Showing papers in "Journal of Materials Science in 2002"

Review Progress in Ostwald ripening theories and their applications to nickel-base superalloys Part I: Ostwald ripening theories

Abstract: The physical basis behind the Ostwald ripening process for two-phase mixture has been reviewed in detail, using the various theories developed to describe this process. The Ostwald ripening, also termed second phase coarsening, is generally thought to be slow, diffusion-controlled process which occurs subsequent to phase separation under extremely small under-saturation levels. The major advance for the description of this process was made when Lifshitz, Slyozov and Wagner (also known as the LSW theory) published their papers more than fourty years ago. This classical LSW theory predicts that the ripening kinetics and the particle size distribution function are applicable to dilute systems only [i.e. when the volume fraction (Q) of second phase approaces zero: Q → 0], in which particle-particle interactions are not important. After the publication of the LSW theory, many experimentalists tested the veracity of the theory. Experimentalists have confirmed the prediction of self-similar ripening behavior at long times. However, virtually none of the reported distributions are of the form predicted by the LSW theory. The reported distributions are generally broader and more symmetric than the LSW predictions. It was later realized that a major problem with the LSW approach was the mean field nature of the kinetic equation. In order to remove the zero volume fraction assumption of the LSW theory, the many theories have been developed based on the statistically averaged diffusion interaction of a particle of given size class with its surroundings, using both analytic and numerical methods. Many attempts to determine the statistically averaged growth rate of a particle either do not account for the long-range nature of the diffusional field surrounding the particle, and/or employed ad hoc assumptions in an attempt to account for the diffusional interactions between particles. The strength of the diffusional interactions between particles stems from the long range Coulombic nature of the diffusion field surrounding a particle. As a result, particle interactions occur at distances of many particle diameters and restrict the validity of LSW theory to the unrealistic limit of zero volume fraction of coarsening phase. More realistic models of the ripening process at finite-volume fractions (Q) of coarsening phase have been proposed by various workers such as Brailsford-Wynblatt (1979), Voorhees-Glicksman (1983), Marqusee-Rose (1984), Tokuyama-Kawasaki (1984), Enomoto-Tokuyama-Kawasaki (ETK) (1986), and Yao-Elder-Guo-Grant (YEGG) (1993) models. Although a great deal of progress has been made in understanding Ostwald ripening, a fully satisfactory approach has not yet been found, and it has remained a vexing problem in the field. At present, it is very difficult to determine which of these theories best describes coarsening at finite volume fraction. The statistical mechanical theories, developed to describe systems in which Q ≪ 1, employed the same microscopic equation to describe the coarsening rates of individual particles, but different techniques to perform the statistical averaging. In addition, these theories can be distinguished on yet a finer scale. All of the theories predict that the rate constant will vary as Q1/2 in this low volume fraction limit. These theories predict that the scaled time-independent particle radius distributions become broader and more symmetric than those predicted by LSW as the volume fraction increases. Clearly more experimental and theoretical work is necessary in order to settle the subtle disagreement now existing between the various Ostwald ripening theories.

Fabrication of soft tissue engineering scaffolds by means of rapid prototyping techniques

Abstract: Scaffolds are of great importance for tissue engineering because they enable the production of functional living implants out of cells obtained from cell culture. These scaffolds require individual external shape and well defined internal structure with interconnected porosity. The problem of the fabrication of prototypes from computer assisted design (CAD) data is well known in automotive industry. Rapid prototyping (RP) techniques are able to produce such parts. Some RP techniques exist for hard tissue implants. Soft tissue scaffolds need a hydrogel material. No biofunctional and cell compatible processing for hydrogels exists in the area of RP. Therefore, a new rapid prototyping (RP) technology was developed at the Freiburg Materials Research Center to meet the demands for desktop fabrication of hydrogels. A key feature of this RP technology is the three-dimensional dispensing of liquids and pastes in liquid media. The porosity of the scaffold is calculated and an example of the data conversion from a volume model to the plotting path control is demonstrated. The versatile applications of the new hydrogel scaffolds are discussed, including especially its potential for tissue engineering.

Tensile and compressive properties of flax fibres for natural fibre reinforced composites

Abstract: Mechanical properties of standard decorticated and hand isolated flax bast fibres were determined in tension as well as in compression. The tensile strength of technical fibre bundles was found to depend strongly on the clamping length. The tensile strength of elementary flax fibres was found to range between 1500 MPa and 1800 MPa, depending on the isolation procedure. The compressive strength of elementary flax fibres as measured with a loop test lies around 1200 MPa. However, the compressive strength can be lowered severely by the decortication process. The standard decortication process induces kink bands in the fibres. These kink bands are found to contain cracks bridged by microfibrils. The failure behaviour of elementary flax fibres under compression can be described as similar to the failure behaviour of a stranded wire.

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Abstract: Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.

Cell wall properties and their effects on the mechanical properties of fibers

Abstract: The properties of the cell wall are determined by its structure and by the properties of the wood polymers. In this study, the influence of the elastic constants of the three wood polymers on the elastic modulus of the cell wall was investigated. Cellulose was found to dominate the properties in the longitudinal direction. In the transverse direction, the effect of the properties of hemicellulose was more pronounced. The results show that it is possible to reduce the discrepancy between experimental and modeled values of the transverse modulus to a large extent by lowering the assumed values of the elastic constants of hemicellulose and lignin. The thickness and fibril angles of the S1- and S3-layers were also found to be important parameters for the transverse properties of the fiber wall. These two layers should not be neglected when transverse elastic properties are related to cell wall structure.

Optical properties of CH3NH3PbX3 (X = halogen) and their mixed-halide crystals

Abstract: Thin films of microcrystalline CH3NH3PbX3 (X = halogen) as well as their mixed-halide crystals were fabricated by the spin-coating technique, and their optical properties were investigated. X-ray diffraction investigation revealed that CH3NH3PbBr3 − x Cl x (x = 0–3) were successfully formed on glass substrate self-assembly and oriented with the a-axis. Owing to due to their large exciton binding energy, these materials showed clear exciton absorption and free-exciton emission in the visible region at room temperature. Replacing Br with CI made it possible to control the band structure of these materials. As a result, the peak position of the exciton band shifted continuously towards blue region with increasing the CI content in the films.

Thermal decomposition of bauxite minerals: infrared emission spectroscopy of gibbsite, boehmite and diaspore

Abstract: Infrared emission spectroscopy has been used to study the dehydroxylation behavior over the temperature range from 200 to 750°C of three major Al-minerals in bauxite: gibbsite (synthetic and natural), boehmite (synthetic and natural) and diaspore. A good agreement is found with the thermal analysis and differential thermal analysis curves of these minerals. Loss in intensity of especially the hydroxyl-stretching modes of gibbsite, boehmite and diaspore as function of temperature correspond well with the observed changes in the TGA/DTA patterns. The DTA pattern of gibbsite clearly indicates the formation of boehmite as an intermediate shown by a endotherm around 500°C. Dehydroxylation of gibbsite is followed by a loss of intensity of the 3620 and 3351 cm−1 OH-stretching bands and the corresponding deformation band around 1024 cm−1. Dehydroxylation starts around 220°C and is complete around 350°C. Similar observations were made for boehmite and diaspore. For boehmite dehydroxylation was observed to commence around 250°C and could be followed by especially the loss in intensity of the bands around 3319 and 3129 cm−1. The DTA pattern of diaspore is more complex with overlapping endotherms around 622 and 650°C. The dehydroxylation can be followed by the decrease in intensity of the OH-stretching bands around 3667, 3215 and 2972 cm−1. Above 550°C only a single band is observed that disappears after heating above 600°C corresponding to the two endotherms around 622 and 650°C in the DTA.

Fracture resistance characterization of chemically modified crumb rubber asphalt pavement

Abstract: The fracture resistance of chemically modified crumb rubber asphalt (CMCRA) pavement was evaluated based on the J-integral concept. The chemical modification process used was developed by the Federal Highway Administration and patented in 1998. The results were compared to that of crumb rubber asphalt (CRA) and control asphalt pavement. Four semi-circular core specimens (76 mm radius and 57 mm thickness) were cut from each gyratory compacted cylinder (GCC) for the fracture resistance tests. Notches with different depth to radius ratios were introduced at the middle of the flat surface of each specimen. Three point bend loading was used to allow the separation of the two surfaces due to tensile stresses at the crack tip. It was found that the CMCRA pavement, had the highest residual strength, at all notch depths tested. The fracture resistance of the CMCRA pavement, based on Jc was found to be about twice that of the CRA and control pavements. The CRA pavement was found to have a slightly higher fracture resistance than that of the control pavement. Scanning Electron Microscopic examination of the fracture surface of each mixture revealed the microstructural origin of the improved fracture resistance of the CMCRA pavement in comparison with the control pavement.

Determination and interpretation of isothermal and non-isothermal transformation kinetics; the effective activation energies in terms of nucleation and growth

Abstract: A general model for the kinetics of solid state phase transformations has been discussed. The model is valid for both isothermal as well as non-isothermal transformations. In certain specific cases, the model can be simplified such that it reduces to so-called Johnson-Mehl-Avrami (JMA) kinetics. The model kinetic parameters are independent of the time-temperature program. In addition, it has been shown that in certain cases where the presented model does not pertain to JMA exactly, the JMA description still holds within practical accuracy. This holds for example, for nucleation of mixed nature. In this case, it is possible to obtain the activation energies for growth and for nucleation, separately, from measurements, if it is possible to vary the nucleation mode, for example by pre-annealing. This determination of the separate activation energies has been tested on a virtual and a real phase transformation: crystallisation of glassy Pd40Cu30P20Ni10.

Review Progress in Ostwald ripening theories and their applications to the γ′-precipitates in nickel-base superalloys Part II Nickel-base superalloys

Abstract: The Lifshitz-Slyozof-Wagner (LSW) theory, which corresponds to a zero volume fraction approximation, was developed to model kinetics of precipitate growth from supersaturated solid solutions. The subsequent modifications of the LSW theory for the incorporations of various factors including volume fractions of precipitates to fit the experimental data from the coarsening precipitates were made by various workers during last twenty five years. The LSW theory and its modifications have been applied on the diffusion-controlled Ostwald ripening of the γ′ precipitate particles [Ni3(Al,Ti)] in nickel-based superalloys. The important Ostwald ripening theories were reviewed in the part I of this paper, and the coarsening characteristics of the γ′-precipitates in Ni-base high temperature superalloys are presented in detail in relation to these theories in the present part. A model developed by D. McLean can be used to predict the γ′ particle growth over service lifetimes in the case of relatively Al-rich nickel-base superalloys. Additional fundamental data (such as the precipitate-matrix interfacial energy, diffusivity of the component species of the particle, and the equilibrium solubility with a particle in nickel-based superalloys) can be obtained from experimental results for coarsening, if the concentration changes during coarsening can be measured precisely, using the methods developed by A. J. Ardell. Furthermore, the factors affecting the shape changes and splitting of the γ′ precipitate particles during the coarsening were also considered seperately since the classical Ostwald ripening theories can not explain the morphological changes.

Stereolithography of structural complex ceramic parts

Abstract: A cost-effective method of complex ceramic parts manufacturing using stereolithography has been developed. The process consists in fabricating ceramic pieces by laser polymerization of an UV curable monomeric system, subsequent removal of organic components and sintering. Highly concentrated suspensions of well dispersed ceramic particles (up to 60 vol%) in a reactive acrylic monomer, with a suitable rheological behaviour for the spreading of thin layers (down to 25 μm) were defined. Adequate cured depth (higher than 200 μm) is obtained even at high scanning speeds. Nevertheless, a compromise has to be found between the cured depth and the cured width to improve the dimensional resolution. The dimensional resolution reached on alumina partterns is about 200 μm with an energy density of 0.05 J · cm−2.

A Review on the various synthesis routes of TiC reinforced ferrous based composites

Abstract: The major thrust underlying the processing of Fe-based composites has been directed towards improving the wear resistance of steel or castiron by incorporating some reinforcing phase, e.g., carbides, oxides, etc. The present article provides a review on the various synthesis routes of TiC reinforced Fe-based composites, i.e., powder metallurgy, conventional melting and casting, carbothermic reduction, combustion synthesis, aluminothermic reduction, electron beam radiation, laser surface melting, and plasma spray synthesis, highlighting the advantages and disadvantages associated with the different routes of synthesis.

Processing and characterization of carbon nanotube/poly(styrene-co-butyl acrylate) nanocomposites

Abstract: Nanocomposite materials were prepared from an amorphouspoly(styrene-co-butyl acrylate) latex as the matrix using an aqueous suspension of carbon nanotubes as the filler. After stirring, the preparations were cast and evaporated. The morphology of the resulting films was examined by scanning electron microscopy and a good dispersion of the filler was observed, except for the 5 wt% filled sample. The electrical conductivity and mechanical behavior in both the linear and non-linear ranges were analyzed. From conductivity measurements, a clear percolation threshold has been observed for a relatively low critical volume fraction around 1.5%. The mechanical characterization displayed a continuous reinforcing effect of the carbon nanotubes without lowering of the elongation at break up to 3 wt%. The thermal stability of the composites was strongly improved by carbon nanotubes loading. For instance, the terminal zone was shifted by 115 K with only 15 wt% of nanotubes.

Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics

Abstract: Because of its outstanding mechanical properties, zirconia-based ceramics are considered as some of the best potential materials within the engineering ceramics field that might be widely used to substitute various metallic parts and specific alloys. Taking into account the transformation toughening mechanisms that operates in their microstructure, important properties can be obtained. Phase transitions as well as transformation toughening in ZrO2 are reviewed briefly with the purpose to understand its effects in some composites and glass systems. Zirconia ceramics as high toughness materials for cutting tool, metal forming applications, mechanically superior ceramics called partially stabilised zirconia (PSZ), solid electrolytes, have been fabricated using the martensitic nature of the tetragonal to monoclinic phase transition.

Characterization of interfacial and mechanical properties of green composites with soy protein isolate and ramie fiber

Abstract: Environment-friendly fiber-reinforced composites were fabricated using ramie fibers and soy protein isolate (SPI) and were characterized for their interfacial and mechanical properties. Ramie fibers were characterized for their tensile properties and the parameters for the Weibull distribution were estimated. Effect of glycerol content on the tensile properties of SPI was studied. Interfacial shear strength (IFSS) was determined using the microbond technique. Based on the IFSS results and fiber strength distribution, three different fiber lengths and fiber weight contents (FWC) were chosen to fabricate short fiber-reinforced composites. The results indicate that the fracture stress increases with increase in fiber length and fiber weight content. Glycerol was found to increase the fracture strain and reduce the resin fracture stress and modulus as a result of plasticization. For 10% (w/w) of 5 mm long fibers, no significant reinforcement effect was observed. In fact the short fibers acted as flaws and led to reduction in the tensile properties. On further increasing the fiber length and FWC, a significant increase in the Young's modulus and fracture stress and decrease in fracture strain was observed as the fibers started to control the tensile properties of the composites. The experimental data were compared to the theoretical predictions made using Zweben's model. The experimental results are lower than the predicted values for a variety of reasons. However, the two values get closer with increasing fiber length and FWC.

Development and measurement of stress in polymer coatings

Abstract: This paper reviews stress development mechanisms and stress measurement techniques for polymer coatings. Most polymeric coatings shrink during and after solidification due to chemical reaction, solvent evaporation, phase separation, or some combination thereof. Coating adhesion, however, prevents shrinkage from occurring freely; this frustration of in-plane shrinkage leads to a tensile stress in the plane of the coating. At the same time, stress accumulates, it may be relaxed by processes such as molecular motion. The measured stress at any time is the result of the competition between stress buildup from frustrated shrinkage and stress relief from relaxation. Accumulation of stress is a problem because it can lead to defects such as cracks. An understanding of stress development in various types of polymeric coating systems will lead to strategies for material selection, process optimization, and defect elimination. In this paper, background on stress development is provided, followed by an overview of stress measurement methods for polymer coatings. The remainder of the paper focuses on stress development during the drying and curing of polymer coatings, drawing many examples from previous stress measurement studies.

Synthesis of silica aerogels from waterglass via new modified ambient drying

Abstract: A new modified ambient drying process for synthesizing silica aerogels cost-effectively from waterglass has been developed. Crack-free silica aerogels were obtained via solvent exchange/surface modification of wet gels using IPA/TMCS/n-Hexane solution. Silica aerogels were heated at different temperatures. The effects of heating temperature on chemical bonding state of aerogels were investigated by means of DTA and FTIR. The surface characteristic of the aerogel was hydrophobic when heat-treated under 350°C. The porosities, densities, and specific surface areas of the silica aerogels were in the range of 93–94%, 0.12–0.15 g/cm3, and ∼630 m2/g, respectively. Distinct spring back phenomena were observed in surface modified wet gels during drying.

Review Processes and influencing parameters of the solid particle erosion of polymers and their composites

Abstract: The solid particle erosion behaviour of polymers and polymeric composites has been reviewed. Attention was paid to the effects of testing variables (e.g., erodent type, size and flux, impact angle) and target material characteristics (e.g., crystallinity, crosslink density, reinforcement content and arrangement). The occurring failure mechanisms were classified and discussed. Various predictions and models proposed to describe the erosion rate (ER) were listed and their suitability was checked. Recommendations were given how to solve some open questions related to the structure—erosion resistance relationships for polymers and polymeric composites.

In-situ deformation of an open-cell flexible polyurethane foam characterised by 3D computed microtomography

Abstract: The deformation behaviour of an open-cell flexible polyurethane foam was observed using X-ray microtomography on the ID19 beamline at the ESRF in Grenoble, France. Tomographs, consisting of 1024 voxels cubed, were collected with a voxel size of 6.6 μm from a small region near the centre of the foam at a range of compressive strains between 0 and 80%. The results show that the initial stages of compression are taken up by small amounts of elastic bending in struts that are inclined to the compression direction. At 23% strain, entirely collapsed bands were observed in the structure. By 63% strain, there was evidence of struts impinging on each other, corresponding to the densification regime. The compression of an irregular foam (i.e. one with strut length and cell size distributions) appears to involve a sudden change in modulus, accompanied by localised increases in density. Observations of this nature would have been extremely difficult to interpret unambiguously without the ability to carry out sequential microtomographic imaging under realistic in situ loading conditions. The process of finite element analysis (FEA) was begun by constructing node-strut models from the experimental data by a mathematical skeletonisation process. These were used to derive node coordination, strut-length and cell-size distributions. However, direct comparison of the elastic properties with FEA was hampered by the absence of periodicity in the experimentally determined foam structures.

Oxidation bonding of porous silicon carbide ceramics

Abstract: A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al2O3 and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al2O3 content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al2O3, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.

Effects of martensite formation and austenite reversion on grain refining of AISI 304 stainless steel

Abstract: The austenite–martensite transformation followed by annealing for austenite reversion in AISI 304 stainless steel has been investigated in order to study the effect of this thermo-mechanical process on grain refinement. In particular the effect of cold reduction, annealing temperature and annealing times have been analysed. After getting ultrafine grains the effect of the grain size on the hardness and on the tensile properties has been evaluated, showing a Petch-Hall dependency in the fully analysed range (down to 0.8 μm grain size).

Review: Improving cement-based materials by using silica fume

Abstract: The effects of silica fume as an admixture in cement-based materials are reviewed in terms of the mechanical properties, vibration damping capacity, freeze-thaw durability, abrasion resistance, shrinkage, air void content, density, permeability, steel rebar corrosion resistance, alkali-silica reactivity reduction, chemical attack resistance, bond strength to steel rebar, creep rate, coefficient of thermal expansion, specific heat, thermal conductivity, fiber dispersion, defect dynamics, dielectric constant and workability. The effects of silane treatment of the silica fume and of the use of silane as an additional admixture are also addressed.

Talc as nucleating agent of polypropylene: morphology induced by lamellar particles addition and interface mineral-matrix modelization

Abstract: The efficiency of talc used as nucleating agent (0.5% by weight) in polypropylene (PP) was determined taking into account the particle size d50, particle morphology and BET specific surface areas. These findings were compared to a mineral with similar properties, pyrophyllite. Talc samples with the finest particle sizes induce a significant increase in the starting crystallization temperature of PP and irrespective of the particle size d50, pyrophyllite was found to be less efficient than talc. X-Ray results show that PP oriented crystallization due to talc or pyrophyllite addition, corresponds to an epitaxial growth whereby the mineral c*-axis is merged with the PP b*-axis. Microscopic observations revealed that in the presence of talc, nuclei density of PP increased strongly. In addition, a large number of nuclei was observed to appear everywhere on the talc surface. A PP-talc interface model is proposed by matching the (001) talc plane and the (010) PP plane. In this model, 3% of PP cell accommodation on talc is necessary with a 15° angle between PP chains elongation and the crystallographic directions of talc. Hexagonal rings on talc surface are believed to represent hydrogen bonds with PP methyl groupings. This fine structure relation between talc and PP is discussed, and is used to characterize the differences observed between the efficiency of talc and that of pyrophyllite.

Synthesis, characterization and photocatalytic property of nano-sized Zn2SnO4

Abstract: Nano-sized Zn2SnO4 materials have been synthesized using the coprecipitation method. The synthetic conditions and the calcination behaviors of nano-sized Zn2SnO4 materials have been studied. The nano-sized Zn2SnO4 materials have been characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry and differential thermal analysis (TG-DTA) and specific surface area. As a result, the kinetic grain growth equation for nano-sized Zn2SnO4 can be expressed as D4.78 = 9.12 × 1023t exp(−40.6 × 103/T), with an activation energy for grain growth of Q = 337.9 KJ/mol. The nano-sized Zn2SnO4 materials have been used as photocatalysts to decompose benzene in water solution. The results show that Zn2SnO4 can photocatalytically decompose benzene, and the photocatalytic capacity for Zn2SnO4 relates to the grain size, which is discussed in terms of the surface effect and the quantum size effect.

The effect of new ferrite content on the tensile fracture behaviour of dual phase steels

Abstract: The effect of new ferrite present with different volume fractions and morphologies of martensite on microvoids formation and tensile fracture behaviour in dual phase steels has been studied for a steel containing 0065% C, 158% Mn and 05% Ni Fine and coarse dual phase microstructures were obtained from two different starting conditions Martensite contents were kept constant at ∼18 and ∼25% and new ferrite content was varied by controlled cooling from intercritical annealing temperature of 740, 750 and 785°C In both fine and coarse dual phase structures microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region Martensite morphology had an influence in determining martensite cracking Coarse and interconnected martensite distributed along ferrite grain boundaries cracked easily Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones Microvoid coalescence was the dominant form of fracture in both structures The specimens with higher new ferrite contents had higher densities of voids In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite

Formation and properties of ZnO nano-particles from gas phase synthesis processes

Abstract: ZnO nano-particles have been synthesized in low pressure flow reactors utilizing Zn(CH3)2 as precursor. Two different synthesis routes have been employed. A low pressure flame reactor and a microwave reactor were used for synthesis of ZnO particle in Zn(CH3)2 doped H2/O2/Ar flames and Zn(CH3)2 doped Ar/O2 plasmas, respectively. The particle formation process has been investigated in situ by a particle mass spectrometer. Also, sampled powders have been investigated ex situ by means of FT-IR, XRD, TEM, and UV-VIS. For both synthesis routes nanometer sized ZnO particles were found with particle diameters in the range between 4 to 8 nm. In cases of the flame reactor the results suggest a strong influence of water on the particle formation process.

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

Abstract: The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y_2O_3-ZrO_2 (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y_2O_3 stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y_2O_3 in the t-zirconia began to decrease as predicted by the Y_2O_3-ZrO_2 phase diagram. The c-ZrO_2 phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO_2, which has a higher thermal conductivity than tetragonal or cubic zirconia.

Review Structure and properties of glass-like carbon

Abstract: The comparison and discussion of the results of the long-term studies in the field of the glass-like carbon electronic and emission properties are presented. The data available from wide range of references and original experiments have given an opportunity to propose a new model of the glass-like carbon microcrystalline structure and its modification due to heat treatment.

Synthesis and microstructure of calcia doped ceria as UV filters

Abstract: Fine particles of calcium oxide doped cerium dioxide, 2–4 nm in diameter, were prepared by the chemical reaction of CeCl3-CaCl2 mixed aqueous solution and NaOH aqueous solution at pH 6–12 and 40°C followed by the oxidation with hydrogen peroxide Doping of CaO with CeO2 resulted in decreasing the particle size and consequently, increasing UV shielding efficiency and transparency to visible light The particle shape of ceria changed significantly depending on the reaction condition, ie, rod-like particles and spherical particles were formed by the H2O2 oxidation of cerium trihydroxide pH above 8 and below 7, respectively

Thermal and morphological characterization of poly(ethylene terephthalate)/calcium carbonate nanocomposites

Abstract: Nanocomposites composed of poly(ethylene terephthalate) (PET) filled with calcium carbonate particles of nanometer scale were prepared by polymerizing the polyester in the presence of the nanosized fillers. Besides plain calcium carbonate, carbonate nanoparticles coated with stearic acid were also used, in order to improve the compatibility between the polymeric matrix and nanofillers. Morphological analysis evidenced a good dispersion of both the nanopowders into the PET matrix, especially in the case of coated calcium carbonate. The strong interfacial adhesion between the two phases is also responsible for the increase of the glass transition and melting temperatures in the nanocomposites compared to plain PET. Finally, non-isothermal crystallization studies revealed that the coated CaCO3 is a good nucleating agent for PET. Analysis of non-isothermal crystallization data with the Ozawa theory was successful for plain PET and PET/un-CaCO3, but this method failed to describe the dynamic solidification of the PET/c-CaCO3 nanocomposite.