Crystal morphology in pristine and doped films of poly (p-phenylene vinylene)

The crystal morphology of oriented films of poly (p-phenylene vinylene) (PPV) has been investigated using electron microscopy and X-ray diffraction. An X-ray diffraction rotation series confirmed the existence of fibre symmetry in bulk oriented films. Dark-field imaging by transmission electron microscopy (TEM) revealed small diffracting regions of the order of 7 nm in size with an aspect ratio near 1. These diffracting regions were shown by high resolution transmission electron microscopy (HREM) to be composed of small crystallites with an average size of 5 nm. Imaging of the lateral packing by HREM allowed the evaluation of local variations in crystallite orientation. This HREM method of orientation function determination compares well to bulk methods (e.g. wide-angle X-ray scattering, infrared dichroism) for PPV of similar draw ratio. A micellar model is presented to describe the crystalline morphology of oriented PPV. The model presents PPV as a highly connected network of small crystallites. The well-formed crystalline regions are thought to compose approximately 50% of the sample volume with the remainder of the volume being grain boundaries. Doping by AsF₅ led to the formation of an electron-dense overlayer, thought to be arsenic oxide, which prohibited darkfield imaging of the crystallites. After doping with H₂SO₄, crystallites of the electrically conductive phase were observed. The general morphological character is preserved in the conversion from insulating to conducting forms. For the conditions employed, the doped diffracting regions were 4 nm in size and retained the orientation initially present in the pristine film.     

Morphological,optical, structural and photoelectrochemical properties of TiO2 nanoflower prepared via PVP surfactant assisted liquid phase deposition technique

This paper reports the synthesis of TiO₂ nanoflowers as photovoltaic materials of the photoelectrochemical cell (PEC) via the phase liquid deposition technique assisted with a polyvinylpyrrolidone (PVP) surfactant. This work investigates the influence of the surfactant concentration on the morphological, optical, phase structure and PEC properties of the TiO₂ PEC cell. The grain size of the TiO₂ sample decreases with increase in PVP concentration. The smallest grain size of 16 ± 2 nm is obtained at 8 mM PVP. The thickness of the sample increases with PVP concentration. The concentration of PVP does not influence the optical absorption and band gap of the sample. The phase structure and crystallinity degree of the sample are not influenced by PVP concentration. The crystallite size is around 20 nm. The short-circuit current density, J_sc, of the PEC cell utilising these TiO₂ samples increases with decrease in grain size which is due to the increase in PVP concentration. The best J_sc was 0.068 mA cm^?2 obtained at 8 mM PVP.     

Dense pure binderless WC bulk material prepared by spark plasma sintering

The phases, microstructures and mechanical properties of binderless WC bulk materials prepared by the spark plasma sintering technique were investigated systematically. The addition of carbon was added to eliminate the impurity phase W₂C. The relative density, Vickers hardness and grain size increase obviously with increasing sintering temperature, but increase weakly with increasing pressure or sintering time. The high relative density of 99·1%, HV30 of 27·5 GPa and fracture toughness K_IC of 4·5 MPa m^1/2 of pure binderless WC bulk with a grain size of 400 nm was obtained by sintering the WC powders with a particle size of 200 nm and the addition of 0·63 wt-%C at 1800°C for 6 min under 70 MPa.     

Composition and crystal structure of resorbable calcium phosphate thin films

Silicon stabilized tricalcium phosphate (Si-TCP) is formed, among other phases, as a result of sintering hydroxyapatite (HA) in the presence of silica (SiO₂) at >800°C. Calcium phosphate films sintered at 1000°C on quartz substrates are examined with and without additional SiO₂ added to the starting precipitate. Data from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) separate the undoped film morphology into a surface layer with a monoclinic crystal structure P2₁/a characteristic of α or Si-tricalcium phosphate and grain size in the range 100–1000 nm and a substrate layer with a crystal structure which is predominantly apatitic P6₃/m and grain size in the range 30–100 nm. The silicon content is greatest in the substrate layer. The addition of SiO₂ to the film material during fabrication induces a more uniform grain size of 10–110 nm and a higher Si content. The structural and phase evolution of these films suggests the nucleation of α-TCP by the local formation of Si-TCP at a SiO₂-hydroxyapatite interface. The results are consistent with X-ray diffraction studies and are explained by a model of nucleation and growth developed for bulk powders.     

Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (H _c) is roughly inversely proportional to the grain size (D _g) for grain sizes exceeding ∼0·1−1 μm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesD _g<∼100 nm,H _c decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.     

Effect of nanostructuration on compressibility of cubic BN

Compressibility of high-purity nanostructured cBN has been studied under quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and angle-dispersive synchrotron powder X-ray diffraction. It has been found that the data fit to the Vinet equation of state yields the values of the bulk modulus B ₀ of 375(4) GPa with its first pressure derivative B ₀?? of 2.3(3), thus, the nanometer grain size (??20 nm) results in a decrease of the bulk modulus by ??9%.     

Microstructure and ionic conductivity of (La1/2Li1/3+x )TiO3 perovskite-like solid solutions

The lattice parameters of (La_1/2Li_1/3+x )TiO₃ (x = 0, 1/10, 1/6, 1/5, 1/4) ceramics have been determined as functions of temperature, and their microstructure and transport properties have been studied. The results indicate that the oxides retain orthorhombic symmetry upon the structural changes at ?925 K. According to atomic force microscopy results, the grains in the ceramics consist of nanocrystallites 60 to 120 nm in size. Analysis of the grain bulk and grain boundary contributions to the ionic conductivity of the (La_1/2Li_1/3+x )TiO₃ solid solutions shows that their bulk conductivity σ_bulk decreases with increasing x because reducing the concentration of A-site vacancies impedes ionic transport.     

Strong quantum confinement effect in nanocrystalline CdS

CdS quantum dots have been prepared by chemical method. The X-ray diffraction results indicated the formation of CdS nanoparticles with hexagonal phase and grain size 2.5 nm. The HRTEM analysis reveals the formation of CdS quantum dots with an average grain size of ~2.5 nm. The X-ray photoelectron spectroscopy spectra exhibit the 3d _5/2 and 3d _3/2 peaks corresponding to cadmium and the S2p _3/2 peak corresponding to sulphur. Optical studies by UV–vis spectroscopy show a blue shifted absorption at 471 nm because of the quantum confined excitonic absorption. The photoluminescence spectra of CdS exhibited a broad green emission band centred at around 494 nm.     

Influence of mechanical milling and thermal annealing on electrical and magnetic properties of nanostructured Ni-Zn and cobalt ferrites

The present article reports some of the interesting and important electrical and magnetic properties of nanostructured spinel ferrites such as Ni_0.5Zn_0.5Fe₂O₄ and CoFe₂O₄. In the case of Ni_0.5Zn_0.5Fe₂O₄, d.c. electrical conductivity increases upon milling, and it is attributed to oxygen vacancies created by high energy mechanical milling. The real part of dielectric constant (?′) for the milled sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The increase in Néel temperature from 538 K in the bulk state to 611 K on the reduction of grain size upon milling has been explained based on the change in the cation distribution. The dielectric constant is smaller by an order of magnitude and the dielectric loss is three orders of magnitude smaller for the milled sample compared to that of the bulk. In the case of cobalt ferrite, the observed decrease in conductivity, when the grain size is increased from 8–92 nm upon thermal annealing is clearly due to the predominant effect of migration of some of the Fe³⁺ ions from octahedral to tetra-hedral sites, as is evident from in-field Mössbauer and EXAFS measurements. The dielectric loss (tan δ) is an order of magnitude smaller for the nano sized particles compared to that of the bulk counterpart.     

Large electrical resistivity and high saturation magnetization nanocrystalline Fe86B13Cu1 alloy prepared by hot isothermal pressing

The effects of hot isothermal pressing (HIP) on the microstructures and magnetic properties of nanocrystalline Fe₈₆B₁₃Cu₁ ribbons were studied. It is shown that the precipitation of Fe₃B phase is suppressed and the grain size of α-Fe phase decreases to 13.2 nm when amorphous Fe₈₆B₁₃Cu₁ ribbons are annealed by HIP under the pressure of 150 MPa. A high electrical resistivity and high saturation magnetization nanocrystalline soft magnetic material is prepared by HIP owing to the suppression of the precipitation of Fe₃B phase and a marked decrease in the grain size of α-Fe phase. The prepared sample exhibits a large electrical resistivity of 183 μΩ cm, a high saturation magnetization of 1.94 T and a low coercive force of 12 A/m.     

Effects of deposition temperature and pressure of the surface roughness and the grain size of polycrystalline Si1−xGex films

The effects of temperature and pressure on the surface roughness and the grain size of poly-Si_1–x Ge _x films, and the effect of the initial surface state on the final film surface roughness and grain size, have been investigated. The deposition temperature and pressure were varied from 450 to 600 °C and from 1 to 50 Torr, respectively. The transition temperature from amorphous to polycrystalline during the deposition was about 525 °C for the Si_0.46Ge_0.54 alloy film and the average grain size of the film deposited at 600 °C and 3 Torr was measured approximately as 180 nm. As the temperature increased, the grain size and the rms (root mean square) surface roughness increased at constant pressure, whereas both were decreased with increasing pressure at constant temperature. The initial surface state of Si_1–x Ge _x film influenced the final film structure such as the surface roughness and grain size. The smooth surface was obtained at higher pressure and lower temperature.     

Study of magnetic properties of thin cobalt films deposited by chemical vapour deposition

Chemical Vapour Deposition (CVD) of cobalt was deposited from a liquid source precursor of cobalt tricarbonyl nitrosyl (Co(CO)₃NO) on to oxidised < 100 > silicon wafers. The cobalt layers were deposited at 450∘C at 1.5 torr chamber pressure of hydrogen for 15 min processing time with various precursor flow rates. X-ray diffraction studies of the cobalt films reveal both hcp and fcc peaks. The vibrating sample magnetometer (VSM) yields coercivity (Hc) 167 Oe and 364 Oe for 46 nm and 30 nm thickness layers respectively at room temperature and squareness (S) M_r/M_s (remanence/saturation of magnetisation) value of ∼ 1. The study of magnetic properties of the cobalt suggests that magnetisation is dependent on grain size and therefore thickness. The grain size was observed by atomic force microscopy (AFM). Magnetic images were observed by magnetic force microscopy (MFM) and analyzed in terms of domain structure. The surface domain structure was recorded with the tip lift height 100 nm so that the magnetic interactions arising produced the topography effect. Where there is repulsive interaction the intensity is recorded as a bright region and where the interaction is attractive the intensity is recorded as a dark region.     

Elastic and elastoplastic response of thin copper foil

The tensile elastic, elastoplastic and low strain plastic parameters of the 12–35 m thick rolled (R) and electrodeposited (ED) copper foils have been characterized between 296 and 573 K and after an anneal exposure at up to 1173 K. At 296 K, all parameters are inversely proportional to grain size but the thermal effects dominate at the higher temperatures. For the R foils, the temperature coefficient of elastic modulus (E) is more or less identical to that for the bulk copper. For the ED foils, the coefficient increases with decreasing grain size; this effect is presumably related to the characteristic point defect structure generated during deposition. A significant grain growth ensues upon annealing but the post-anneal grain size has little effect on E; for the R foil, however, E decreases sharply with the anneal induced emergence of new crystallogrpahic textures with orientations near (100). The tangent modulus (E _t) in the elastoplastic regime decreases with strain and with temperature, a very large drop in E _t with anneal temperature suggests that both the diminution of dislocation activity and the texture modification are the contributory factors. It is argued that the strain hardening parameter (n) in the elastoplastic regime, and the strain rate hardening parameter (m) and the flow stress in the low strain plastic regime influence the handling damage to the thin foil.     

Oxygen inhomogeneities in YBa2Cu3O7−x single crystals withx>0.5 (oxygen inhomogeneities in YBa2Cu3O7−x …)

The microstructure of HTS YBa₂Cu₃O_7−x single crystals withx⩾0.5 has been investigated by TEM and selected area diffraction. An inhomogeneous oxygen distribution over the crystal was observed. Large differences between the bulk crystal structure and the surface have been established. The bulk structure was presented by orthorhombic blocks (≈100×10×20 nm in size) embedded in tetragonal matrix withx≈1. The bulk structure was not of a superconductive type. The crystal surface was enriched by oxygen and had the usual orthorhombic twinned structure. The superconductive properties of the whole specimen were determined by surface layer about 5 μm thick.     

Preparation of crystal-controlled Y-TZP/Al2O3 nanocomposites

ZrO₂-Y₂O₃-Al₂O₃ nanocrystalline powders with different grain sizes have been synthesized using a chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a vacuum hot-pressing furnace. Density, pore size distribution, grain size and composition of the composites were determined by various techniques, including BET gas absorption, field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). It has been shown that the porosity, grain and pore size of the ceramics can be controlled by the initial powder size and sintering temperature. Fully densified ceramics with narrow grain size distribution in the range of 100 ?? 500 nm could be obtained.     

Production of aluminum-matrix carbon nanotube composite using high pressure torsion

In this study, an Al-based composite containing carbon nanotubes (CNTs) was fabricated using a process of severe plastic deformation through high pressure torsion (HPT). Neither heating nor sintering was required with the HPT process so that an in situ consolidation was successfully achieved at ambient temperature with 98% of the theoretical density. A significant increase in hardness was recorded through straining by the HPT process. When the composite was pulled in tension, the tensile strength of more than 200 MPa was attained with reasonable ductility. Transmission electron microscopy showed that the grain size was reduced to 100 nm and this was much smaller than the grain size without CNTs and the grain size reported on a bulk sample. High resolution electron microscopy revealed that CNTs were present at grain boundaries. It was considered that the significant reduction in grain size is attributed to the presence of CNTs hindering the dislocation absorption and annihilation at grain boundaries.     

Improvement of Low‐Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg₃Sb₂‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T^?3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg₃Sb₂‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm² V^?1 s^?1 in annealed 800 °C sintered Mg_{3 + δ}Sb_1.49Bi_0.5Te_0.01, the highest ever reported for Mg₃Sb₂‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi₂Te₃) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.     

Growth of nanocrystalline diamond films in CCl4/H2 ambient

We investigated the growth characteristics of the nanocrystalline diamond films using CCl₄/H₂ as gas sources in a hot-filament chemical vapor deposition (CVD) reactor. Successful growth of nanocrystalline diamond at typical growth condition of 1.5-2.5% CCl₄ and 550-730 °C substrate temperature has been demonstrated. Glancing angle X-ray diffraction (XRD) clearly indicated the formation of diamond in the films. Typical root-mean-square surface roughness of 10-15 nm and an optimal root-mean-square surface roughness of 6 nm have been achieved. Transmission electron microscopy (TEM) analyses indicated that nanocrystalline diamond film with an average grain size in the range of 10-20 nm was deposited from 2.5% CCl₄/H₂ at 610 °C. Effects of different source gas composition and substrate temperature on the grain nucleation and grain growth processes, whereby the grain size of the nanocrystalline film could be controlled, were discussed.     

Study of infrared spectroscopy and elastic properties of fine and coarse grained nickel–cadmium ferrites

The elastic properties of Cd _x Ni_1−x Fe₂O₄ (x = 0.2, 0.4, 0.6 and 0.8) spinel ferrite system synthesized by wet-chemical technique, have been studied by infra-red spectroscopy and X-ray diffraction pattern analysis before (W) and after high temperature annealing (AW). The average particle size for wet-samples was within the range 4–5 nm, which is much lower than the average particle size found for AW samples (≈85 nm). The force constants for tetrahedral and octahedral sites determined by infrared spectral analysis, lattice constant and X-ray density values by X-ray diffraction pattern analysis; have been used to calculate elastic constants. The elastic moduli for W-samples are found to be larger as compared to AW samples, which are explained on the basis of grain size reduction effect. The average crystallite size calculated from elastic data is in agreement to that determined from X-ray diffraction data analysis.     

Bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared by the phase transition of amorphous Nd9Fe81Co3Nb1B6 under high pressure

In the present study, the authors succeeded in preparing the bulk α-Fe/Nd₂Fe₁₄B nanocomposite magnets with a nearly full density by the phase transition of amorphous Nd₉Fe₈₁Co₃Nb₁B₆ powders under 1 GPa at a temperature of 750 °C. Compared to the magnets prepared from partly amorphous or nanocrystalline powders, the magnets show quite homogeneously distributed nanocrystals with a small grain size, 28 nm for the α-Fe phase and 35 nm for the Nd₂Fe₁₄B phase, which results in enhanced magnetic properties.