A metastable phase in thermal decomposition of Ca-deficient hydroxyapatite

We investigated the microstructural changes on an atomic length scale during thermal decomposition process of Ca-deficient hydroxyapatite (Ca-def HAp) by high-resolution transmission electron microscopy (HRTEM). Ca-def HAp was prepared by hydrolysis of -tricalcium phosphate. The Ca-def HAp had a whisker-like morphology 2–5 m in length and 0.1 m in diameter that was elongated along c-axis. Thicker planer defects parallel to the (100) plane of the HAp matrix were observed as precipitation in the sample annealed at 700 and 800 °C by HRTEM observation. Thickness of the precipitation was about 10 nm and the boundaries between the precipitation and HAp matrix was coincident. The periodicity in the precipitation was parallel to the (100) plane of the HAp matrix and measured to be 1.42 nm. Since the precipitation was observed only in the sample annealed at a narrow temperature range of 700–800 °C, it was regarded as a metastable phase formed on the thermal decomposition process. Absorption peaks in IR spectra of annealed Ca-def HAp containing the metastable phase appeared at 744 and 3538 cm^–1 due to non-stoichiometric HAp with high Ca/P molar ratio. Furthermore, the results of energy dispersive X-ray spectroscopy showed that the metastable phase had higher Ca/P molar ratio than that of the matrix and stoichiometric HAp. Therefore, the metastable phase could be identified as Ca-rich metastable phase. The presence of Ca-rich metastable phase was confirmed to be associated with the thermal decomposition process.     

Transmission electron microscopic observation of dislocations in x-phthalocyanine crystals

Transmission electron microscopy has been utilized to directly reveal the defects that are present in thin single crystals of x-phthalocyanine polymorph. The bright field images are characteristic of dislocation arrays while the associated diffraction patterns indicate that the parent monoclinic structure of x-phthalocyanine may undergo a stress-induced phase transformation into a daughter orthorhombic structure. The transformation is akin to a martensitic process as a result of the operation of an invariant plane strain. The dislocation arrays observed have been interpreted in terms of slip dislocations.     

Kinetics of thermal decomposition of hydroxyapatite bioceramics

The mechanism and kinetics of thermal decomposition of injection moulded hydroxyapatite ceramics were studied over the temperature range of 1473–1758 K. At temperatures below 1473 K the sintering and transformation of hydroxyapatite to hydroxyoxyapatite proceeded to a conversion degree of 0.4 to 0.5. At temperatures between 1473 and 1758 K the hydroxyapatite was decomposed to -TCP, H₂O and CaO. The decomposition of HOA started on the surface of the HOA ceramics. The rate of increase in the thickness of the reaction products (-TCP) was described by the parabolic law. The kinetic analysis of the time dependence of HOA conversion to TCP by means of the J-M-A-J-K equation also showed that the thermal decomposition of HOA ceramics was controlled by diffusion of water from the reaction zone to the surface of the ceramic sample. The activation energy of the thermal decomposition of HOA ceramics amounted to 283.5 kJ/mol.     

Transmission electron microscopic observations of a commercial Nb-Ti superconducting alloy

Microstructures in a commercial Nb-Ti alloy heat-treated or repeatedly aged and cold worked have been investigated by transmission electron microscopy. In the aged samples after solution treatment,-phase forms at ageing temperatures below 673 K and-phase precipitates at 773 K. Cold work after solution treatment or heavy cold work without prior solution treatment results in accelerated precipitation of-phase on ageing at 623 K and of-phase on ageing at 773 K, while it gives-precipitation instead of on ageing at 673 K. Repeated ageing and cold work make the microstructures very fine. The refinement is due to formation of dislocation cell structures and subgrains and precipitation of- and-phases.     

Transmission electron microscopic study of multiwalled carbon nanotubes

Multiwalled carbon nanotubes have been studied by transmission electron microscopy, selectedarea electron diffraction, and transmission electron energy loss spectroscopy (EELS). The results demonstrate that the walls of carbon nanotubes consist of graphite-like layers with an increased interlayer spacing compared to graphite. The local density distribution in the CNTs has been evaluated from EELS scans over the nanotubes.     

Transmission electron microscopic investigation of the microstructure of Fe-Cr-Al alloys

The microstructure of different Fe-Cr-Al alloys in the composition range of 0 to 18 at % Cr and 7 to 25 at % Al was investigated by transmission electron microscopy (TEM). By systematically varying the compositions, the location of the order-disorder transition was determined. Dark field images confirmed that many samples consisted of a two-phase mixture of small DO₃-ordered particles in a disordered A2 matrix. The mottled contrast exhibited by some samples and sometimes taken as a proof of the existence of ordered particles is shown to be produced by an artefact due to the TEM sample preparation. This revised version was published online in September 2006 with corrections to the Cover Date.     

Transmission electron microscopic investigations on SiC- and BN-coated carbon fibres

Carbon fibres were coated with layers of silicon carbide (SiC) and boron nitride (BN) by conventional chemical vapour deposition. The SiC films were deposited by thermal decomposition of methyltrichlorosilane, whereas the BN films were deposited using the stepwise disproportion reaction of boron chloride with ammonia. Samples for electron microscopic investigations were prepared by separating film from fibre or by conventional mechanical thinning and subsequent ion milling of cross sections of coated fibres. Bright- and dark-field images of both planar and cross-sectional electron microscopic investigations on the fibre coatings gave detailed information on film thickness and morphology. High-resolution images improved the structural information of electron diffraction patterns. Crystal dimensions in the SiC film vary between 10 and 40 nm. Electron diffraction revealed the crystal structure to be a mixture of disordered hexagonal 2H-SiC and cubic -SiC. High-resolution images showed the (1 1 1)-planes to be preferred for deposition. In BN films, a hexagonal turbostratic structure similar to turbostratic carbon was observed. Apart from amorphous regions, nanocrystalline parts were detected, which have a higher structural perfection in the stacking sequence of their (0 0 2)-planes compared to the (0 0 2)-planes of the turbostratic carbon fibre. High-resolution images located the film-fibre interface that was confirmed by electron energy loss spectroscopy.     

The phase diagram and spinodal decomposition of metastable states of Lennard-Jones system

The method of molecular dynamics is used to treat the metastable states of Lennard-Jones crystal and liquid at different temperatures and pressures, including the negative-pressure region. Analysis is made of the pattern of relative position of surfaces of the equation of state of the crystal and liquid phases in the metastable region. The limits of stability of metastable states of liquid and crystal on the phase diagram are determined. The mechanism of crystal decomposition in the vicinity of the stability limit is treated. The stochastic properties of a many-particle system are investigated in the region in the vicinity of the spinodal, and the dynamic memory time and K-entropy are calculated.     

Transmission electron microscopic characterization of zirconia reinforced with silicon carbide fibres

The microstructures of zirconia reinforced with silicon carbide fibres prepared by the sol–gel process have been examined using a transmission electron microscope. The characteristic feature of highly oriented grains of monoclinic zirconia, with its unique b-axis as well as c-axis of the tetragonal structure nearly all parallel to the hot pressing plane, shows the formation of matrix texture. Twinning in the monoclinic phase was well developed and highly dominated by twinning with the (1 0 0) plane as the interface. Alternative twinning with the interfaces parallel to the (0 0 1) plane has also been revealed and a possible model was suggested based upon the basic structure to be coincident with the [1 0 0] rotation twinning. Strong intergrowth of the tetragonal and monoclinic phases was frequently found and the orientation relationship was determined. The possible orientation variants resulting from the tetragonal to monoclinic phase transformation were proposed based upon such orientations of the lattices. Two of them with the misorientation angle of 9°15′ and 80°45′, respectively, were also found to coexist with the two kinds of twinning. Such texture configuration may have a close relation to the improvement of toughness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Transmission electron microscopic study of three-dimensional polyacrylonitrile fibre-mesophase pitch matrix carbon-carbon composite

Conventional and high resolution transmission electron microscopy performed in this study provides some detailed microstructural information of a polyacrylonitrile (PAN) fibremesophase pitch matrix carbon-carbon composite which has not been published in open literature. The PAN fibre in this composite possesses a turbostratic structure throughout the fibre. The structure of the mesophase pitch matrix is graphitic and anisotropic. Near-fibre matrix crystallites are aligned roughly parallel to the fibre surface, exhibiting a flow-type morphology. The fibre-matrix interface in this composite is microfissured. Numerous microcracks exist both within the matrix and along partially bonded interfaces. The irregularly shaped interfacial microcracks readily expose the fibre surface topography. Microcracks within the matrix are formed between, and parallel to, the basal planes of the graphitic platelets. Such submicron-sized matrix cracks appear smaller and denser near the fibre-matrix interface.     

Direct electron microscopic observation of transcrystalline layers in microfibrillar reinforced polymer-polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight ratio of PET/LDPE = 50/50. Dog bone samples with MFC structure were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy. SEM observations on cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils. By means of TEM on stained ultrathin slices one observes the formation of transcrystalline layers of LDPE matrix on the surface of the PET microfibrils. In these layers the crystalline lamellae are aligned parallel to each other and are placed perpendicularly to the fibril surfaces. This is in contrast to the bulk matrix where the lamellae are quasi-randomly arranged.     

Specific heat of a new metastable phase of scandium-chromium

Specific heat and magnetic susceptibility measurements are reported for a new metastable phase of scandium-chromium with the E9₃ crystal structure. The results show that this phase exhibits bulk type II superconductivity with an onset near 6.7 K.     

Transmission electron microscope observation of Si deposited on W(110) surface

The (110) surface of a thin W crystal deposited with Si has been investigated by using a transmission electron microscope (TEM). It was found that an ordered Si–W alloy layer with a regular periodic arrangement of antiphase boundaries (APBs) is formed, as a result of intermixing of deposited Si with the W substrate at room temperature (RT). A crystal structure model of this W–Si superstructure is proposed in this study. After 800°C annealing of Si deposited at RT, grains of tetragonal WSi₂ are formed on the W–Si ordered alloy layer. Grains of WSi₂ are also formed when Si is deposited on a W substrate maintained at 900°C. These WSi₂ grains have epitaxial orientation relationships with the W(110) substrate as follows: (110)W//(01 ) WSi₂ and [001]W//[100]WSi₂.     

Effect of glass phase on the dissolution of hydroxyapatite

Nano size defect formation at grain boundary during the dissolution of hydroxyapatite in water was evaluated by adding several sintering additives for sinterability enhancement. In the case of sintered pure hydroxyapatite, significant dissolution occurred after immersion in distilled water or in simulated body fluid. The dissolution initiated at the grain boundaries creating nano-size defects like small pores that afterwards grew up to micro scale by increasing immersion time. This dissolution resulted in grain separation at the surfaces and finally in fracture. The dissolution concentrated on the grains adjacent to pores rather than those in the dense region. So hydroxyapatite ceramics containing glass powders were prepared to prevent the dissolution by strengthening grain boundary. Calcium silicate and phosphate glasses were added at 0 to 10 mass% and sintered at 1200 degrees C for 2 h in air with moisture protection. Glass phase was incorporated into hydroxyapatite to act as the sintering aid followed by crystallization in order to improve the mechanical properties without reducing biocompatibility. Dissolution tests, as well as X-ray diffraction and SEM showed little decomposition of hydroxyapatite to secondary phases and the fracture toughness increased compared to pure hydroxyapatite.