XPS study of the process of apatite formation on bioactive Ti—6Al—4V alloy in simulated body fluid

Bioactive Ti—6Al—4V alloy, which spontaneously forms a bonelike apatite layer on its surface in the body and bonds to living bone through this apatite layer, can be prepared by producing an amorphous sodium titanate on its surface by NaOH and heat treatments. In this study, the process of apatite formation on the bioactive Ti—6Al—4V alloy was investigated in vitro, by analyzing its surface with X-ray photoelectron spectroscopy as a function of soaking time in a simulated body fluid 4SBF). Thin-film X-ray diffractometry of the alloy surface and atomic emission spectroscopy of the fluid were also performed complementarily. It was found that immediately after immersion in the SBF,the alloy exchanged Na¹ ions from the surface sodium titanate with H₃O¹ ions in the fluid to form Ti-OH groups on its surface. The Ti-OH groups, immediately after their formation,incorporated the calcium ions in the fluid to form calcium titanate. The calcium titanate thereafter incorporated the phosphate ions in the fluid to form an amorphous calcium phosphate, which was later crystallized into bonelike apatite. This process of apatite formation on the alloy was the same as on the pure titanium metal, because the alloy formed the sodium titanate free of Al and V by the NaOH and heat treatments. The initial formation of the calcium titanate is proposed to be a consequence of the electrostatic interaction of negatively charged units of titania dissociated from the Ti-OH groups with the positively charged calcium ions in the fluid. The calcium titanate is postulated to gain a positive charge and interact with the negatively charged phosphate ions in the fluid to form amorphous calcium phosphate, which eventually stabilizes into crystalline apatite.     

XPS study of the process of apatite formation on bioactive Ti–6Al–4V alloy in simulated body fluid

Bioactive Ti–6Al–4V alloy, which spontaneously forms a bonelike apatite layer on its surface in the body and bonds to living bone through this apatite layer, can be prepared by producing an amorphous sodium titanate on its surface by NaOH and heat treatments. In this study, the process of apatite formation on the bioactive Ti–6Al–4V alloy was investigated in vitro, by analyzing its surface with X-ray photoelectron spectroscopy as a function of soaking time in a simulated body fluid (SBF). Thin-film X-ray diffractometry of the alloy surface and atomic emission spectroscopy of the fluid were also performed complementarily. It was found that immediately after immersion in the SBF, the alloy exchanged Na⁺ ions from the surface sodium titanate with H₃O⁺ ions in the fluid to form Ti-OH groups on its surface. The Ti-OH groups, immediately after their formation, incorporated the calcium ions in the fluid to form calcium titanate. The calcium titanate thereafter incorporated the phosphate ions in the fluid to form an amorphous calcium phosphate, which was later crystallized into bonelike apatite. This process of apatite formation on the alloy was the same as on the pure titanium metal, because the alloy formed the sodium titanate free of Al and V by the NaOH and heat treatments. The initial formation of the calcium titanate is proposed to be a consequence of the electrostatic interaction of negatively charged units of titania dissociated from the Ti-OH groups with the positively charged calcium ions in the fluid. The calcium titanate is postulated to gain a positive charge and interact with the negatively charged phosphate ions in the fluid to form amorphous calcium phosphate, which eventually stabilizes into crystalline apatite.     

Formation of bioactive functionally graded structure on Ti-6Al-4V alloy by chemical surface treatment

An Al- and V-free sodium titanate hydrogel layer with a graded structure where the sodium titanate gradually decreases toward the interior, was formed on the surface of Ti-6Al-4V alloy, when the alloy was exposed to 5M NaOH solution at 60 °C for 24 h. This gel layer was transformed into an amorphous sodium titanate layer without giving considerable change in the graded structure, except a little increase in the depth of the oxygen distribution by a heat treatment at 600 °C for 1 h. The sodium titanate layer formed Ti-OH groups on its surface by exchanging its Na⁺ ion with H₃O⁺ ion in simulated body fluid when soaked in the fluid, and thus formed Ti-OH groups induced the apatite nucleation. The apatite layer also formed a graded structure toward the substrate. The strong bond of the apatite layer to the substrate was attributed to this graded structure.     

Self-radiation damage in Gd2Ti2O7

The effects of self-radiation damage from alpha decay in Gd₂Ti₂O₇ were investigated by studying specimens doped with ²⁴⁴Cm. The radiation-induced microstructure consists of individual amorphous tracks from both the alpha-recoil particles and the spontaneous fission fragments. The eventual overlap of the tracks at higher doses leads to a completely amorphous state. The self-radiation damage increases the volume, dissolution rate, and fracture toughness. Electron-beam recrystallization of the amorphous state results in the formation of fine microcrystallites on the order of 0.05 μm in size.     

Phase transformations in nanostructural anatase TiO<Subscript>2</Subscript> under shock compression conditions studied by Raman spectroscopy

Shock-wave-induced phase transformations in nanostructural titanium dioxide TiO₂ (anatase) powders of two types have been studied by Raman spectroscopy. It is established that, at a shock-wave compression to 42 GPa, anatase particles can either transform into a columbite phase or exhibit amorphization.     

Formulation of tablets containing an ‘in-process’ amorphized active pharmaceutical ingredient

The aim of this work was a preliminary study of the “in-process” amorphization of clopidogrel hydrogensulfate (CLP) as model drug during the production of tablets as dosage form. A solvent method was used for amorphization and the crystalline phase of CLP was detected by differential scanning calorimetry; the physical parameters of fresh and stored tablets were investigated. For the amorphous form, Aerosil 200 was selected as crystallization inhibitor as the most suitable of eight auxiliary agents. The optimum composition of the product for amorphization in the scaling-up process (100-fold) was 7 parts of CLP to 3 parts of Aerosil 200. In this scaled-up product, the amorphous CLP was fixed on the surface of microcrystalline cellulose. The tablet form further stabilized the amorphous form. Finally, the steps of an “in-process” amorphization are given as a protocol, which can promote stabilization of an amorphized active pharmaceutical ingredient.     

Formulation of tablets containing an 'in-process' amorphized active pharmaceutical ingredient

The aim of this work was a preliminary study of the "in-process" amorphization of clopidogrel hydrogensulfate (CLP) as model drug during the production of tablets as dosage form. A solvent method was used for amorphization and the crystalline phase of CLP was detected by differential scanning calorimetry; the physical parameters of fresh and stored tablets were investigated. For the amorphous form, Aerosil 200 was selected as crystallization inhibitor as the most suitable of eight auxiliary agents. The optimum composition of the product for amorphization in the scaling-up process (100-fold) was 7 parts of CLP to 3 parts of Aerosil 200. In this scaled-up product, the amorphous CLP was fixed on the surface of microcrystalline cellulose. The tablet form further stabilized the amorphous form. Finally, the steps of an "in-process" amorphization are given as a protocol, which can promote stabilization of an amorphized active pharmaceutical ingredient.     

Behavior of GeSbTeBi phase-change optical recording media under subnanosecond pulsed laser irradiation

We investigated the variations in reflectivity during the phase transition between amorphous and crystalline states of a Bi-doped GeTe-Sb2Te3 pseudobinary compound film with subnanosecond laser pulses, using a pump-and-probe technique. We also used a two-laser static tester to estimate the onset time of crystallization under 2.0-micros pulse excitation. Experimental results indicate that the formation of a melt-quenched amorphous mark is completed in approximately 1 ns, but that crystalline mark formation on an as-deposited amorphous region requires several hundred nanoseconds. Simple arguments based on heat diffusion are used to explain the time scale of amorphization and the threshold for creation of a burned-out hole in the phase-change film.     

Effect of heat treatment on apatite-forming ability of Ti metal induced by alkali treatment

The present authors previously showed that titanium metal forms a bone-like apatite layer on its surface in a simulated body fluid (SBF), when it has been treated with a NaOH solution to form a sodium titanate hydrogel layer on its surface. This indicates that the NaOH-treated Ti metal bonds to living bone. The gel layer as-formed is, however, mechanically unstable. In the present study, the NaOH-treated Ti metal was heat treated at various temperatures in order to convert the gel layer into a more mechanically stable layer. The gel layer was dehydrated and transformed into an amorphous sodium titanate layer at 400–500°C, fairly densified at 600°C and converted into crystalline sodium titanate and rutile above 700°C. The induction period for the apatite formation on the NaOH-treated Ti metal in SBF increased with the transformation of the surface gel layer by the heat treatment. Ti metal heat treated at 600°C, however, showed a fairly short induction period as well as high mechanical stability, since it was covered with a fairly densified amorphous layer.     

Improvement in crystallinity of apatite coating on titanium with the insertion of CaF2 buffer layer

In the apatite coatings on Ti the heat treatment process is necessary to crystallize the apatite structure for improved chemical stability and biological properties. However, the heat treatment normally degrades the mechanical strength of the coating layer associated with thermally induced stress. In this study, we aimed to improve the crystallization of apatite coating by using calcium fluoride (CaF₂) as a buffer layer. The insertion of a thin layer of CaF₂ (0.2–1 μm) between apatite and Ti significantly improved the crystallization behavior of apatite. Moreover, this crystallization was more enhanced as the thickness of CaF₂ was increased. When a 1 μm-thick CaF₂ was inserted, the crystallization of apatite initiated at a temperature as low as 320 °C, being a dramatic improvement in the crystallization when considering the crystallization initiation temperature of a bare apatite coating on Ti was ∼450 °C. As a result of this crystallization enhancement, the dissolution behavior of CaF₂-inserted apatite coatings was more stable than that of the bare apatite coating, showing much reduced initial-burst effect. Preliminary cellular assay showed the CaF₂-inserted apatite coating provided a substrate for cells to spread and grow favorably, as being similar to the bare apatite coating. This novel way of apatite coating on Ti using CaF₂ buffer layer may be useful in the coating systems particularly requiring low temperature processing and increased crystallinity with high chemical stability.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Investigation of crystallization and amorphization dynamics of phase-change thin films by subnanosecond laser pulses

We report experimental results on amorphization and crystallization dynamics of reversible phase-change (PC) thin-film samples, GeSbTe and GeBiTe, for optical disk data storage. The investigation was conducted with subnanosecond laser pulses using a pump-and-probe configuration. Amorphization of the crystalline films could be achieved with a single subnanosecond laser pulse; the amorphization dynamics follow closely the temperature kinetics induced in the irradiated spot. As for crystallization of the samples initially in the amorphous state, a single subnanosecond pulse was found to be insufficient to fully crystallize the irradiated spot, but we could crystallize the PC film (in the area under the focused spot) by applying multiple short pulses. Our multipulse studies reveal that the GeSbTe crystallization is dominated by the growth of nuclei whose initial formation is slow but, once formed, their subsequent growth (under a sequence of subnanosecond pulses) happens quickly. In the case of GeBiTe samples, the crystalline nuclei appear to be present in the material initially, as they grow immediately upon illumination with laser pulses. Whereas our amorphous GeSbTe samples required approximately 200 pulses for full crystallization, for the GeBiTe samples approximately 15 pulses sufficed.     

REVIEW Bioactive metals: preparation and properties

Some ceramics, such as Bioglass, sintered hydroxyapatite, and glass-ceramic A-W, spontaneously form a bone-like apatite layer on their surface in the living body, and bond to bone through the apatite layer. These materials are called bioactive ceramics, and are clinically important for use as bone-repairing materials. However, they cannot be used at high-load sites, such as is found in femoral and tibial bones, because their fracture toughness values are not as high as that of human cortical bone. Titanium metal and its alloys have high fracture toughness, and form a sodium titanate layer on its surface when soaked in a 5 M-NaOH solution at 60 degrees C for 24 h, followed by a heat treatment at 600 degrees C for 1 h. On moving toward the metal interior, the sodium titanate layer gradually changes into the pure metal within a distance of 1 microm from the surface. The mechanical strength of the titanium metal or a titanium alloy is not adversely affected by these chemical and thermal treatments. The titanium metal and its alloys resulting from the above treatment can release Na+ ions from its surface into a surrounding body fluid via an ion exchange reaction with H3O+ ions, resulting in many Ti-OH groups forming on its surface. These Ti-OH groups initially combine with Ca2+ ions to form amorphous calcium titanate in the body environment, and later the calcium titanate combines with phosphate ions to form amorphous calcium phosphate. The amorphous calcium phosphate eventually transforms into bone-like apatite, and by this process the titanium metals are soon tightly bonded to the surrounding living bone through the bone-like apatite layer. The treated metals have already been subjected to clinical trials for applications in artificial total hip joints. Metallic tantalum has also been found to bond to living bone after it has been subjected to the NaOH and heat treatment to form a sodium tantalate layer on its surface.     

Fast amorphization and crystallization in Al-Fe binary system by high-energy ball milling

Large amount of amorphous phase of Al-Fe binary system was obtained by MA of elemental powders using a high-energy ball mill at milling intensity of 150G (G is the gravitational acceleration). XRD, HRTEM and DSC were used to analyze the process of amorphization and crystallization. The time required achieving almost complete amorphous state is only 4.2 ks for Al-25 at.%Fe system and 3 ks for Al-30 at.%Fe system, respectively. The time of amorphous formation is very shorter than that of previous reports on Al-Fe binary system. Further milling causes rapid crystallization of the amorphous phase. By analysis of S(Q), the presence of a strong Al-Fe chemical short-range order in the amorphous matrix is suggested. Moreover, the superstructure of these Al-Fe clusters in the amorphous matrix is similar to the solid structure of Al₅Fe₂, and the clusters transform into the nucleus of Al₅Fe₂ intermetallic compound under the action of milling energy.     

Neutron spectrometry using CR-39 track etch detectors

Track-size distributions were measured for chemically etched CR-39 foils exposed to monoenergetic neutrons with energies ranging from 0.144 to 19 MeV and to various broad-spectrum neutron sources including spontaneous fission neutrons from (238)Pu. These tracks are due to energetic charged particles resulting from interactions of the neutrons with the CR-39. The tracks are visible with an optical microscope after chemical etching and vary in size and configuration depending on the particle, energy and angle of incidence. The foils were analysed using an automatic analysis system that scans the foils, identifies valid tracks and records the track-size parameters. The track-size distributions vary with neutron energy for the monoenergetic sources and with the hardness of the broad-spectrum sources. The distribution from the (238)Pu fission source is readily distinguishable from the other sources measured and from distributions owing to the background.     

Electrical properties of hydrogen bombarded silicon surfaces

The effect of low energy hydrogen bombardment on the electrical properties of crystalline silicon Schottky diodes has been investigated. While a low dose irradiation produces a high number of donor-type defects which strongly modify the diodes' behaviour, a high dose irradiation leads to the formation of a metal-hydrogenated amorphous silicon-silicon structure due to silicon amorphization in a close surface region.     

In situ HVEM studies on the effects of electron-irradiation on the thermal stability of Ni-based amorphous alloys

In situ high voltage electron microscope (HVEM) studies on the thermal stability of splat-cooled Ni-based amorphous alloys were made and three kinds of accelerated crystallization modes were observed during bombardment by focused 1000 keV electrons. In one case the crystalline grains induced by irradiation were coarser near the edge of the irradiated region (IR) than in other parts. This type of crystallization was observed in Ni₇₅B₁₇Si₈ and Ni₇₅B₁₅Si₈C₂ amorphous alloys subjected to a continuous increase in temperature of irradiation. The second case was one where there was no appreciable difference in size distribution of the crystalline grains throughout the IR. This type was observed in a Ni₈₀P₁₀B₁₀ amorphous alloy which again was subjected to a continuous increase in irradiation temperature and also observed in Ni₇₅B₁₇Si₈ amorphous alloy irradiated during isothermal annealing. In these two cases, the crystalline grains induced during irradiation did not cover the whole of the IR before crystallization started in the unirradiated region. In the third case, however, the amorphous phase completely disappeared from the IR before crystallization in the unirradiated region occurred. This type of crystallization was observed in Ni₈₀P₂₀ amorphous alloy whilst the temperature was being increased continuously during irradiation.     

Solid-state reaction induced by milling of a mixture of cobalt and boron powders

A mixture of polycrystalline elemental cobalt and boron powders in the atomic ratio Co₆₇B₃₃ was mechanically milled for 150 h. The milled powders were examined by X-ray diffraction, extended X-ray absorption fine structure spectroscopy and differential scanning calorimetry. Two steps where energy was regulated by different full/void volume ratio in the vials were carried out. In the first milling step (f/v=1/1) amorphization involves only a portion of the starting powder and reaches a steady state after 25 h. More energetic conditions (f/v=1/5) lead to almost complete amorphization of the sample after 70 h, and in the final stages the formation of crystalline t-Co₂B is observed. Then a steady state, in which both amorphous and crystalline phases coexist, is reached.     

Needle-like apatite-leucite glass-ceramic as a base material for the veneering of metal restorations in dentistry

A needle-like apatite-leucite glass-ceramic was prepared in the SiO₂-Al₂O₃-Na₂O-K₂O-P₂O₅-F system. Nucleation and crystallization processes were studied in bulk and powdered samples. The crystallization of leucite follows the mechanism of surface crystallization. After the precipitation of NaCaPO₄ crystals and another unknown crystal phase, the formation of needle-like apatite is based on a volume nucleation and crystallization process. The mechanism of the formation of needle-like apatite differs to those of apatite precipitation in glass-ceramics. The morphology of needle-like apatite is comparable to that of apatite in natural teeth.The properties of the glass-ceramic, especially the good chemical durability, the optical properties, as well as mechanical and thermal properties allow glass-ceramic to be used as a main component in a bio-material for the veneering of metal restorations in dentistry.     

Structure and mechanical properties of a Fe90Zr10 amorphous alloy

The structure of a Fe₉₀Zr₁₀ amorphous alloy was investigated by means of small angle X-ray scattering as well as large-angle diffraction measurements. For as-quenched specimens, SAXS was found to be relatively weak, but spread over a wide scattering angle. After quantitative analysis, it was concluded that a compositional fluctuation occurs on a fine scale of about 0.6 nm. When the specimen was heat treated below the crystallization temperature, the amorphous structure changed to a more stable dual structure consisting of pure iron and a structure similar to Fe₃Zr. By prolonged heat treatment, the iron-rich regions crystallized initially from the amorphous state. An apparent correspondence was found to exist between the changes in the amorphous structure and in the mechanical properties. The microscopic phase separation within the amorphous state resulted in an increase of ultimate tensile strength and fracture toughness. The deterioration of mechanical properties was suggested to be attributed to the gradual crystallization of iron-rich regions.