Bio-inerte Implantatkeramiken. Teil II. Zur Darstellung von nanoskaligen Feinstpulvern im Al2O3-SiO2-TiO2-System

Bio-inert implant ceramics Part II. Preparation of nanoscaled ultra-fine powders in the Al₂O₃-SiO₂-TiO₂-system Medical demands from the basis for the development of binary and ternary bioinert ceramics via nanoscaled powders. This powder preparation by hydrolysis – and sol-gel-methods is described and compared, amongst them mullite-powder, tialite-powder and numerous compositions in the ternary region between aluminiumtitanate, silica and titania. Optimal conditions in order to prepare these powders are concluded and summarized.     

Bioinerte Implantatkeramiken Teil I. Technologisch relevante Phasenuntersuchungen im Al2O3-SiO2-TiO2-System

Bioinert implant ceramics Part I. Technological relevant phase investigations in the Al₂O₃-SiO₂-TiO₂-system The Al₂O₃-SiO₂-TiO₂-system forms the basis for developing variable, bioinert implant ceramics. This is why its components and their binary systems as well as the ternaryphase diagram for the solid (< 1750 K) and solid-liquid region (1775 K) have been investigated and compared with literature data.     

Study of the preparation process for BaO-Al2O3-SiO2 powders by a two-step method

BaO-SiO₂ and Al₂O₃-SiO₂ dried gel powders were prepared by the sol-gel process using TEOS, aluminium nitrate and barium acetate as raw materials. By using these starting materials, -BaAl₂Si₂O₈ (hexacelsian) powders with small particle size were obtained after calcining at 1000 °C. The effects of solvents and prehydrolysis of TEOS on the gelation were studied. The physical and chemical changes, and phase transformation of the gel powders for BaO (Al₂O₃)-SiO₂ and BaO-Al₂O₃-SiO₂ systems during heating were examined using differential thermal analysis, gas chromatography and X-ray diffraction. The particle sizes of the gel powders obtained were also observed by SEM and TEM.     

Microstructural homogeneity improvement in Si3N4 by a powder coating method

The efficiency of a powder coating technique has been quantitatively evaluated through a comparison of the densification behaviour, green compact and dense material microstructural homogeneity in terms of a homogeneity dimension, and mechanical properties, using coated powders and mixed powders in the case of Si₃N₄ powder densified by hot-pressing with the liquid-forming additive system Al₂O₃-TiO₂-SiO₂. For coated powder, a significantly smaller value of the homogeneity dimension was obtained. The oxide phases became re-distributed during densification, with the aluminium-containing phase distributed on a finer scale, and the titanium-containing phase on a coarser scale, compared with the green body. Materials prepared by hot-pressing of coated powders showed a more homogeneous microstructure, higher bend strength and higher Weibull modulus, compared with materials prepared from mixed powders. There were no differences in fracture toughness and hardness between the two types of material.     

Preparation of composite Al2O3-TiO2 particles from organometallic precursors and transformations during heating

Equimolar Al₂O₃-TiO₂ composite powders were prepared via controlled hydrolysis of organometallic precursors, sometimes in the presence of submicrometre commercial-Al₂O₃ or anatase-TiO₂ particles. Variations in the chemical procedures were used aimed at different submicrostructures within the resulting powders. Heating such powders in air shows that structural behaviour is influenced by the micromorphology of the composite particle. Transformation temperatures of the titania phases seem to depend upon some size parameter which would represent their morphology within the powders. Studies performed on a series of non-equimolar Al₂O₃-TiO₂ composite powders showed that the temperature of -Al₂O₃ formation may be decreased by 210° C possibly due to a seeding effect of rutile. Pseudobrookite Al₂TiO₅ was never detected at 1300° C in air.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Effects of silica and titania modification additions on the microstructure of sintered alumina

The way in which silica and titania modification additions are introduced into alumina is shown to have a strong effect on the microstructure of sintered Al₂O₃-SiO₂-TiO₂ materials. By combining molecular layering and mechanical mixing, materials with a controlled pore structure can be obtained.     

Influence of TiO2 incorporation in HfO2 and Al2O3 based capacitor dielectrics

Atomic layer deposition was applied to fabricate metal oxide films on planar substrates and also in deep trenches with appreciable step coverage. Atomic layer deposition of Ru electrodes was realized on planar substrates. Electrical and structural behaviour of HfO₂-TiO₂ and Al₂O₃-TiO₂ nanolaminates and mixtures as well as Al₂O₃ films were evaluated. The lowest leakage current densities with the lowest equivalent oxide thickness were achieved in mixed Al₂O₃-TiO₂ films annealed at 700 °C, compared to all other films in as-deposited state as well as annealed at 900 °C. The highest permittivities in this study were measured on HfO₂-TiO₂ nanolaminates.     

Preparing of nanostructured Al2O3–TiO2–ZrO2 composite powders and plasma spraying nanostructured composite coating

Nanostructured Al₂O₃–TiO₂–ZrO₂ composite powders for plasma spraying were prepared by spray drying granulation technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. The results show that with increasing the slurry solid content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the spray drying temperature, the particle size of powders increases, and the bulk density and flowability of powders increases firstly and then decreases. The most appropriate spray drying parameters are the slurry solid content of 40 wt.%, the binder content of 2.0 wt.% and the spray drying temperature of 250 °C. The nanostructured composite coating was successfully prepared by using the as-prepared nanostructured Al₂O₃–TiO₂–ZrO₂ composite powders as feedstocks. The nanostructured coating possessed higher hardness and toughness compared with the conventional microstructured one, which was attributed to the use of the nanostructured composite powders feedstocks.     

Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

The structure of Al₂O₃-SiO₂ sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al₂O₃ and partially amorphous in the range 52 to 88 wt % Al₂O₃. The crystalline phase was mullite up to 75 wt % Al₂O₃ but at higher Al₂O₃ contents a metastable solid solution of SiO₂ in -Al₂O₃ was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al₂O₃ end of the Al₂O₃-SiO₂ system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al₂O₃ contents up to 60 wt %. The viscosity change is relatively small as the Al₂O₃ content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al₂O₃ concentrations greater than approximately 80 wt %, -Al₂O₃ is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al₂O₃. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO₂ in -Al₂O₃, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO₂ in solution in metastable Al₂O₃ increases the temperature of transformation to -Al₂O₃ to greater than 1500° C compared with 1230° C for pure Al₂O₃.     

Atomic force microscopy study on the anatase crystallization of long term stored Al2O3-TiO2-SiO2 coatings on glass

Atomic force microscopy was used to study the surface morphology of sol-gel derived Al₂O₃-TiO₂-SiO₂ coatings on microscope glasses. The coatings freshly annealed at 500°C for 1 h were homogeneous on a nanoscale and showed typical glass pattern. After 10-month storage in air either before or after annealing, tetragonal-like crystallites were observed in coatings. The effects of chemical compositions and the storage process on the surface topography and the crystallization of anatase were studied and discussed in combination with the XRD results of corresponding annealed gels.     

Processing and properties of Y-TZP/Al2O3 composites

The processing and property measurement of Y-TZP/Al₂O₃ ceramic-ceramic composites was investigated. The wet chemical synthesis route was adopted for the preparation of 3Y-TZP matrix dispersed with Al₂O₃ in three different volume fractions. Characterization of the resultant powders was carried out and their densification behaviour was studied by sintering in air in the temperature range 1200–1600 °C. The role of alumina as grain-growth inhibitor for Y-TZP, and the mechanical response of these ultrafine-grain ceramic composites in terms of K_lc characteristics, have been discussed.     

Fabrication and characterization of heat and plasma treated SiC/Al2O3-YSZ feedstocks used for plasma spraying

The SiC/Al₂O₃-YSZ (ZrO₂ + 8 wt.% Y₂O₃) powders with different SiC particle sizes were fabricated and treated from spray drying, heat treatment, and plasma spraying. The morphology, phase composition, flowability and density of powders were analyzed. The sphericity and flowability of powders treated by plasma flame are increased greatly, and the particle surface is very smooth. The flowability and density of powder with nano SiC were evident better than those of powder with submicron SiC. The optimum flowability and compactness of powder with submicron SiC is obtained when the critical plasma spray parameter is 341 and 325, respectively. For nano size SiC, the optimum flowability and the maximum compactness of powders are obtained with critical plasma spray parameter of 341. The grain size of powders is increased after heat treatment and plasma spraying. The SiC is oxidized to SiO₂ in the powders after heat treatment and plasma spraying. The Y₂O₃ dissolved from 8YSZ solid solution at higher critical plasma spray parameter. Besides, there is no phase transformation of ZrO₂ for powders. The metastable phase of Al₂O₃ appeared in feedstocks with submicron SiC, but no metastable phase was formed in feedstocks with nano SiC particles, which nano SiC can hinder the formation of Al₂O₃ metastable phase. The densification process and mechanism of reconstituted particles used for plasma spraying were analyzed from surface morphology, cross section and simulation.     

The effect of MgO additions on the kinetics of hot pressing in Al2O3

The kinetics of hot pressing of Al₂O₃ with and without MgO additives have been measured at 1475 and 1630° C and at 5 to 20 MPa using Al₂O₃ powders of different grain size and using different additive levels. Data obtained within the solid solution regime are interpreted in terms of diffusional creep processes. MgO additions accelerate densification within this regime; the consequent reduction in pore size, and hence in pore drag, can explain the function of MgO as sintering additive for Al₂O₃.     

High Coercive Barium Hexaferrite Powders by Partial Ba2+‐ and Fe3+‐Substitution for Magnetic Sensor Applications

The synthesis of the modified barium hexaferrite (BHF) powders and the adjustment of the magnetic properties for sensoric applications are realized by the glass crystallization technique in the system BaO‐Fe₂O₃‐B₂O₃. Two ways for modification of the BHF‐powders were used. The first possibility is the partial substitution of the oxides in the basic melt by other oxides (e.g. Fe₂O₃ by Al₂O₃ and BaO by SrO). The second way is the variation of the process conditions. The obtained modified powders were analyzed for their crystallographic structure, chemical composition, particle morphology and magnetic properties. The nanoscaled powders with modified magnetic properties open a wide application field. One application is, to use it in flexible powder filled cellulose fiber sensor for position detection in the micro processing technology and micro measurement technology.     

Study of Al2O3 effect on structural change and phase separation in Na2O-B2O3-SiO2 glass by NMR

The effect of Al₂O₃ on the structure change and the phase separation in Na₂O-B₂O₃-SiO₂ glass was investigated using ¹¹B nuclear magnetic resonance (NMR), ²⁹Si MAS NMR, and ²⁷Al MAS NMR together with infrared absorption spectroscopy and field emission scanning electron microscopy (FE-SEM). The results show that the structure change from the introduction of Al₂O₃ contributes greatly to the inhibition of phase separation. First, the introduction of Al₂O₃ imparts an ionic character to the boron-oxygen network, resulting in the formation of B-O-Al-O-Si bonds and thus increases the compatibility of the silicon network with the boron-oxygen network. Second, the addition of Al₂O₃ causes the sodium ion to transfer from the boron-oxygen network to AlO₄ tetradedra, changing a number of four-coordinated borons into three-coordinated borons. As the bond energy of the four-coordinated boron is weaker than that of the three-coordinated boron, the -B-O-Si- bond with the four-coordinated boron in Na₂O-B₂O₃-SiO₂ glasses is easily broken and results in severe phase separation during heat treatment. However, the -B-O-Al- bond with the three-coordinated boron formed in Na₂O-B₂O₃-SiO₂-Al₂O₃ glasses is difficult to be broken due to the high bond energy. In addition, the silicon network in Na₂O-B₂O₃-SiO₂-Al₂O₃ glasses is also strengthened by the addition of Al₂O₃, which prevents [BO] groups from further aggregation. As a result, the tendency of the glass towards phase separation is greatly suppressed in the Na₂O-B₂O₃-SiO₂-Al₂O₃ system.     

Effect of calcination on characteristics and sintering behaviour of Al2O3-ZrO2 composite powders

The effect of calcination on the characteristics and sintering behaviour of zirconia-toughened alumina (ZTA) composite powders has been investigated. TiO₂ was selected as an additive to promote the sinterability of ZTA powders. The starting materials were Al₂O₃ powder, Zr(OC₃H₇)₄ and Ti(OC₃H₇)₄, and homogeneous ZTA powder containing Zr-O-Ti bonding was prepared. Calcination affected the tetragonalmonoclinic phase transformation temperature of ZrO₂ crystallizing from the gels. Calcination improved the densification rate of ZTA powder compact during sintering, which was attributed to the optimal ZrO₂ particle size and distribution on the surface of alumina. A ZTA specimen with high bulk density and high tetragonal ZrO₂ content was obtained under the conditions of 850°C/1 h calcination and 1500°C/1 h sintering.     

The structure of plasma-prepared Al2O3-TiO2 powders

The morphology and phase constitution of sub-micron Al₂O₃-TiO₂ powders prepared by oxidation of mixtures of Al₂Br₆ and TiCl₄ in an oxygen-argon high-frequency plasma have been studied. The particle size and distribution were consistent with formation of liquid particles by rapid nucleation and surface reaction followed by growth by coalescence of droplets. The particle size of the powders is related to the concentration of reactants in the gas stream and the temperature difference between condensation and solidification. A metastable solution of TiO₂ in -Al₂O₃ was formed in the range 0 to 7 wt% TiO₂, at higher TiO₂ concentrations particles consisted of a dispersion of rutile particles (~ 10 nm) within single crystals of -Al₂O₃. A metastable phase identified as 3Al₂O₃·TiO₂ was also formed in powder with compositions in the range 14 to 40 wt% TiO₂. Over the composition range 40 to 80 wt% TiO₂ the powder consisted predominantly of crystals with a two-phase Al₂TiO₅-rutile structure. Pure TiO₂ consisted largely of anatase and the addition of Al₂O₃ resulted in the formation of rutile as the major phase. The phase constitution of the powders is interpreted in terms of the nucleation kinetics of the various phases.     

Sol–gel processing and phase characterization of Al2O3 and Al2O3/SiC nanocomposite powders

Monolithic Al₂O₃ and Al₂O₃/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the precipitation of Al(NO₃)₃·9H₂O with NH₄OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al₂O₃. The ²⁷Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al₂O₃ formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al₂O₃) is marked by the abrupt disappearance of tetrahedral Al. Al₂O₃/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the ²⁹Si NMR spectra of the Al₂O₃/SiC powders reveal α-SiC, Al₂O₃·xSiO₂ and SiO₂ phases. Stable α-Al₂O₃ and α-Al₂O₃/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al₂O₃ in the nanocomposites by stabilising the Al₂O₃ phases with concomitant oxidation of SiO₂.     

Corrosion properties of plasma-sprayed Al2O3-TiO2 coatings on Ti metal

Three different oxides of CrO₂-TiO₂, Al₂O₃ and Al₂O₃-TiO₂ were plasma-sprayed on Ti substrate to evaluate the crystal structure and the corrosion properties of the coatings. No phase change of the coatings after corrosion test in 0.5 M H₂SO₄ solution at 25°C was found regardless of the presence of the NiCoCrAlY bond layer. Electrochemical measurements and SEM results revealed that the single coatings without the bond layer were always effective against corrosion resistance due to lower current density within the passive region. Pitting corrosion of the surface was observed for the Al₂O₃ coating. It can be concluded that the Al₂O₃-TiO₂ coating without the bond layer may be the best oxide among the oxides investigated due to low porosity (5.4%), smooth surface roughness (4.5 μm), low current density (6.3 × 10^‒8 A/cm²) in the passive region, low corrosion potential (E_corr, ‒0.55 V) and no pitting corrosion.