in the properties with deposition parameters have been explained in terms of the growth kinetics. The n-type μc-Si:H thin film, thus developed, has been applied in the first cell of a double-junction amorphous silicon solar cell. The prepared p-i-n–p-i-n stacked cell employing the n-type μc-Si:H film has exhibited appreciable improvement in open-circuit voltage, fill factor, and efficiency compared to the one with amorphous n layer in the inner n–p contact. Degradation of the cells prepared with and without μc-n layer has been studied.

On the pressure-induced phase transformation in the structure II clathrate hydrate of tetrahydrofuran

A high-pressure phase of the clathrate hydrate of tetrahydrofuran was prepared by freezing a liquid phase of overall composition THF · 7 H2O under a pressure of 3.0 kbar, or by pressurizing the solid structure II THF hydrate of 255K to 3.4 kbar. Unfortunately, the products recovered at 77K were always mixed phase materials as shown by X-ray powder diffraction. A number of diffraction lines could be indexed in terms of the cubic structure I hydrate with a slightly expanded lattice parameter, 12.08 A, giving some support to Dyadin's idea that the high pressure phase transition involves a conversion of Structure II to Structure I. Other phases observed in the recovered product include Ice IX and amorphous materials. The reversion of the high pressure sample to the structure II hydrate was followed by differential scanning calorimetry. At ambient pressure, the high pressure sample converts slowly back to Structure II hydrate event at 77K.

New method of Bravais lattice determination

A new method of Bravais lattice determination from a single convergent beam electron diffraction (CBED) pattern is proposed. The method is based on the principle that a primitive reciprocal lattice cell can be measured accurately from the deficiency line of medium or high order reflections or high order Laue zone (HOLZ) line positions within the central disk of a single CBED pattern taken near an arbitrary zone axis. This primitive cell can be reduced to the unique Niggli cell and thus the Bravais cell by Krivy and Gruber reduction procedures. This method could be very useful in identifying phase of mixed phase materials or as the first step toward determination of a new structure or as an automated procedure of indexing diffraction patterns of known structure.

β-Cyclodextrin/Diol Phase Silica Materials for Direct Injection Analysis of Drug Enantiomers in Serum by Liquid Chromatography

β-Cyclodextrin (CD)/diol phase silica materials have been designed for direct serum injection assays of drug enantiomers. The mixed functional phase (MFP) materials having β-CD and diol phases were synthesized in four steps: introduction of a 3-chloropropyl phase, introduction of a β-CD phase by ether linkage, introduction of a 3-glycidoxypropyl phase and hydrolysis of the oxirange ring to a diol phase. The obtained MFP materials were more stable against continuous flow of an eluent including phosphate buffer and organic solvent and /or acid hydrolysis than only β-CD-bonded materials were. These MFP materials could be used for direct serum injection assays of drug enantiomers. The recovery of drug enantiomers from serum was almost 100%.

OBSERVATION OF THERMALLY RECYCLEABLE ZERO-RESISTANCE STATES IN YBaCuO ABOVE DRY ICE TEMPERATURES

Thermally recycleable zero-resistance states with transition temperatures above 200 K are observed in mixed-phase YBaCuO materials that are treated by a low-temperature (< 150° C ) oxygenation process and enclosed in an oxygen atmosphere during electrical and magnetic measurements. The zero-resistance state is verified through multiple lead and contact arrangements as well as by measurements of the current-voltage characteristics for several paths with the highest transition temperature (zero resistance) for any sample being 265 K and with a zero-voltage current exceeding 4.3 mA at 240 K. Although a full Meissner effect is not obtained, diamagnetic-like deviations and hysteretic behavior are observed to occur at the same temperatures as the resistive transitions. This indicates that the zero resistance, if related to superconductivity, is due to filamentary superconducting paths or materials of lower dimensionality.

Structural and superconducting properties of Bi 2Sr 2(M′ 1−yM y″) 2Cu 3O x(M′=Ca,Y; M″=Na,Ca) including hydrided materials

Structural and superconducting properties of materials of nominal composition Bi 2Sr 2(M′ 1−yM″ y) n−1Cu nO x of the type Bi 2Sr 2(Ca 1−yY y) 2Cu 3O x, Bi 2Sr 2(Y 1−yNa y) 2Cu 3O x and Bi 2Sr 2(Ca 1−yNa y) 2Cu 3O x have been investigated. Mixed phase materials are observed with superconducting onset temperatures as high as T c ~ 107K when M′=Ca, M″=Na. Non-superconducting mixed phase materials are found when M′=Y, M″=Na and y = 0.5 or 0.6. For Bi 2Sr 2(Ca 1−yY y) 2Cu 3O x values of T c for the n = 2 phase decrease with y. They can be partly restored through insertion of H. The results are discussed in terms of stability of the competing Bi 2Sr 2CaCu 2O x phase and modification of bond valence and hole concentration. In particular, a mechanism is discussed based on a lower bond valence than 3 on Bi. Accordingly, Bi can act as a hole reservoir. Partial Y substitution for Ca reduces T c as it increases x and Bi bond valence. H insertion restores the low Bi bond valence and characteristic Cu hole concentration through partial bonding to O.

EPR and NMR measurements on high-temperature superconductors

Some EPR and NMR experiments at the Cu sites in the mixed-phase materials Ba0.6Y0.4CuOy and Ba0.2Y0.8CuOy, and the pure-phase compounds YBa2Cu3Oy (1:2:3), BaCuO2 (0:1:1), Y2Cu2O5 (2:0:2), Y2BaCuO8 (2:1:1), YBa3Cu2Oy (1:3:2) and CuO are presented and discussed. In particular, it is shown that any EPR signal in nominally pure 1:2:3 superconducting material is probably due to a small amount of impurity 2:1:1 (greenphase) compound. For the 2:1:1 and 1:3:2 compounds the authors find g/sub /// approximately=2.23 and gperpendicular to approximately=2.09 with a linewidth of approximately=0.025 T, whereas for the 2:0:2 compound g=2.12(2) with a linewidth of 0.047(3)T. Some suggestions are also made concerning the absence of both NMR and EPR signals in the 1:2:3 compound, in terms of (i) Anderson's resonating bond model and (ii) recent FLAPW band-structure calculations. In particular, it is argued that the delocalised nature of the Cu d electrons probably gives rise to exceptional spin-spin dipolar broadening of both the NMR and EPR transitions.

Mechanisms for the Deposition of Thin Metallic Films by Laser Driven Gas Phase Reactions

AbstractGas phase processing rakes laser deposition over large areas possible but homogeneous nucleation of large atomic clusters must be avoided if films are to be produced. Clusters can be highly variable in size from a few atoms to significant fractions of a micrometer. If conditions do not allow for complete quenching of the clusters produced in the gas phase, these clusters can arrive at the substrate with sufficient energy to self sinter into homogeneous films which are substantially different from metallic films grown by thermal techniques. Using transmission electron microscopy (TEM), we have characterized the microstructure of thin metallic films deposited by laser breakdown chemical vapor deposition and identified a range of deposition conditions which can lead from powders to homogeneous polycrystalline films and mixed phase materials. Gas phase nucleation is dependent on reactant partial pressures and the gas phase quench rate which car be varied in part by adjusting the H2 content of the source gas. Manipulation of these parameters can vary powder size from about one micrometer to less than 2 nanometers. Variation of the quench rate during the deposition of polycrystalline films varies the grain size in the films. heating the substrate drastically changes the conditions under which the film is formed and as a consequence, can radically alter the microstructure of the film itself.