Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 2010"

Sol–gel-deposited ZnO thin films: A review

Abstract: During the last years, ZnO thin films have been studied extensively due to their potential applications in e.g. piezoelectric and optoelectronic devices or photovoltaic cells. Ordered c-axis orientation of ZnO crystallites is desirable for applications where crystallographic anisotropy is a prerequisite such as for short-wavelength semiconductor diode lasers (SDLs), and piezoelectric surface acoustic wave or acousto-optic devices. Many works were dedicated to c-axis oriented ZnO thin films elaboration and the study of their properties, including physical and chemical methods. For instance, sol–gel processes are particularly well adapted to produce ZnO films in a simple, low-cost and highly controlled way. This review summarizes the main chemical routes used in the sol–gel synthesis of undoped ZnO thin films and highlights the chemical and physical parameters influencing their structural properties. In this process, the ZnO films synthesis includes three principal steps: (i) solution preparation, (ii) coating and (iii) heat treatment. For the first step, the particle formation is discussed including nucleation and growth, particle size, morphology and colloids stability. These three steps involve several parameters such as: (i) nature and concentration of precursor, solvent and additive, and solution aging time, for the chemical system, (ii) coating method, thickness and substrate for the coating step, and (iii) pre-and post-heat treatment for the last step. The influence of these steps and synthesis parameters on ZnO thin films orientation is discussed.

Synthesis and characterization of Pt/BiOI nanoplate catalyst with enhanced activity under visible light irradiation

Abstract: A series of Pt/BiOI nanoplate catalysts have been synthesized by a solution combination with photodeposited method at room temperature. The as-synthesized products have been investigated by photocatalytic reaction tests and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) emission spectroscopy, and UV–vis diffuse reflectance spectra (DRS). The results show that platinum has important role in promoting the photocatalytic performance of BiOI in degradation of acid orange II in an aqueous solution under visible light (λ > 420 nm) irradiation. The deposition of optimal amount of 0.2 wt% Pt brings about a two-time increase in the photocatalytic performance. The dispersed platinum nanoparticles over BiOI nanoplate could decrease the recombination rate of the e−/h+ pairs, hence promoting the photocatalytic activity.

Optical and structural properties of thermally evaporated cadmium sulphide thin films on silicon (100) wafers

Abstract: Cadmium sulphide (CdS) polycrystalline thin films were deposited on ultrasonically cleaned silicon wafers (1 0 0) by thermal evaporation technique in a vacuum of about 2 × 10 −5 Torr. X-ray diffraction (XRD), Fourier transforms infrared (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), and spectroscopic ellipsometry (SE) were used to characterise the polycrystalline thin films. The thickness of the films as estimated by the ellipsometry, found to be in the range of 263–282 nm. The values of refractive index ( n ) and extinction coefficient ( k ) of thin films are found to decrease with wavelength. XRD results indicated the formation of cadmium sulphide thin films with hexagonal phase. The films have preferred orientation along (0 0 2) plane. The lattice parameters are calculated as a = 4.135 A and c = 6.742 A, which are in good agreement with the reported data. The optical properties of the polycrystalline thin films are investigated systematically by spectroscopic ellipsometry (SE) and a blue shift compared with bulk cadmium sulphide is observed. The optical band gap of the thin film is estimated to be 2.50 eV. The structural and optical properties of the films fabricated by the thermal evaporation technique are found to be desirable for optoelectronic devices.

The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

Abstract: Aluminium nitride (AlN) reactively sputter-deposited from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. A good c -axis orientation is essential for obtaining high piezoelectric coefficients. Therefore, the influence of different sputtering conditions on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. In this study it is demonstrated that highly c -axis oriented AlN thin films can be deposited on nominally unheated (1 0 0) silicon substrates, most preferentially when using a pure nitrogen atmosphere. The degree of c -axis orientation increases with higher nitrogen concentration and with decreasing the sputtering pressure, whereas the influence of plasma power on the microstructure was found to be negligible. A low sputtering pressure is also useful for minimizing the amount of oxygen contaminations in the deposition chamber and hence for reducing the incorporation of impurities into the AlN films. Intrinsic stress values of AlN thin films were determined by wafer bow measurements and were found to be between −3.5 and 750 MPa depending on choice of deposition parameters. Finally, the piezoelectric coefficients d 33 and d 31 were determined experimentally by laser scanning vibrometry in conjunction with a theoretical model. Effective values in c -axis oriented 500 nm films with FWHM of 0.33° are 3.0 and −1.0 pm/V. For a film of 2.4 μm thickness, values of 5.0 and −1.8 pm/V were measured, which are near the bulk values.

Photocatalytic activity of TiO2 doped with Zn2+ and V5+ transition metal ions: Influence of crystallite size and dopant electronic configuration on photocatalytic activity

Abstract: Anatase TiO 2 was prepared by sol–gel method through the hydrolysis of titanium tetrachloride and doped with transition metal ions like V 5+ and Zn 2+ . The photocatalysts were characterized by various analytical techniques. Powder X-ray diffraction studies revealed only anatase phase for the doped samples. The band gap absorption for the doped samples showed red shift to the visible region (∼456 nm) as confirmed by UV–vis absorption spectroscopy and diffuse reflectance spectral studies. The surface area of the Zn 2+ doped samples were higher than the V 5+ doped samples as observed by BET surface area measurements due to their smaller crystallite size. Scanning electron microscopy showed almost similar morphology, while energy dispersive X-ray analysis confirmed the presence of dopant in the TiO 2 matrix. The photocatalytic activities of these catalysts were tested for the degradation of Congo Red under solar light. Although both the doped samples showed similar red shift in the band gap, Zn 2+ (0.06 at.%) doped TiO 2 showed enhanced activity and its efficiency was five fold higher compared to Degussa P-25 TiO 2 . This enhanced activity was attributed to smaller crystallite size and larger surface area. Further completely filled stable electronic configuration (d 10 ) of Zn 2+ shallowly traps the charge carriers and detraps the same to the surface adsorbed species thereby accelerating the interfacial charge transfer process.

Study of nanobiomaterial hydroxyapatite in simulated body fluid: Formation and growth of apatite

Abstract: Hydroxyapatite (HAp) is main mineral component of hard tissues. It is widely used in biomedical applications due to its excellent bioactivity and biocompatibility. Nanosized HAp is synthesized by wet chemical process. The synthesized HAp is characterized by XRD, FTIR, AFM and SEM for structural, morphological and functional groups analysis. The Simulated Body Fluid (SBF) is prepared by using chlorides, carbonates, oxides, and sulphates of alkali metals at 37 °C. The ion exchange process is carried out to exchange calcium cation by sodium and potassium. The pure HAp and ion exchanged HAp pellets are used as source of nucleating agent for apatite layer formation, in SBF maintained at 37 °C using incubator for different periods of time to study the bioactivity. The dielectric study is carried out on incubated pure and ion exchanged HAp pellets. XRD analysis confirms the hexagonal phase of hydroxyapatite. FTIR shows the presence of functional groups. SEM observations reveal that the growth of highly porous apatite layer on HAp surface increases with time. The dielectric constant is found to be in the range 3–12. It is seen that the synthesized HAp bioceramic nano material not only supports the growth of apatite layer but also accelerates the growth onto itself.

Microwave-assisted synthesis of perovskite ReFeO3 (Re: La, Sm, Eu, Gd) photocatalyst

Abstract: A novel microwave-assisted method was applied to the synthesis of perovskite-type samples ReFeO 3 (Re: La, Sm, Eu, Gd). Rare earth nitrates were blended with Fe(NO 3 ) 3 ·9H 2 O to form the precursors, active carbon was used to enhance microwave absorption, polyvinyl alcohol (PVA) and urea were introduced as the inhibitor and the homogeneous precipitator, respectively. The whole assembly was then exposed to microwave to complete the reaction. Compared with traditional methods, samples prepared by this microwave-assisted method are nanosized powder. All the samples were characterized by X-ray diffraction (XRD), UV-visible diffuse reflection spectroscopy (UV-vis DRS), Brunauer-Emmett-Teller (BET) specific surface area measurements, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Photocatalytic property was evaluated by degradation of Rhodamine B (RhB) aqueous solution under visible light irradiation. All these samples exhibit good photocatalytic activity due to their perovskite structure.

Uniformity of fully gravure printed organic field-effect transistors

Abstract: Fully mass-printed organic field-effect transistors were made completely by means of gravure printing. Therefore a special printing layout was developed in order to avoid register problems in print direction. Upon using this layout, contact pads for source–drain electrodes of the transistors are printed together with the gate electrodes in one and the same printing run. More than 50,000 transistors have been produced and by random tests a yield of approximately 75% has been determined. The principle suitability of the gravure printed transistors for integrated circuits has been shown by the realization of ring oscillators.

Investigation on Fe-doped ZnO nanostructures prepared by a chemical route

Abstract: Zn 1− x Fe x O ( x = 0.03, 0.05 and 0.07) nanoparticles synthesized by a chemical route were characterized by different techniques. The structural characterization by XRD and TEM confirmed the phase purity of the samples and indicated a reduction in particle size with increase in the dopant (Fe) concentration in ZnO. The optical characterization of the nanoparticles by FTIR, PL and UV–visible spectroscopy confirmed the formation of wurtzite structure and incorporation of Fe in the ZnO lattice. Magnetization measurements by VSM and Faraday balance techniques indicate presence of room temperature ferromagnetism in the Fe-doped ZnO samples. Local environment around the Fe atoms has been probed by 57 Fe Mossbauer spectroscopy and the measured isomer shifts confirmed the charge state of iron as Fe 3+ . Positron annihilation lifetime spectroscopy (PALS) measurements confirm the presence of cation vacancies in the nanoparticles and indicate a reduction of overall defect concentration with incorporation of Fe atoms in the ZnO structure.

A novel anion exchange membrane from polystyrene (ethylene butylene) polystyrene: Synthesis and characterization

Abstract: We look forward for an eco-friendly hydrocarbon polymer with higher molecular weight for the preparation of an anion exchange membrane. Polystyrene ethylene butylene polystyrene (PSEBS) was chosen as the polymer matrix. The anion exchange membrane was prepared from PSEBS tri-block co-polymer and then the properties were characterized for alkaline fuel cell application. The preparation of anion exchange polymer involved two steps namely chloromethylation and quaternization. The anion exchange membrane with high conductivity has been prepared by introducing quaternary ammonium groups in to the polymer. Finally, the membrane was prepared using solution casting method. The solution casting method yields highly hydrophilic membranes with uniform structure that were suitable for electrochemical applications. The efficiency of the entrapment was monitored by swelling ratio, chemical stability and ion exchange measurement. The characteristic structural properties of the membrane were investigated by FT-IR spectroscopy and 1H NMR spectroscopy. The thermal stability of the membrane was characterized by TGA, DSC and DMA (dynamic mechanical analysis). The prepared uniform electrolyte membrane in this study has high thermal and chemical stability. The surface morphology and elemental composition of the quaternized PSEBS was determined by SEM-EDXA techniques, respectively. The measured hydroxyl ion conductivity of the synthesized alkaline PSEBS polymer electrolyte membrane showed ionic conductivity in the range of 10−3 S/cm in deionized water at room temperature. It was found that the substitution provided a flexible, chemically and thermally stable membrane. Hence, the membrane will have potential application in the alkaline fuel cell.

The properties of ethanol gas sensor based on Ti doped ZnO nanotetrapods

Abstract: An ethanol gas sensor was fabricated based on Ti doped ZnO nanotetrapods which were prepared by chemical vapor deposition (CVD) of ZnO nanotetrapods followed by co-annealing with TiO 2 powder. X-ray diffraction (XRD), Raman spectra and scanning electron microscopy (SEM) were used to characterize the morphology and structure of the as-obtained sample and the ethanol-sensing characteristics of the device were investigated. ZnO:Ti sensors show higher gas response than ZnO counterparts towards 100 ppm ethanol gas at a temperature of 260 °C. The recovery times of the devices are 3.1 min for ZnO:Ti and 10.1 min for ZnO, respectively. The enhancement of sensing properties of ZnO:Ti tetrapods indicates the potential application for fabricating low power and highly sensitive gas sensors.

Antibacterial characteristics of CaCO3–MgO composites

Abstract: Dentifrices, such as tooth-paste, are pastes containing insoluble abrasives that aid in the removal of plaque from the teeth and help to polish them. Composite powders contributing to oral hygiene application, i.e., nano-scale MgO crystallite dispersed in CaCO 3 grain, were fabricated by the thermal decomposition of dolomite. The composite obtained by heating at 800 °C consisted of CaCO 3 grains including 20 nm MgO fine crystallite, being the purpose powder in this study. The antibacterial activity of these powders related to gram-positive and gram-negative bacteria was evaluated in vitro . The thermal decomposition above 800 °C resulted in the mixture of CaO and MgO. Antibacterial activity of the composite enhanced with increasing powder concentration. Though antibacterial action toward Staphylococcus aureus was greater than towards Escherichia coli , the death rate constant was identical in both bacteria. It can be concluded that the obtained composite possesses two functions able to improve the oral hygiene: as a tooth abrasive and as an antibacterial agent.

Room temperature Young's modulus, shear modulus, Poisson's ratio and hardness of PbTe–PbS thermoelectric materials

Abstract: Two-phase PbTe–PbS materials, in which PbS is a nanostructured phase, are promising thermoelectric materials for the direct conversion of heat energy into electricity. In this study, a Vickers indentation mean hardness of 1.18 ± 0.09 GPa was measured for hot pressed specimens Pb 0.95 Sn 0.05 Te–PbS 8% while the mean hardness of cast specimens was 0.68 ± 0.07 GPa. The mean fracture toughness of the not pressed specimens was estimated as 0.35 ± 0.04 MPa m 1/2 via Vickers indentation. Resonant Ultrasound Spectroscopy (RUS) measurements on hot pressed specimens gave mean values of Young's modulus, shear modulus and Poisson's ratio of 53.1 GPa, 21.4 GPa and 0.245, respectively while for the cast specimens the Young's and shear moduli were about 10% lower than for the hot pressed, with a mean value of Poisson's ratio of 0.245. The differences between the hardness and elastic moduli values for the cast and hot pressed specimens are discussed.

Mechanical properties of filled antimonide skutterudites

Abstract: Time-of-flight and resonant ultrasound spectroscopy techniques were employed for elastic moduli measurements on a set of various Fe 4 Sb 12 - and Co 4 Sb 12 -based skutterudites filled by mischmetal, didymium, or alkaline earths (Ca, Sr, Ba). A weak temperature influence on the longitudinal modulus C 11 indicates weak degradation of elastic properties within the thermoelectric working temperature range. Elastic moduli for Co 4 Sb 12 -based skutterudites are only slightly higher than for Fe 4 Sb 12 -based skutterudites, and the influence of various filler atoms or filling fractions on the elastic moduli is even smaller. Ball milled and hot pressed samples (grain size ∼250 nm) illustrate an obvious improvement of elastic properties in relation to those hot pressed from hand milled powders (grain size ∼50 μm). Debye temperatures calculated from sound velocity measurements are comparable to the values obtained from the parameters fitted to thermal expansion, which indicate that Co 4 Sb 12 -based skutterudites having slightly higher values than Fe 4 Sb 12 -based skutterudites. Vickers hardness is increased by Co or Ni substitution and demonstrates a linear dependence on density, Young's modulus, and shear modulus.

Synthesis of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite and study of its magnetic behavior at different temperatures

Abstract: Ni0.5Zn0.5Fe2O4 ferrite nanocrystals with average diameter in the range of 1–2 nm have been synthesized by reverse microemulsion. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) are used to characterize the structural, morphological and magnetic properties. X-ray analysis showed that the nanocrystals possess cubic spinel structure. The absence of hysteresis, negligible remanence and coercivity at 300 K indicate the superparamagnetic character and single domain in the nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite materials. The nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite were annealed at 600 °C. As a result of heat treatment the average particle size increases from 2 nm to 5 nm and the corresponding magnetization values have increased to 21.69 emu/g at 300 K. However, at low temperature of 100 K, the annealed samples show hysteresis loop which is the characteristic of a superparamagnetic to ferromagnetic transition. In addition, a comparative study of the magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion has been carried out with those obtained from the general chemical co-precipitation route.

Hydrothermal synthesis, characterization, formation mechanism and electrochemical property of V3O7·H2O single-crystal nanobelts

Abstract: Single-crystal V 3 O 7 ·H 2 O nanobelts have been successfully synthesized in a large-scale by ethanol reducing of the commercial V 2 O 5 powder via a facile hydrothermal approach, without any templates and surfactants. The as-prepared V 3 O 7 ·H 2 O nanobelts are up to several tens of micrometers in length, about 100 nm in width and about 20 nm in thickness in average, respectively. The “Hydrating–Reducing–Exfoliating–Splitting” (HRES) mechanism was proposed to describe the formation of the V 3 O 7 ·H 2 O nanobelts. In our research progress, it was found that the ratio of EtOH/H 2 O, the reaction time and categories of the reducing agents had significant effects on the morphology and composition of as-obtained products. Furthermore, the electrochemical properties of V 3 O 7 ·H 2 O nanobelts were preformed and the results revealed that a lithium battery using those nanobelts as the positive electrode exhibited a high initial discharge capacity of 373 mAh/g.

High temperature polymer film dielectrics for aerospace power conditioning capacitor applications

Abstract: Polymer dielectrics are the preferred materials of choice for capacitive energy-storage applications because of their potential for high dielectric breakdown strengths, low dissipation factors and good dielectric stability over a wide range of frequencies and temperatures, despite having inherently lower dielectric constants relative to ceramic dielectrics. They are also amenable to large area processing into films at a relatively lower cost. Air Force currently has a strong need for the development of compact capacitors which are thermally robust for operation in a variety of aerospace power conditioning applications. While such applications typically use polycarbonate (PC) dielectric films in wound capacitors for operation from −55 °C to 125 °C, future power electronic systems would require the use of polymer dielectrics that can reliably operate up to elevated temperatures in the range of 250–350 °C. The focus of this research is the generation and dielectric evaluation of metallized, thin free-standing films derived from high temperature polymer structures such as fluorinated polybenzoxazoles, post-functionalized fluorinated polyimides and fluorenyl polyesters incorporating diamond-like hydrocarbon units. The discussion is centered mainly on variable temperature dielectric measurements of film capacitance and dissipation factor and the effects of thermal cycling, up to a maximum temperature of 350 °C, on film dielectric performance. Initial studies clearly point to the dielectric stability of these films for high temperature power conditioning applications, as indicated by their relatively low temperature coefficient of capacitance (TCC) (∼2%) over the entire range of temperatures. Some of the films were also found to exhibit good dielectric breakdown strengths (up to 470 V/μm) and a film dissipation factor of the order of

Complex permittivity, permeability and microwave absorbing studies of (Co2−xMnx) U-type hexaferrite for X-band (8.2–12.4 GHz) frequencies

Abstract: In order to develop a new microwave absorbing material over the X-band (8.2–12.4 GHz) frequency range, polycrystalline samples of U-type hexaferrite series: Ba 4 Co 2− x Mn x Fe 36 O 60 , with x varying from 0 to 2.0 in steps of 0.5, were prepared through conventional solid state reaction route. X-ray diffraction analysis was done to find the structural change with composition. The variation of electromagnetic parameters (complex permittivity ɛ * = ɛ ′ − jɛ ′′ and complex permeability μ * = μ ′ − jμ ′′) and microwave absorbing properties were studied over the X-band frequency range with varying composition. Both dielectric loss ( ɛ ′′) and magnetic loss ( μ ′′) were found to decrease with increasing x , whereas the dielectric constant ( ɛ ′) and magnetic permeability ( μ ′) variations are reciprocal with Mn addition. Composition with x = 1, presenting a maximum value of microwave absorption of 99.84% (or reflection loss R L = −28.4 dB) at 8.45 GHz when sample thickness is 1.7 mm. This composition also offered wide band absorption (>96%) throughout the X-band frequencies where it can be used for suppressing electromagnetic interference and radar signature.

Effect of milling time on the synthesis of magnetite nanoparticles by wet milling

Abstract: In this study, nanosize magnetite (Fe3O4) particles have been prepared directly from metallic iron (Fe) powder within distilled water (H2O) by using a planetary ball mill, and the effect of milling time has been investigated. According to Rietveld refinement result obtained from X-ray diffraction (XRD) analyses, the amount of Fe decreases from 98.2% to 0.0%, and it is transformed into Fe3O4, from 1.8% to 100.0%, with the increasing milling time from 1 to 48 h. Due to similar crystal structure of the magnetite and maghemite (γ-Fe2O3), FTIR and Raman spectroscopies as well as a chemical analysis method was used to verify the magnetite structure. FTIR spectra have clearly revealed absorption peaks around 628, 581 and 443 cm−1, which are in good agreement with the characteristic absorption peaks of Fe3O4. In addition Raman analysis verified the formation of magnetite phase with a clear main band peak at 671 cm−1. Chemical analyses have shown that the total amount of Fe in the milled sample for 48 h is 73.04%, which contains 24.10% Fe2+ and 49.34% Fe3+. These results are consistent with the theoretically estimated values of the magnetite. It has been observed that the saturation magnetization decreased from 146.02 to 63.68 emu/g with increasing milling time due to the formation of the ferrimagnetic magnetite phase.

Facile synthesis of ZnO nanoparticles based on bacterial cellulose

Abstract: ZnO nanoparticles with a pure wurtzite structure have been successfully synthesized through decomposing bacterial cellulose infiltrated with zinc acetate aqueous solution at high temperature. The effects of the concentration of zinc acetate aqueous solution, the calcination temperature, and the templates on the average particle size and morphology of the ZnO nanoparticles were investigated. The prepared ZnO nanoparticles were characterized by FESEM, XRD, FTIR and TG–DTA. The results suggest that bacterial cellulose plays an important role in preventing the ZnO nanoparticles from aggregating under optimized conditions. The calcination temperature has great effects on the morphologies of ZnO nanoparticles. When calcinating at 600 °C and using BC as the template with 1 wt.% zinc acetate aqueous solution, well-dispersed and regular ZnO nanoparticles with a narrow size distribution of 20–50 nm and high photocatalytic activity were obtained.

Influence of pH on structural morphology and magnetic properties of ordered phase cobalt doped lithium ferrites nanoparticles synthesized by sol–gel method

Abstract: Cobalt doped lithium ferrite nanoparticles were synthesized at different pH by sol–gel method. The effect of pH on the physical properties of cobalt doped lithium ferrite nanoparticles has been investigated. The nanoparticles synthesized at different pH were characterized through X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman spectroscopy (RS), Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and vibrating sample magnetometer (VSM). The XRD patterns were analyzed to determine the crystal phase of cobalt doped lithium ferrites nanoparticles synthesized at different pH. The XRD results show the formation of impurity free cobalt doped lithium ferrites having ordered phase spinel structure. A similar kind of conclusion was also drawn through the analysis of Raman spectra of the nanoparticles synthesized at different pH. SEM micrographs show that the structural morphology of the nanoparticles is highly sensitive to the pH during the synthesis process. The magnetic properties such as; saturation magnetization (Ms), remnant magnetization (Mr) and coercivety (Hc) have been also investigated and found to be different for the nanoparticles synthesized at different pH, which may be attributed to the different size and surface morphology of the nanoparticles.

Optical properties and thermal stability of germanium oxide (GeO2) nanocrystals with α-quartz structure

Abstract: Germanium dioxide (GeO 2 ) crystals were prepared by a chemical precipitation method at a relatively low-temperature (100 °C). The grown crystals were characterized by studying their microstructure, optical properties and thermal stability. The results indicate that the grown GeO 2 crystals exhibit α-quartz type crystal structure. The lattice parameters obtained from XRD were a = 4.987(4) A and c = 5.652(5) A. Electron microscopy analysis indicates a high structural quality of GeO 2 crystals grown using the present approach. Optical absorption measurements indicate a direct bandgap of 5.72 eV without any additional bands arising from localized or defect states. Thermogravimetric measurements indicate the temperature stability of the grown GeO 2 nanocrystals. Microscopic analysis coupled with energy dispersive X-ray spectroscopy of the GeO 2 crystals with α-quartz type crystal structure indicates their stability in chemical composition up to a temperature of 400 °C. The surface morphology of GeO 2 crystals, however, found to be changing with the increase in temperature.

Theoretical and experimental study of La/Ni co-doped SrTiO3 photocatalyst

Abstract: The effects of Ni and La co-doping on the electronic structure of SrTiO 3 were investigated by theoretical (DFT) and experimental work. The calculation results revealed that doping sites of La and Ni impurities had a great influence on geometric structure and electronic properties of SrTiO 3 . Pure SrTiO 3 and doped SrTiO 3 were synthesized through citrate sol–gel method. The as-synthesized photocatalysts were characterized by XRD, UV–visible absorption spectroscopy and nitrogen adsorption, and employed to degrade methylene blue. La/Ni co-doped SrTiO 3 exhibited the highest activity. The experimental results were in good agreement with the calculation results.

A novel synthesis of graphene by dichromate oxidation

Abstract: A novel synthetic route has been developed to produce very stable aqueous dispersed graphene sheets from the pristine graphite by oxidation with dichromate followed by reduction with hydrazine The particle size, physical feature and zeta potential of graphene oxide show better than that of the modified Hummer's method

Hard coating performance enhancement by using [Ti/TiN]n, [Zr/ZrN]n and [TiN/ZrN]n multilayer system

Abstract: In order to increase the performance of hard coatings for industrial application, Ti/TiN, Zr/ZrN and TiN/ZrN multilayers were deposited onto Si(1 0 0) and AISI 5160 steel substrates via r.f. (13.56 MHz) multi-target magnetron sputtering technique. The crystallography structures of Ti/TiN, Zr/ZrN and TiN/ZrN multilayers were evaluated via X-ray diffraction analysis (XRD). The composition, chemical-depth distribution and cross-sectional morphology of the films were characterized by EDX, SIMS, and SEM. The enhancement of mechanical properties and adherence strength behavior were determined via nanoindentation and scratch test, respectively. From these results, it was found that hardness and critical load of all multilayer films depends on the bilayer period (� ) and bilayer number (n). Moreover, the TiN/ZrN multilayer films exhibit the highest hardness (30 ± 1 GPa) and critical load (60 ± 1 N) in relation to Ti/TiN and Zr/ZrN multilayers. The different mechanical performances and mechanisms in the multilayers group were observed and discussed in this paper, suggesting the possibility to use AISI 5160 steel coated with multilayers as cutting tools. © 2010 Elsevier B.V. All rights reserved.

Preparation, characterization and photocatalytic activity of TiO2 towards methylene blue degradation

Abstract: Titanium dioxide (TiO2) powders were prepared by simple hydrolysis of titanium tetrachloride (TiCl4) followed by subsequent drying at 100 °C. The as prepared powders were calcined at different temperatures ranging from 400 to 900 °C. The as synthesized as well as calcined TiO2 powders were characterized using X-ray diffractometry, scanning electron microscopy and BET surface area measurements. X-ray diffractometry revealed anatase to rutile phase transformation above 800 °C for the TiO2 powders. Photocatalytic activity of these samples was evaluated by studying the degradation of 10 ppm aqueous methylene blue dye under 400 W high pressure mercury vapor lamp with 50 mg of TiO2 powders. Highest photocatalytic activity was observed for TiO2 powder calcined at 750 °C due to presence of anatase and rutile phases in 80:20 ratio. The activity of powder calcined at 750 °C was comparable with commercial Degussa P-25 TiO2 powders owing to similar phase composition.

Biocompatibility and sp3/sp2 ratio of laser created DLC films

Abstract: Thin diamond-like carbon (DLC) films of various diamond/graphitic content were created using pulsed laser deposition method. “Diamond” sp3 concentration in region from 32% to 62% was determined using X-ray photoelectron and Auger spectroscopy (XPS/XAES). Zeta potential test was conducted to measure electric surface properties. In vitro test of DLC layers was arranged using normal human fibroblasts. The study was stressed on cytotoxicity, adhesion and cell proliferation. Correlations between sp3, sp2 bonds and surface and biological properties of DLC layers are discussed.

Microwave absorption properties of carbon nanotubes and tetrapod-shaped ZnO nanostructures composites

Abstract: CNTs/T-ZnO/EP composites were fabricated using carbon nanotubes (CNTs) and tetrapod-shaped ZnO (T-ZnO) nanostructures as absorbents and epoxy resin (EP) as binder. The electromagnetic characteristics and microwave absorption properties of the composites were investigated in the frequency range of 2–18 GHz. The influences of absorbents concentration and composite thickness on microwave absorption properties were studied. When the content of CNTs and T-ZnO nanostructures are 12 wt% and 8 wt%, respectively, and the composite thickness is 1.5 mm, the value of the minimum reflection loss for CNTs/T-ZnO/EP composites is −23.00 dB at 12.16 GHz, and the bandwidth corresponding to reflection loss below −10 dB is 5 GHz. Moreover, the mechanism of microwave absorbing for CNTs/T-ZnO/EP composites was discussed. The results indicate that CNTs/T-ZnO/EP composites have the potential applications as microwave absorbents with the thin thickness and light weight.

Nanostructured CrN thin films prepared by reactive pulsed DC magnetron sputtering

Abstract: High quality chromium nitride (CrN) films have been deposited onto silicon (1 0 0) substrates using pulsed DC magnetron sputtering of pure Cr target at different gas mixtures of argon and nitrogen and in the substrate temperature range 303–973 K. At low N 2 flow rates ( 2 N and CrN) are obtained. At higher flow rates, in the range of 10–25 sccm, only cubic CrN phase is obtained. The films prepared at different substrate temperatures and at 10 sccm of nitrogen flow rate indicated the formation of cubic CrN phase at room temperature and the phase formed is found to be stable up to 973 K. The deposition of the films as a function of nitrogen flow rate and substrate temperatures indicated that the good quality crystalline films could be formed at 773 K, 10 sccm of nitrogen flow rate. The Cr 2p 3/2 and N 1s of XPS spectra also confirmed the formation of CrN phase. Determination of the texture coefficients of the CrN films as a function of substrate temperature showed that the preferred orientation changes from [1 1 1] to [1 0 0]. Deposition as a function of nitrogen flow rates and substrate temperatures showed significant changes in the morphology and RMS surface roughness, which could be related to the difference in the growth mechanism of the CrN films. Measurement of nanomechanical properties on typical films deposited on titanium modified stainless steel substrates at optimum conditions show hardness of 12 ± 1.81 GPa, Young's modulus of about (250 ± 51.28 GPa) and coefficient of friction of 0.16.

Allylhydridopolycarbosilane (AHPCS) as matrix resin for C/SiC ceramic matrix composites

Abstract: In present study, partially allyl-substituted hydridopolycarbosilane (5 mol% allyl) [AHPCS] has been characterized by spectral techniques and thermal analysis. The DSC studies show that, the polymer is self-cross-linking at lower temperatures without any incorporation of cross-linking agents. The spectral and thermal characterizations carried out at different processing stages indicate the possibility of extensive structural rearrangement accompanied by the loss of hydrogen and other reactions of C and Si containing species resulting in the conversion of the branched chain segment into a 3D SiC network structure. AHPCS gave ceramic residue of 72% and 70% at 900 and 1500 °C respectively in argon atmosphere. XRD pattern of 1500 °C heat-treated AHPCS, indicates the formation of silicon carbide with the particle size of 3–4 nm. AHPCS was used as matrix resin for the preparation of C/SiC composite without any interfacial coating over the T-300 carbon fabric reinforcement. Flexural strength value of 74–86 MPa for C/SiC specimen with density of 1.7 g/cm 3 was obtained after four infiltration and pyrolysis cycles.