Crystallisation kinetics of hydroxyapatite thin films prepared by sol-gel process

Abstract

Abstract

In this study, the crystallisation of nano hydroxyapatite (HA) films on stainless steel 316L was studied. The film was prepared by sol-gel technique. The process was started with preparation of an HA sol. After aging of the sol at room temperature, a stainless steel 316L substrate was dip coated and then was heat treated from 350 to 450°C at different periods of time in air. The crystallisation behaviour and the transformation-temperature-time diagram of HA films were achieved and analysed using the avrami equation. The results showed that the crystallisation of HA began at 250°C and was increased up to 450°C. The obtained HA film showed a nanostructure character with a suitable crystalinity after heat treatment.     

Optimizing the processing conditions of sodium potassium niobate thin films prepared by sol-gel spin coating technique

In the present study, potassium sodium niobate (KNN) thin films were synthesized by means of sol-gel spin coating method. Along with the synthesis, the effects of annealing temperature and various number of coating layers on both the structural and electrical properties were looked into. The results of the study revealed that the annealing temperature had a great impact on the properties of KNN. In addition, the XRD diffractograms and texture coefficient of the synthesized films confirmed that a highly oriented orthorhombic perovskite structure was obtained at 650 °C, whereas at a relatively higher temperature (700 °C), a spurious phase of K₄Nb₆O₁₇ was evolved. In addition, the growth of KNN at 650 °C exhibited a reasonable resistivity value for piezoelectric applications. Looking into the results, it was discovered that the KNN thin films also found to be dependent on a number of coating layers. Field emission scanning electron microscopy (FESEM) showed that KNN with five coating layers was highly crystalline, cracks-free, and had significantly more homogenous surface morphology and the size of grains being uniform, the resistivity of KNN thin films improved with the increasing number of coating layers i.e., up to five.     

溶胶-凝胶法制备疏水型SiO2薄膜的研究

采用溶胶-凝胶工艺,以正硅酸乙酯（TEOS）为前驱体,结合三甲基氯硅烷（TMCS）对胶粒的修饰作用,利用浸渍提拉法在玻璃表面制备了具有一定疏水能力的SiO2薄膜.考察了TMCS的掺杂量、醇硅比、加水量及热处理温度对薄膜疏水性的影响.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Pd-based catalysts for catalytic wet oxidation of combined Kraft pulp mill effluents in a trickle bed reactor

Catalytic wet oxidation of combined pulp mill effluents was studied in a trickle bed reactor. Two types of supported Pd-based catalysts, eggshell and uniform catalysts, and a supported Pd–Pt uniform catalyst were employed. Supported Pd catalysts showed high activity for total organic carbon (TOC) and color removal at temperatures of 353–448 K and 1.84 MPa. Compared to uniform catalysts, the eggshell catalysts with a 0.2 wt.% Pd loading show promise for treatment of combined pulp mill effluents. High activity for TOC and color removal can be attributed to the high density of active Pd sites in the shell area and to the decrease of diffusion path for reactants and intermediates. No apparent deactivation of eggshell catalysts was observed after 40 h on stream. Potential application of the supported Pd catalysts for treatment of combined pulp mill effluents is discussed.     

Parameter setting on growth of carbon nanotubes over transition metal/alumina catalysts in a fluidized bed reactor

This work investigates the synthesis of multilayered carbon nanotubes (CNTs) using the catalytic decomposition of acetylene at 700-850 °C over Fe- and Ni-supported Al₂O₃ catalysts in a fluidized bed reactor. Thermogravimetric analysis showed that the CNTs grown in a fluidized bed reactor have better thermal stability and higher production yield, compared to that in a fixed bed reactor. The CNT production yield increased with the growth temperature, and Fe-catalyst exhibited greater activity than Ni-catalyst in the formation of CNTs. According to Arrhenius plots, the apparent activation energies for the growth of CNTs were estimated to be 25.6 kJ/mol for Fe-catalyst and 65.6 kJ/mol for Ni-catalyst. The as-grown CNT products were characterized by high-resolution transmission electron spectroscopy, N₂ physisorption, Raman spectroscopy, and X-ray diffraction. After purification, the CNT products were of the multilayered type, which were composed of perfect graphene layers. The results of this study demonstrate that the fluidized bed technology favors the large-scale production of CNTs with uniformity and at low cost.     

A model for Kolbe electrolysis in a parallel plate reactor

A parallel-plate reactor model is developed for the Kolbe electrolysis of acetate to ethane and carbon dioxide with hydrogen evolution as the counterelectrode reaction. The parallel-plate reactor is considered to consist of three zones: a turbulent bulk region in which streamwise convection is the dominant mass-transport mechanism (plug-flow model) and a thin diffusion layer at each electrode where diffusion and migration mass transport are dominant (Nernst diffusion-layer model). The acetic acid solution is supported with sodium hydroxide, and the reactor is under steady cell-potential control. Gaseous products are tracked by a hypothetical gas layer which increases in thickness in the streamwise direction. The gas phase is assumed to be an ideal, three-component mixture of hydrogen, carbon dioxide and ethane; the liquid phase consists of acetate, proton, acetic acid, and sodium and hydroxyl ions. The model predicts streamwise profiles of concentration, current density, gas-void fraction, and gas and liquid velocities in addition to reactant conversion, and cell-polarization characteristics. The average current density exhibits a maximum at a base-to-acid ratio of 0.96 due to the weak-acid/strong-base chemistry and a broad maximum at an interelectrode spacing of 0.37 cm resulting from minimized ohmic losses.     

Characterization of indium tin oxide (ITO) thin films prepared by a sol-gel spin coating process

Indium tin oxide (ITO) thin films were prepared by a sol-gel spin coating method, fired, and then annealed in the temperature range of 450-600°. The XRD patterns of the thin films indicated the main peak of the (2 2 2) plane and showed a higher degree of crystallinity with an increase in the annealing temperature. Upon annealing the films at 500 and 600°, two binding energy levels of Sn⁴⁺ ion of 486.9 eV and 486.6 eV, respectively, were measured in the XPS spectra. The ITO film that was annealed at 600° contained two oxidation states of Sn, Sn²⁺ and Sn⁴⁺, and it had a higher sheet resistance based on a rather low doping concentration of Sn⁴⁺. The film that was annealed at 500° and subsequently treated with 0.1 N HCl solution for 40 s showed a sheet resistance of 225 Ω/square. The surface treatment by the acidic solution diminished the RMS (root mean square) roughness value and the residual carbon content (XPS peak intensity of carbon) of the ITO films. It seems that the acid-cleaning of the ITO thin films led to a decrease of the surface roughness and sheet resistance.     

Magnetic and electrical properties of single-phase multiferroic (1-x)Pb(Zr0.52Ti0.48)O3–xPb(Fe0.5Nb0.5)O3 thin films prepared by sol-gel route

Single-phase multiferroic (1-x)Pb(Zr_0.52Ti_0.48)O₃-xPb(Fe_0.5Nb_0.5)O₃ (0≤x≤0.5) thin films were synthesized by sol-gel route and characterized to understand their structural, electrical, and magnetic properties. The films were thermally treated by conventional furnace (CFA) and rapid thermal annealing (RTA). A pyrochlore-free perovskite phase is stabilized only by RTA in samples with high Fe³⁺/Nb⁵⁺ content. The films displayed excellent dielectric and ferroelectric properties in the whole concentration range, with saturated hysteresis loops and remanent polarization values of ～15μC/cm². Films with x>0.3 showed ferromagnetic behavior at room temperature. Consequently, the multiferroic behavior in the films occurs in a different concentration range than that observed in bulk ceramics. The origin of the weak ferromagnetism is discussed.     

Entrance effect on mass transfer in a parallel plate electrochemical reactor

The influence of different fluid inlet types, slits or tubes, on mass transfer in a rectangular reactor was studied. Measurements of mass transfer coefficients were made using the limiting diffusion current technique based on ferricyanide ion reduction at a large nickel electrode located downstream of abrupt expansions. The overall mass transfer coefficients obtained were 3 to 13 times greater than those obtained in fully developed flows. Overall mass transfer coefficients were correlated for Reynolds numbers ranging from 400 to 3500 by a unique equation by introducing a nondimensional expansion factor defined by the ratio of the fluid inlet cross-section to that of the reactor. The correlation equation obtained was compared with literature data.     

Effect of sulphur poisoning on perovskite catalysts prepared by flame-pyrolysis

ABO₃ perovskite-like catalysts are known to be sensitive to sulphur-containing compounds. Possible solutions to increase resistance to sulphur are represented by either catalyst bed protection with basic guards or catalyst doping with different transition or noble metals. In the present work La_(1−x)A^′_xCoO₃, La_(1−x)A^′_xMnO₃ and La_(1−x)A^′_xFeO₃, with A′ = Ce, Sr and x = 0, 0.1, 0.2, either pure or doped with noble metals (0.5 wt% Pt or Pd), were prepared in nano-powder form by flame-pyrolysis. All the catalysts were tested for the catalytic flameless combustion of methane, monitoring the activity by on-line mass spectrometry. The catalysts were then progressively deactivated in operando with a new procedure, consisting of repeated injection of some doses of tetrahydrothiophene (THT), usually employed as odorant in the natural gas grid, with continuous analysis of the transient response of the catalyst. The activity tests were then repeated on the poisoned catalyst. Different regenerative treatments were also tried, either in oxidising or reducing atmosphere.Among the unsubstituted samples, higher activity and better resistance to poisoning have been observed in general with manganites with respect to the corresponding formulations containing Co or Fe at the B-site. The worst catalyst showed LaFeO₃, from both the points of view of activity and of resistance to sulphur poisoning. La_0.9Sr_0.1MnO₃ showed, the best results, exhibiting very high activity and good resistance even after the addition of up to 8.4 mg of THT/g of catalyst. Interesting results were attained also by adding Sr to Co-based perovskites. Sr showed a first action by forcing Mn or Co in their highest oxidation state, but, in addition, it could also act as a sulphur guard, likely forming stable sulphates due to its basicity. Among noble metals, Pt doping proved beneficial in improving the activity of both the fresh and the poisoned catalyst.     

Effect of rare earth doping on sol-gel derived PZT thin films

We have studied the effect of rare earth dopants (Nd, Gd and Ce) on the phase formation behavior and electrical properties of sol-gel derived Pb_1.05(Zr_0.53Ti_0.47)O₃ thin films. In all these films the perovskite phase is obtained up to 5 at% doping and beyond that pyrochlore phase was found to coexist with the perovskite phase. Ce and Gd doping(1-2 at%) exhibited improved ferroelectric and dielectric properties as compared to the undoped PZT films. Nd doping (2 at%) was found to be effective to increase the retained switchable polarization of undoped PZT from 63% to 84%. The transition temperature of undoped PZT film was found to be reduced with Nd doping. The Nd doped films also exhibited typical relaxor behavior and a diffuse phase transition, characteristic of the relaxor material. Introduction of Nd into the PZT lattice probably introduces disorder in the B site of ABO₃ lattice which causes the observed relaxor behavior     

Synthesis and thermal expansion behaviors of spin-coated Sc2Mo3O12 thin films

Orthorhombic Sc₂Mo₃O₁₂ films have been successfully prepared via spin coating technique followed by annealing at 500–750 °C. The phase composition, microstructure, morphology and negative thermal behavior of the synthesized Sc₂Mo₃O₁₂ films were investigated. XRD and XPS analysis indicate that as-deposited film is amorphous. Orthorhombic Sc₂Mo₃O₁₂ films can be prepared after post-annealing at 500–750 °C for 1 h. The crystallinity of Sc₂Mo₃O₁₂ films gradually improved with the increase of post-annealing temperature. SEM analysis shows as-deposited film is smooth and compact, and the grain size of Sc₂Mo₃O₁₂ film grows up as the post-annealing temperature increases. Variable temperature XRD analysis demonstrates that the synthesized orthorhombic Sc₂Mo₃O₁₂ films show stable thermo-chemical and anisotropic NTE property in 25–700 °C. The corresponding coefficients of thermal expansion (CTEs) of the orthorhombic Sc₂Mo₃O₁₂ film in a, b and c directions are ?6.68 × 10^?6 °C^?1, 5.08 × 10^?6 °C^?1 and ?4.76 × 10^?6 °C^?1, respectively. The whole unit cell of the orthorhombic Sc₂Mo₃O₁₂ film shrinks and the volumetric CTE of the Sc₂Mo₃O₁₂ thin film is ?6.36 × 10^?6 °C^?1, and the linear CTE is about ?2.12 × 10^?6 °C^?1 (α_v = 3α_l).     

Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/γ-alumina, CuAl₂O₄/γ-alumina, NiO/γ-alumina, NiAl₂O₄/γ-alumina and bulk CuAl₂O₄ have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature of 413 K. The copper and nickel catalysts supported on γ-alumina have surface areas of the same order as the support γ-alumina of ca. 190 m²/g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m²/g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/γ-alumina and NiO/γ-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/γ-alumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation reaction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of CO₂ formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process.     

Wood surface modification by in-situ sol-gel deposition of hybrid inorganic–organic thin films

Interest in the use of nanoparticles of iron, titanium, aluminum, and zinc oxides in transparent coatings for wood is increasing. Such nano-composite coatings have the potential of not only preserving the natural color of the wood, but also stabilizing the wood surface against the combined degradative effects of sunlight and moisture. The nanoparticles can be used as additives to coating formulations or deposited directly as thin films on a substrate. Thin film deposition can be accomplished by plasma-enhanced chemical vapor or by sol-gel deposition. This paper describes sol-gel deposition of a hybrid inorganic–organic thin film on wood using a mixture of metal–organic precursors and its effect on weathering properties of the wood surface. Presented at the 5th International Woodcoatings Congress: Enhancing Service Life, sponsored by the Paint Research Association (PRA), in Prague, Czech Republic, on October 17–18, 2006.     

Optimization and kinetic modeling of waste lard methanolysis in a continuous reciprocating plate reactor

Continuous biodiesel production from a waste pig-roasting lard, methanol and KOH was carried out in a reciprocating plate reactor (RPR) using a factorial design containing three process factors, namely methanol/lard molar ratio, catalyst loading, and normalized height of the reactor. The main goals were to optimize the influential process factors with respect to biodiesel purity using the response surface methodology and to model the kinetics of the transesterification reaction in order to describe the change of triacylglycerols (TAG) and fatty acid methyl esters (FAME) concentrations along the RPR height. The first-order rate law was proved for both the reaction and the mass transfer. The model of the changing reaction mechanism and mass transfer of TAG was also applicable. Both kinetic models agreed with the experimental concentrations of TAG and FAME determined along the RPR height.     

Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 μm and peeled away from the substrate due to the difference in thermal expansion between SiO₂ and Al₂O₃ at temperatures above 1500 °C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 μm to Ra 1.83 μm, and no peeling was observed when heated to 1550 °C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by sol-gel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 °C and withdraw rate of 5 mm/min. While a uniform, 4–5 μm thick HfO₂ reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.     

Low-temperature oxidation resistance of the silica-coated MoSi2 powders prepared by sol-gel preoxidation method

MoSi₂ has been regarded as one of the most promising structural materials, due to its excellent high-temperature oxidation resistance and high emissivity. However, as the most commonly used emittance agent in high emissivity coatings, it can be easily oxidized when the operating temperature is below 600 °C. To address this issue, the silica-coated MoSi₂ powders with good low-temperature (400–600 °C) oxidation resistance were fabricated by a sol-gel method followed by a preoxidation process. Here in, by the combination of the non-isothermal and cyclic-isothermal oxidation analysis, we demonstrate that the silica gel film coated on MoSi₂ can prevent oxidation to some extent, but the preoxidation treatment plays a more critical role in improving its oxidation resistance at low temperature. Both the sol-gel and preoxidation processes enable the integrated and compact encapsulation of the silica coating. The synthesized silica-coated MoSi₂ exhibits excellent oxidation resistance with slightly weight gains (<1 wt%) after cyclic-isothermal oxidation at 400–600 °C for 12 h.     

High dielectric tunability with high thermal stability of the (111) highly oriented 0.85Pb(Mg1/3Nb2/3)-0.15PbTiO3 thin film prepared by a sol-gel method

Ferroelectric thin films with high tunability, low dielectric loss, high figure-of-merit (FOM) and high thermal stability are attractive to military and civil tunable devices such as phase shifters, filters, etc. In this work, the (100), (110) and (111) highly oriented 0.85Pb(Mg_1/3Nb_2/3)-0.15PbTiO₃ (PMN-15PT) relaxor ferroelectric thin films were prepared by a sol-gel method. The (111) highly oriented PMN-15PT thin film has high dielectric tunability (η) value of ∼ 80% at 1500 kV/cm, high thermal stability (±2% variation) from room temperature to ∼ 450 K and the dielectric tunability is independent from 500 Hz to 10 kHz. The high dielectric tunability of the (111) highly oriented PMN-15PT thin film is associated to the reorientation of polar nanoregions (PNRs) and intrinsic lattice phonon polarization. Meanwhile, the excellent thermal stability may be attributed to the large dielectric diffuseness degree (γ ∼ 1.81) derived from the modified Curie-Weiss law. It is believed that the (111) highly oriented PMN-15PT thin film is an attractive material in the application of the next-generation tunable devices.     

Modelling propane dehydrogenation in a rotating monolith reactor

The catalytic dehydrogenation of propane is equilibrium limited, strongly endothermic and normally carried out at high temperatures. The catalyst deactivates due to the laydown of carbonaceous species on the surface. This is conventionally countered by subjecting the catalyst to periodic regeneration. In commercially available processes, the catalyst time on line for a given cycle is in the order of 10–10,000 min.

In this study, the catalyst has been observed to exhibit very high activity and selectivity in the short period after regeneration. Conceptual and model development of a reactor with structured catalyst to capitalise on this beneficial early activity is presented.

The preferred reactor comprises a cylindrical block of honeycomb monolith that rotates past various feed zones, subjecting the catalyst successively to propane and regenerating gas. The exothermic nature of the regeneration reactions is used at least in part to provide heat to the endothermic dehydrogenation reaction via the regenerative heat transfer facilitated by the movement of the solid monolith. Specifically, it is noted that an oxidisible catalyst provides operating advantage due to the additional exotherms associated with the regeneration stage.

The process modelling shows the design to be feasible in terms of matching the heats of reactions and achieving high conversions, but questions are raised over its practicability from mechanical design and process stability viewpoints.