Showing papers in "Journal of Materials Science Letters in 1994"

Thermal decomposition of hydroxyapatite structure induced by titanium and its dioxide

Abstract: Titanium (Ti) implants coated with hydroxyapatite (HA), combining the ability of HA to bond with natural bone and excellent mechanical properties of titanium, have been successfully used in clinics. To improve the crystallinity of the coatings, post-heattreatment was applied on the HA coating-titanium system. Several groups [1-4] have post-heat-treated plasma-sprayed and sputtered HA coatings, to increase their crystallinity and lower their solubility compared with the as-received coatings. Ducheyne et al. [5] employed a vacuum heat treatment on electrophoretically deposited HA coatings to investigate the induced changes on their structures. Besides the benefits produced by heat treatment, there also exist some negative side-effects. Ducheyne et al. [5] found that the heat treated layers contain distinctively different compositions from the as-received coatings. The present authors [3] also reported that the plasma-sprayed HA coatings began to decompose at 800 °C during vacuum heat treatment. It is well known that the HA structure is thermally stable up to 1250 °C in air [6] and to 1050 °C in vacuum [7]. So it is hypothesized that the destruction of HA structure in H A T i or HA-TiO: systems at lower temperatures than 1000°C is completely related to the titanium and its dioxide. In the present study, HA powder was pre-heat-treated at 1200 °C for 120 min (the XRD pattern showing no decomposition) and then mechanically mixed with titanium and titanium dioxide powders. The mixtures were pressed into circular plates and sintered for 30 rain in vacuum and for 60 min in air, separately, at different temperatures. The stability of the starting H A powder was also proved by heating it in vacuum (<1.333 x 10 -3 Pa) at 1000 °C for 30 min, showing the same reflection patterns as those of the standard HA structure. The XRD patterns were determined on a RIGAKU diffractometer (D/max-y A) with Cu-Ko~ radiation at 45-50 kV and 140160 mA. The XRD patterns in Fig. 1 exhibit phase compositions of the mixture of HA and titanium sintered at different temperatures in vacuum. The decomposition of HA induced by titanium begins at 800 °C with the appearance of traces of o: tricalcium phosphate (oL-TCP) and tetracalcium phosphate (TCPM). With the increase of temperature, this process becomes more extensive and no other phase is produced besides cr-TCP and TCPM. The results are identical with our previous reports [3] concerning the thermal

Polyimide-silica hybrid materials by sol-gel processing

Abstract: Recently, there has been increasing interest in the development of hybrid materials consisting of an organic polymer and a ceramic type (inorganic) material [1]. There are mostly produced by a sol-gel technique [2], which can be viewed as a two-step network forming process, the first step being the hydrolysis of a metal alkoxide and the second consisting of a polycondensation reaction. The resulting hybrid materials, known also as ceramers [3], exhibit some characteristics and properties of both ceramics (e.g. heat resistance, retention of mechanical properties at high temperatures and low thermal expansion) and organic polymers (e.g. toughness, ductility and processability). This letter describes an experimental study on the preparation and properties of hybrid systems based on a condensation type polyimide and silica, prepared according to the scheme in Fig. 1. In ceramer SE2 films containing small amounts of 7-glycidyloxypropyltrimethoxysilane (GOTMS) coupling agent (see Table I), the transition from cloudy to hazy and later to a transparent appearance with increasing mixing time at 80 °C prior to casting the films at the same temperature, results from the precipitation of particles, which later start to adhere to the continuous matrix, eventually leading to a continuous silica network intermingled with the polyimide chains (Table II and Fig. 2). On the other hand, for ceramer S, which does not contain GOTMS ,coupling agent, cloudy films were observed for all mixing times at 80 *C, owing to the persistence of a particulate structure dispersed in a continuous phase (Table II and Fig. 2a). It is presumed from control viscosity measurements that the compatibilization of the polyimide and the silicate phases for ceramer SE2 is due to simultaneous reactions between the epoxy groups in the coupling agent GOTMS with themselves, which increases the viscosity of the siloxane component, and with the carboxylic acid groups of the polyamic Alkoxide I > '~ H20

The difference between transcrystallization and shear-induced cylindritic crystallization in fibre-reinforced polypropylene composites

Abstract: Institut f(Jr Verbundwerkstoffe GmbH, Universit#t Kaiserslautern, Pf.3049, D-67663 Kaiserslautern, Germany It was recognized early on that the fibre/matrix interphase (or interface) strongly influences the mechanical properties of composite materials. One of the current research directions in the field of composites with semicrystalline thermoplastic ma- trices is devoted to the question of whether or not particular supermolecular arrangements of the ma- trix in the vicinity of reinforcing fibres can improve the overall mechanical performance. Interest is focused particularly on the potential benefits of a transcrystalline interphase [1-4], since it is believed that this kind of "physical coupling" would enhance the stress transfer between the fibre and the matrix. On the effects of the transcrystalline interlayer, however, rather contradictory reports have appeared. The formation of the transcrystalline interphase layer, originated by fibre surface induced heterogeneous nucleation, is generally studied by single fibre model composites [2, 3]. This has also been the case for glass fibre (GF) reinforced polypropylene (PP) [5-8], where a transcrystaUine- like supermolecular structure could only be pro- duced when the crystallizing PP melt was sheared by pulling the embedded GF. In our recent work [9, 10], it was demonstrated that the columnar structure developed by melt shearing is not tran- scrystalline, but row-nucleated cylindritic. We have shown further [9, 10] that when both the crystalliza- tion (Tc) and pulling temperature (Tpull) are set in the range of the formation of the/3-PP, i.e. between T,~ ~ 100 °C and T~, ~ 140 °C [11, 12], a /3-rich columnar structure appears. Partial melting of this supermolecular formation revealed a layer attached to the pulled GF. This sheared layer of oi-PP contained ol-row nuclei and originated/3-crystalliza- tion (c~-fl bifurcation of growth, [11]). So under the conditions To~ < To, Tpun < T~o: melt shearing by pulling of GF resulted in a complex polymorphous structure containing both the o~- (row-nuclei along the GF) and fl-form of PP (grown up onto the oL-row nuclei). Due to this "/3-overgrowth" the oc-layer along the fibre surface can be resolved on the optical level only after separate melting of the/3-phase [10]. In the cited works on the shear induced crystalliza- tion of PP by pulling the fibre [5, 6, 9, 10] the GF proved to be "inert", i.e. it did not show any o-nucleation ability. Therefore it seemed that it would be very interesting to study how the above scenario changes when ol-nucleating fibres are used. 0261-8028 © 1994 Chapman & Hall In this case the conditions of both transcrystalliza- tion (due to heterogeneous 0l-nucleation) and row- induced cylindritic crystallization (due to melt shear- ing) are met. One can easily differentiate between them by taking into account the polymorphism of the PP and thus choosing the following crystalliza- tion conditions: T~ < To, Tpull < T~. Under these conditions the development of an o~-transcrystalline front is evidence of transcrystallization, while the formation of a fl-PP rich columnar structure indic- ates cylindritic crystallization [9, 10]. As oL-nucleating fibre poly(ethylene-terephtha- late) (PET) fibre taken from a commingled PET/PP yarn supplied by Toyobo Co. (Japan) was used. The oL-nucleating ability of PET has been reported in several works (cf. [7, 13, 14]). The isotactic PP used in this study was a general-purpose injection-mould- ing grade (Tipplen H-523, Tisza Chemical Works, Hungary). The crystallization and melting behaviour of the GF/PP model composites were studied with a Leitz polarizing optical microscope equipped with a Mettler hot stage. All further experimental details can be found in our previous work [9, 10]. Figs 1-3 show the effect of PET fibre on the isothermal crystallization of PP in its quiescent melt. Fig. 1 demonstrates the supermolecular structures formed in 2-step isothermal crystallization. At higher T0 (Td = 134 °C, t~ = 30 min) only spherul- ites were grown sporadically (Fig. la). Reducing T~ after 30 min crystallization time (t~) to To2 -- 124 °C gives rise to o:-transcrystallization of PP along the PET fibre (Fig. lb and c). A well developed transcrystalline layer can be obtained in 1-step isothermal crystallization, provided Tc is low enough (To = 124 °C, Fig. 2). This indicates that for the transcrystallization growth necessary high surface nucleation density on the PET fibre is guaranteed only at Tc ~< 130 °C in the given PP/PET composi- tion. The strong ol-nucleating ability of the PET fibre can be demonstrated by using a/3-nucleated PP matrix (Fig. 3). Fig. 3 shows that the PET fibre preserves its c~-nucleating ability even in fl-PP (achieved by using a proprietary selective fl-nucleat- ing agent (in 0.1 wt %). The oL-transcrystalline layer on the PET fibre is obvious, whereas/3-spherulites characterize the bulk (Fig. 3a). This structure becomes even more striking after selective melting (cf. later) of the /3-phase at Tf = 158 °C (Fig. 3b). Figs 1-3 show that the PET fibre acts as an

Protection of 316L stainless steel against corrosion by SiO2 coatings

Abstract: Oxide films prepared by sol-gel methods and presenting high resistance to heat, corrosion, friction and wear, as well as excellent mechanical properties, have recently been developed and put into practical use as structuraI materials [1-5]. The process of preparation offers potential advantages for modifying the properties of surfaces by low-temperature treatment without altering the original properties of strength and toughness of the substrates. A number of reports on sol-gel coatings concerning the prevention of chemical corrosion and oxidation have been published [6-10]. All of these films increase the protection of metal substrates from air corrosion (tested up to 800 °C) and acid attack (tested up to 80 °C). The most promising corrosion prevention for stainless steel has been studied by our group using sol-gel films of ZrO2, SiO2, SiO2-TiO2 and SiO 2A1203 prepared by dip-coating using sonocatalysed sols [11-14]. The. properties of these coatings have been studied by electrochemical techniques in NaC1 and H2SO 4 solutions. Although preliminary measurements have shown that SiO2 films are not the best protective coatings [13], they provide a very adequate model system to correlate corrosion protection with the physical structure of the sol-gel films. In this work, amorphous coatings of SiO 2 were deposited on 316L stainless steel by the dip-coating technique using a sol preparation involving sonocatalysis. The films were prepared through hydrolysis polymerization of metal alkoxide solutions and conversion to an oxide layer by heating at relatively low temperatures. The effect of the time of heat treatment of the SiO2 films on the corrosion resistance of stainless steel was studied in 15% H2SO 4 through potentiodynamic polarization curves at 25 °C. The substrate used in the experiments was 316L stainless steel (SS 316L, Caseurop, France) with chemical composition (wt %): 67.25 Fe, 18.55 Cr, 11.16 Ni, 2.01 Mo, 0.026 Cu, 1.71 Mn and 0.028 C. The specimens were machined into dimensions of 30 mm × 15 mm x I mm, degreased ultrasonically in acetone, cleansed by distilled water then dried in air. For silica films, tetraethylorthosilicate Si(OC2H5) 4 (TEOS) was used as the source of silica, absolute ethanol (C2HsOH) as solvent and glacial acetic acid CH3COOH as catalyst. The silica sonosol was prepared by dissolving Si(OCzHs) 4 in absolute ethanol to which a small amount of acetic acid CH3COOH was added. The volume ratios of Si(OC2Hs)4/C2HsOH and Si(OC2Hs)4/CH3COOH were, respectively, 1 and 5. The mixture was submitted to intense ultrasonic irradiation (20 kHz) produced by a transducer (Heat Systems Ultrasonics W385). After 25 min the resulting sol was homogenized and remained stable for about five weeks at room temperature when kept in a closed vessel. Coating films were formed on the substrates by dipping into the clear sonosol and withdrawing at a speed of 10 cmmin -1. The resulting gel films were dried at 60 °C for 15 min and densified in a furnace with air atmosphere by increasing the temperature at a rate of 5 °C min -1 up to 450 °C when an isothermal holding of 1 h was applied in order to remove the organic residues. The temperature was then increased again at the same rate up to either 600 or 800 °C and maintained at that value for variable lengths of time to complete the densification and obtain adherent coatings. The average thickness of the heat-treated films at 800 °C was around 0.4/zm. X-ray diffraction (XRD) analysis of the substrate and coatings was done with a Philips diffractometer using CuKo, The diffractogram of SS 316L shows the existence of a crystalline phase which corresponds to the cubic phase of the alloy containing Cr, Fe and Ni [4]. When the steel was heated at 800 °C for 2 h in air, additional XRD peaks appear corresponding to a mixture of cubic and hexagonal CrzO 3 [4]. In contrast, samples coated with SiO2 analysed after oxidation tests in air at 800 °C for 2 h showed only the peaks of the original substrate, indicating that the silica coating remains amorphous and inhibits any oxidation of the base material. A Bomem Fourier transformation infrared (F-FIR) spectrometer was used to obtain high resolution spectra of the coatings in the 400-4000 cm -1 range; the measurements were carried out at room temperature by reflection at an incident angle of 30 ° . The spectrum of a coating deposited on SS 316L and

The influence of alumina dopant on the structural transformation of gel-derived nanometre titania powders

Abstract: Nanometre materials, characterized by an ultrafine grain size, have attracted much attention in the past few years because of their unusual chemical, mechanical, optical, electrical and magnetic properties and their wide applicability [1]. Thermodynamically, however, they are metastable. Under certain conditions, the grains comprising the material may grow to a larger scale, which may consequently result in the disappearance of some unique properties of the nanometre material. Therefore, it is important to investigate the thermal stability of a nanostructural material. For the past two decades, sol-gel routes to ultrafine metallic oxide powders have been widely investigated [2]. Among these oxides, titania is a very important material for its humidity [3], hydrogen[4], and oxygen[5] sensitive properties and some catalysis applications [6]. Usually, titania has three different structures: brookite, anatase and futile. The former two phases are both metastable, and the futile phase is of a thermodynamic stable state. Some properties of titania may strongly depend on its microstructure; for example, many studies have suggested that the anatase phase of titania is the superior support of VzOs/TiO4 catalyst for the selective partial oxidation reaction compared to the rutile phase [6]. It is well known that many properties of ceramic materials can be improved by a small amount of doping. For gas-sensitive oxide materials, doping is often necessary to increase the sensitivity and selectivity [7] of the material. In this letter, nanometre titania powders with and without alumina dopant were prepared by a sol-gel method. The structural development of these powders was studied systematically, and the influence of a small amount of alumina dopant on the structural changes was also investigated. Tetrabutyl titanate and aluminium isopropoxide were used as the precursors of titania and alumina, respectively. In preparing TiO2 sol, Ti (OBu)4 was dissolved in ethanol, and then HC1 + H 2 0 solution was dropped into Ti(OBu)4 solution with continuous stirring for an hour. The molar ratio of these reactants was Ti(OBu)4:EtOH:HCl:H20=l:15: 0.3:1. After a week, a transparent orange gel was obtained. To form the alumina-doped titania sol, a given amount of aluminium isopropoxide was added to the Ti(OBu)4 solution (molar ratio AI(C3H70) to Yi(OBu)4, 0.12) and stirred thoroughly using a magnetic mixer and ultrasonic wave successively before the addition of HC1 + H 2 0 solution. The gelation time for alumina-doped titania sol is also about a week. After drying in a vacuum tube (10 -1 Pa) furnace at 333 K for 5 h, the gels were heat treated at different temperatures for 2 h under oxygen atmosphere (O2 flow rate: 40mlmin-1). Changes in the structures of these powders with temperature were investigated by thermogravity analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) experiments. Both TiO2 dry gels without and with A1203 dopant were confirmed to be of amorphous structure by XRD experiments, as shown in Fig. la and b. For pure titania gel (Fig. la), partial crystallization occurred after annealing at 523 K for 2 h, and the powders annealed at a temperature below 773 K are of anatase structure. A phase transformation from anatase to rutile occurred for annealing temperatures above 823 K and was completed at 1073 K. On the other hand, the alumina-doped TiO2 gel (Fig. lb) remained amorphous after annealing at 623 K for 2 h. When the annealing temperature was 723 K, crystallization began, and the phase transformation from anatase to rutile did not occur until the annealing temperature was elevated to 1073 K. Moreover, a few anatase crystallites still existed at an annealing temperature of 1223 K. This leads to the conclusion that a little alumina addition

Conversion of calcium aluminate cement hydrates re-examined with synchrotron energy-dispersive diffraction

Abstract: Previous studies [l-41 on calcium aluminate cement (CAC), formerly known as high alumina cement (HAC), have shown that on reaction with water the initial hydrate formed at lower temperatures, par- ticularly below 15 "C, is the metastable, hexagonal hydrate CAHlo (see Table I for cement nomen- clature system) CA

New metal-ceramic composites grown by metalorganic chemical vapour deposition

Abstract: While exploring new and effective routes to composites of ceramics and metals we have used the heterometallic alkoxide barium bis[tert-butoxistannate(II)], BaSn2(OtBu)6, as molecular precursor in metalorganic chemical vapour deposition (MOCVD) techniques. The solid material obtained is characterized by highly dispersed metal in a ceramic matrix as determined by electron microscopy, electron spectroscopy and X-ray diffraction and therefore has peculiar chemical and physical properties. Furthermore the material is exclusively built of globular particles. In the one-component CVD process at low pressure heating is achieved by a microwave system. To yield the new solid composite we have designed a CVD apparatus as shown in Fig. 1 which is especially useful for alkoxide precursors. The decomposition takes place in the temperature range 300-500 °C. This temperature is sufficient to decompose the precursor completely and is low enough to prevent carbide formation (the highest C-content found is 5% by photoelectron spectroscopy (XPS)). The precursor barium bis[tert-butoxi-stannate (II)], obtained as described [1] and recrystallized from toluene, is incorporated into the system and the substrate is heated by induction to the temperature of decomposition which is 350 °C for Ba Sn2(OtBu)6. The temperature of the outer furnace is successively raised until a continuous flow of precursor gas is reached which can be monitored by the unchanging mass spectrum of the reaction gases. If

Performance of electrochromic cells of polyaniline in polymeric electrolytes

Abstract: Polyaniline is currently considered to have good device potential [1-5]. One of the devices for which polyaniline is seriously considered is the electrochromic display. A number of studies on the electrochromism (EC) of polyaniline covering various aspects, such as the mechanism of the colour change [6-8] and the effect of cell parameters on the switching time [9], have been done, but these and similar studies have generally been conducted in liquid media in electrochemical cells. In practical devices it is preferable to employ solid materials in order to minimize the problems of sealing in hazardous liquids. Furthermore, EC devices are usually required to have a thin layer configuration. With these considerations in mind, we have performed experiments with thin layer cells of polyaniline containing a supporting electrolyte such as LiC10 4 dissolved in polyethylene oxide (PEO) or polyvinyl alcohol (PVA). We have also studied cells containing urea dissolved in glycerol as the medium. We report the performance of such cells in this letter. A polymeric electrolyte such as PEO was chosen because the amorphous nature of PEO leads to good ionic conductivity and redox stability up to +3 V [10]. In addition, the combination of PEO/ LiC104 is known to be a very fast ionic conductor, with the Li ÷ being the mobile species [11]. Recently, reports have also appeared which describe solid state electrochromic cells of methylene blue using polyacrylamide [12] and gels of polymethyl methacrylate in electrochromic cells of WO3 [13]. The composition of the electrochromic cells used in this study is given in Table I. We have studied the effects of the medium on the various parameters such as switching time, cycle lifetime and applied voltage on the electrochromic display. The current transients for the switching reaction of polyaniline were analysed to understand the influence of mass transport on the switching reaction in polyaniline. Cells for electrochromic studies were constructed as follows. Polyaniline (nominal thickness 1 #m) was coated with indium tin oxide (ITO) plate by application of alternating voltage ( -0 .1 V to 1 V) in HC1 medium containing 0.1 M aniline. The film was dried and dip coated with the appropriate electrolyte then covered with another plate of ITO. The whole assembly was then sealed with wax to exclude air. The schematic of such an electrochromic cell is shown in Fig. 1. Electrical contacts were made using alligator clips after ensuring that there was minimal contact resistance. The cell was mounted on a stand in an optical bench and illuminated at 632 nm with a tungsten-halogen lamp (Oriel Corp., USA) through a monochromator (Oriel). The transmitted light was monitored by an Si photodiode (Oriel). The diode