Emilija Tkalčec

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O2C8H15)2) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P ≤ 1.67, β-tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca10−2x/3[(PO4)6−x(CO3)x][(OH)2−x/3−2y(CO3)y]). A release of CO2 substituting PO43− occurs between 900°C and 1100°C yielding carbonate apatite, Ca10(PO4)6[(OH)2−2y(CO3)y], whereas CO2 substituting OH− groups in the apatite structure is released above 1200°C. In sols with Ca/P ≥ 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca10−2x/3[(PO4)6−x(CO3)x](OH)2 is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO2) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.

Correlation of the precursor type with densification behavior and microstructure of sintered mullite ceramics

Abstract The effect of alumina component in diphasic mullite precursors containing alkoxy-derived silica on the crystallization and sintering behavior of compacts was studied. The phenomena observed were characterized using differential thermal analysis (DTA), powder X-ray diffraction (XRD), dilatometry and transmission and scanning electron microscopy (TEM, SEM). In order to change the characteristics of as-prepared gels, the alumina source was varied while keeping the silica source constant. Al(NO 3 ) 3 ·9H 2 O, γ-Al 2 O 3 and boehmite (γ-AlOOH) were used as the alumina source and TEOS as the silica source. Clear differences were found in the microstructure of sintered samples derived from the precursors with aluminum nitrate nonahydrate in comparison to the samples containing γ-Al 2 O 3 or boehmite (γ-AlOOH). The former exhibited elongated mullite grains embedded into the “equiaxial mullite matrix”. This morphology is due to the overlapping of mullite crystallization and viscous flow sintering temperatures. Transient alumina, either added as γ-Al 2 O 3 or formed in situ by decomposition of boehmite, shifts the mullite formation above the sintering temperature, and enables formation of equiaxial mullite. The smaller are the transient alumina particles, the smaller are mullite grains of sintered bodies.

Crystallization kinetics of mullite from single-phase gel determined by isothermal differential scanning calorimetry

Abstract Transformation kinetics of single-phase gel with mullite composition was studied by isothermal differential calorimetry (DSC) in temperature range from 937 to 959°C. Single exotherm was observed for annealing temperatures below 947°C, and two overlapped exothermic peaks were seen above this temperature. According to XRD analysis, mullite was the only phase crystallized either under non-isothermal or isothermal heat treatment. Johnson-Mehl-Avrami (JMA) equation for nucleation and growth could not describe mullite crystallization adequately, even below 947°C. Using bimodal JMA-type model, that proposes mullite crystallization in two steps, the fitting was remarkably good in the whole temperature range. Obtained kinetics data do not allow one to characterize the gel, either as typical single phase one (nucleation-controlled process with two rate constants and small apparent activation energies), or as hybrid gel (mullite formation via spinel and high apparent activation energies). The rate constants were an order of magnitude smaller than is proposed for single phase gel. The apparent activation energies, however (Ea1 = 1053 ± 51 kJ/mol, and Ea2 = 1028 ± 22 kJ/mmol, were in great discrepancy to those already cited for single phase gels, but they were in very good agreement with data evaluated for diphasic and hybrid gels. Mullite a-axis length and effective fraction of mullite that is formed in the first and second step of the process provided an insight in the mechanism of mullite crystallization. It is assumed that not the nucleation and crystallization limitations, rather the phase separation is the controlling process in mullite formation from single phase gel under applied experimental conditions.

Inverse spinel structure of Co-doped gahnite

Abstract Powder ZnAl2O4 (gahnite) samples doped with 0-100 at% Co were obtained by a sol-gel technique. X-ray powder diffraction was used to characterize the samples. Gahnite samples are cubic with the normal spinel structure, space group Fd3̄m. Cobalt doping caused a nonuniform increase of unit-cell parameter. The structure of the gahnite samples was refined by the Rietveld method. The location of Co2+ was determined by EPR spectroscopy. Cobalt doping of gahnite induces the inverse spinel structure at only 4 at% Co, and the inversion parameter increases with Co2+ doping level. Metal-oxide distances in the (Al,Co)O6 octahedra dominantly influence the unit-cell parameter of Co-doped gahnite

Crystallization kinetics of mullite formation in diphasic gels containing different alumina components

Abstract The crystallization kinetics of mullite formation in diphasic gels containing TEOS-derived amorphous silica and alumina component in different crystalline form and particle size has been studied using quantitative X-ray diffraction (QXRD) with 3:2 mullite as a standard and transmission electron microscopy (TEM). As the source of alumina component, aluminum nitrate nonahydrate, commercial γ-Al2O3 and boehmite (γ-AlOOH) were used. The results were consistent with previous studies which indicated that mullite forms initially by nucleation and growth. During the second slow stage of transformation, mullite with approximate 2:1 composition subsequently converts further into 3:2. This process is the fastest in the precursor with the smallest particle size of alumina, and the slowest in the sample with boehmite-derived alumina. In very fine scale of morphology the conversion of 2:1 mullite started even in the first stage of transformation.

Microstructure and mechanical properties of slip cast sol-gel derived mullite ceramics

Abstract Mullite ceramics were processed by pressureless sintering (1620 °C, 2h) of slip cast mullite derived from single-phase gel calcined at 1070 °C and attrition milled with ZrO 2 balls. The four-point bending strength was determined from room temperature up to 1400 °C. Creep behaviour in compression was determined at stresses of 20 and 100 MPa and temperatures of 1200 to 1450 °C. Microstructural and microchemical characterisation of mullite material was performed using scanning (SEM) and high resolving transmission electron microscopy (HRTEM) in conjuction with energy dispersive X-ray spectrometry (EDX). A remarkable abrasion of ZrO 2 , and the leaching of magnesium from ZrO 2 grains occurred in milling process. The bending strength and the creep behaviour is determined by residual glassy phase, observed in triple points and at the grain boundaries of mullite/mullite and mullite/ ZrO 2 grains. When ZrO 2 grain participates in the formation of triple points, the composition of the glassy phase differs from that at the junction of three mullite grains. The EDX line microanalyses across the mullite/mullite grain boundaries revealed 2 -rich glassy film. There are some grain boundaries at which only a compositional gradient of SiO 2 occurred. The creep below 1300 °C is most likely controlled by grain boundary sliding accommodated by solution-precipitation mechanism, and above by the grain boundary sliding accommodated by viscous flow of SiO 2 -rich phase.

Cobalt incorporation in mullite

Abstract Mullite samples doped with cobalt were derived from diphasic gels with constant atomic ratio (Al + Co)/Si = 3:1, where 0, 1, 2, and 3 at% of aluminum was replaced by cobalt. X-ray powder diffraction showed that the samples contained mullite phase and some amount of α-Al2O3 (for pure and doped samples) and CoAl2O4 (for doped samples). Cobalt doping caused an increase in unit-cell parameters of the mullite phase. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used for sample microanalysis and determination of the chemical composition of the Co-doped mullite phase. The Rietveld method was performed for quantitative phase analysis of the samples and for structure refinement of the mullite phase in the samples. It was found that a small amount of Co2+, 0.36 wt%, substituted for Al3+ in the AlO6 octahedra of the mullite structure. Simultaneously, the same amount of tetrahedral Al3+ ions was likely substituted by Si4+ in the (Al,Si)O4 tetrahedra for the purpose of charge compensation. The remaining cobalt reacted with alumina forming CoAl2O4, and dissolved in the glassy phase. The proposed formula for Co-doped mullite is CoyAl4+2x-2ySi2-2x+yO10-x

Structural changes of mullite precursors in presence of polyethyleneimine

Abstract Thermal properties of two powders derived from the same calcined gel with a stoichiometric mullite composition (atomic ratio Al/Si =3/1) wet milled in different solutions were studied by simultaneous differential thermal analysis, thermogravimetry and evolved gas analysis (DTA,TG,EGA, respectively), by density measurements and by 29 Si MAS NMR spectroscopy and X-ray diffraction (XRD). Calcined gel was milled either in ethanol or in ethanol with the addition of 0.03 g of polyethyleneimine (PEI) per gram of mullite. DTA, TG and mass spectrometry revealed esterification of unhydrated surface of particles formed by milling in ethanol. The difference in the structural evolution of powders and their dependence on annealing temperature seen in 29 Si MAS NMR spectra and XRD patterns is attributed to different degrees of segregation of alumina and silica components, and to differing compositions of transitory spinel phase. In both samples the segregation of silica and alumina was observed at temperatures less than that for mullite crystallization. The degree of segregation was much smaller in the sample milled in ethanol. Therefore, in the latter sample spinel with larger silica content incorporated in γ-Al 2 O 3 will crystallize by annealing at 900°C, and Al-rich mullite at 1100°C. Due to greater segregation of silica and alumina component in the presence of PEI, the crystallization of spinel with smaller amounts of silica incorporated into γ-Al 2 O 3 is shifted to 1100°C, and orthorhombic mullite to about 1200°C. The influence of PEI is indicated by a larger sintering density at lower temperatures.

Highly Porous Hydroxyapatite Ceramics for Engineering Applicatios

Highly porous hydroxyapatite (Ca10(PO4)6(OH)2, HA) was prepared through hydrothermal (HT) transformation of aragonitic cuttlefish bones (Seppia Officinalis L. Adriatic Sea) in the temperature range from 140°C to 220°C for 20 minutes to 48 hours. Mechanism of hydrothermal transformation of bones was investigated by DTA/TG analyzer coupled online with FTIR spectrometric gas cell equipment (DTA-TG-EGA-FTIR analysis), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). DTA-TG-EGA-FTIR analysis have shown the release of CO2 at about 400°C, 680°C and 990°C. The first release could be attributed to organics not completely removed from the heat treated bones, and the second release to decomposition of unconverted aragonite, whereas, the third one could be attributed to CO3 2– groups incorporated in the structure of HA. The interconnecting porous morphology of the starting material (aragonite) was maintained during the HT treatment. The formation of dandelion-like HA spheres with diameter from 3 to 8 μm were observed, which further transformed into nanoplates and nanorods with an average diameter of about 200-300 nm and an average length of about 8-10 μm.

Crystallization of high-quartz solid solution in gahnite glass-ceramics

Abstract The crystallization of ZnO–(MgO)–Al2O3–B2O3–SiO2 glasses, having molar ratio ZnO/Al2O3 = 1 and variable content of MgO, with TiO2 or ZrO2 or both as nucleating agents, has been studied. Phase composition and microstructure of the obtained glass-ceramics were examined by XRD, SEM, EDX and TEM analyses. The main crystalline phase obtained is gahnite, ZnAl2O4. High-quartz solid solution, as an additional phase, crystallizes in the glasses containing ZrO2 or ZrO2 + TiO2 as nucleating agents. When ZrO2 is present alone, the unit cell parameter, a, of high-quartz solid solution decreases linearly with temperature. However, if TiO2 is added as another nucleating agent, a non-linear dependence of the parameter, a, on temperature occurs. Inside the primary separated droplet-shaped phase, from which gahnite precipitates, B2O3 is concentrated in the centre and extends radially to the phase boundary, inducing a specific microstructure of gahnite.