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Iron nitride (Fe3N) and iron carbide (Fe3C) nanoparticles can be prepared via sol−gel synthesis. While sol−gel methods are simple, it can be difficult to control the crystalline composition, i.e., to achieve a Rietveld-pure product. In a previous in situ synchrotron study of the sol−gel synthesis of Fe3N/Fe3C, we showed that the reaction proceeds as follows:
Fe3O4 → FeOx → Fe3N → Fe3C. There was considerable overlap between the different phases, but we were unable to ascertain whether this was due to the experimental setup (side-on heating of a quartz capillary which could lead to thermal gradients) or whether individual particle reactions proceed at different rates. In this paper, we use in situ wide- and small-angle X-ray scattering (wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS)) to demonstrate that the overlapping phases are indeed due to variable reaction rates. While the initial oxide nanoparticles have a small range of diameters, the size range expands considerably and very rapidly during the oxide−nitride transition. This has implications for the isolation of Rietveld-pure Fe3N, and in an extensive laboratory study, we were indeed unable to isolate phasepure Fe3N. However, we made the surprising discovery that Rietveld-pure Fe3C nanoparticles can be produced at 500 °C with a sufficient furnace dwell time. This is considerably lower than the previous reports of the sol−gel synthesis of Fe3C nanoparticles.
Modified Yoldas sols can be used to prepare crack-free well adherent layers on different substrate materials such as sodalime glass, corundum and metal (IN-718). These layers having thicknesses of 0.6 µm are strain tolerant and withstand thermal cycling between 810 °C and room temperature. Introducing alpha-Al2O3 filler particles into modified Yoldas sols allows the preparation of thicker layers of 1.6 µm, which as well can be thermally cycled without noticeable delaminations. Chemical bonding seems to be the predominant layer bonding mechanism.
Improved wetting of bare and pre-coated steels by aqueous alumina sols for optimum coating success
(2010)
Alumina coatings are a promising candidate for
the protection of metals prone to high temperature corrosion.
If applied via sol–gel process, especially by using so
called aqueous modified Yoldas-sols, the deposition is
hardened by differences in surface free energy of substrate
and sol. In this paper the apparent surface properties of
samples to be coated and sols were examined by contact
angle measurements, IR-spectroscopy and tensiometry.
The results imply, after consideration of possible uncertainties,
the use of surface tension reducing 2-butanol
already present during hydrolysis of the sols to obtain an
easy and successful coating process.
Silica coatings filled with nanoscaled inorganic fullerene-like tungsten disulphide (IF-WS2) have been prepared through a solgel process on stainless steel substrates, and the structure and mechanical properties have been investigated. The precursor was prepared from a mixture of colloidal silica, 3-glycidoxypropyltrimethoxysilane (GLYMO), water and ethanol, adjusted to pH 4 with HNO3. In this solution WS2 is dispersed and in some cases immediately before coating ethylenediamine (ED) is added. The stainless steel substrates are dip-coated, dried in air and heat-treated in the temperature range of either 150360 °C in air or up to 900 °C in vacuum. The solidification process is followed by differential thermal analysis (DTA). The resulting brown coloured coatings have a thickness of 1.54 µm. Scanning electron microscopy investigations (SEM) show that the WS2 nanoparticles are embedded as small aggregates in a hybrid silica matrix. X-ray diffraction (XRD) measurements prove that most of the tungsten disulphide embedded in the matrix can be protected against oxidation even after curing the samples at temperatures up to 900 °C. Hardness and modulus of the hybrid silica films were measured through an instrumented indentation test. Increasing the temperature of the heat treatment yields an increase of hardness from 0.3 to 1 GPa and of modulus from 3 to 17 GPa. The amount of up to 10 wt.% WS2 in the coatings has no remarkable influence on hardness and modulus of the samples.
The aluminium oxide films on austenitic steel are prepared from sols of re-dispersed boehmite nano powders in water. After dip-coating of the sol, a heat treatment including drying, calcination and annealing in vacuum at temperatures up to 1100°C is performed to obtain crack-free coatings of a thickness up to 6 µm. XRD measurements detect α- and γ-alumina, a TiOx-phase at the metal/coating interface and a gradient of phase formation in the coating. The strong adhesion on the substrates is due to the layered assembly and gradient composition of the coating caused by an inter-diffusion of metal cations and oxygen in the metal/oxide interface during heat treatment. Residual stress measurements by X-rays result in compressive stresses of 24 GPa in the alumina coatings. The pin-on-disc test shows a remarkable improvement of wear resistance obtained by sol-gel coatings. The α-alumina content and the compressive stress of the coatings correlate with wear resistance of the coatings.
The electrical properties of solgel-derived films can be tailored by embedding conductive particles of ruthenium dioxide or carbon black in an insulating amorphous SiO2 silica matrix. The preparation process included an acid hydrolysis of tetraethoxysilane and methyltrimethoxysilane. Then alcohol solutions of ruthenium chloride or carbon black were added. Films of filler concentration up to 60 vol.% were prepared by dip coating and then dried and heat-treated at various temperatures up to 600_°C. The D.C. resistance of the films can be varied within the range of 109 to 102 OHgr sdot cm. A non-linear dependence on filler composition in the films was observed for both systems, which is explained by a modified percolation theory. A percolation threshold of 5.5 vol.% for SiO2-RuO2 or 50 vol.% for SiO2-C films, whereby the resistance drastically decreases, was determined. Moreover the temperature dependency of resistance and the current-voltage characteristics of the films can also be explained by this geometric model.
The preparation of sol-gel derived silica-based nanomaterials containing electrical conductive carbon fillers in an extensive composition range is described and their electrical properties are presented. Nanomaterials of carbon filler concentrations up to 60% (v/v) were obtained by dip coating or screen-printing from precursors of hydrolysed alkoxysilanes. Nanostructured morphology could be identified to consist of homogeneously dispersed carbon black particles or carbon fibres of 30 to 500 nm in size in a modified silica matrix. The electrical resistivity of the films changes drastically from 1010 to 10?1 O?cm, according to the amount of dispersed conductive particles. A threshold between 5 and 50% (v/v), at which the resistance abruptly decreases, was determined. A geometrical model related to percolation theory explains this non-linear dependence on the filler composition in the materials. Moreover the temperature dependence of resistance and the current-voltage characteristics of the nanomaterials can also be illustrated using this geometric model.
The solgel process was applied to enhance properties of pine sapwood. For this purpose wood prisms were soaked in nanoscaled precursor solutions prepared from titanium(IV) n-butoxide and titanium(IV) iso-propoxide, respectively, using vacuum impregnation technique. The wet composites were cured by special program with final heat treatment at 103 °C. Weight percent gains (WPG) of the wood specimen in the range of 19-25% were obtained due to these procedures. SEM investigations show that precursor solutions penetrate into the whole wood body and the titania formed after heat treatment in the composites is deposited in the pores (lumen) and partly in the cell walls of the wooden matrix. The moisture sorption was investigated in long term tests for a period of some months by storage at 20-23 °C in humid air (relative humidity of 99%) and ambient atmosphere (relative humidity 40-60%), respectively. For untreated reference samples the moisture sorption results in increasing of mass and volume according to saturation values of 24 and 13%, respectively, after about 15 days. The incorporation of titania reduces the saturation values of the moisture sorption by up to 12% in mass and by up to 5% in volume at a relative humidity of 99%. Thus, an enhancement of the dimensional stability of about 60% is obtained at best. The results demonstrate that modification of wood with sol-gel derived precursors can enhance its dimensional stability, which prevents the formation of cracks. Because of that reduced moisture sorption biological attacks should be delimited. Additionally, thermal analyses show a retarded combustion of the wood matrix due to titania infiltration.
MgF2 and TiO2 single layers and layer stacks were produced by a spin-coating sol-gel process. The final temperature treatment was carried out at 100 °C. The layers were deposited onto silicon and fused silica substrates and were analysed by means of atomic force microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ellipsometry, and UV-vis transmission spectroscopy. MgF2 and TiO2 single layers have morphological and optical properties comparable with physical vapour deposited layers. By using spectroscopic mapping ellipsometry, a good inter- and intra-sample homogeneity was confirmed. Multiple deposition steps result in a linear increase of layer thickness. Various films were deposited with thicknesses between 25 nm and 350 nm.
It was shown that the low temperature sol-gel process results in films of optical quality. Anti-reflective and high reflective layer stacks consisting of MgF2 and TiO2 were designed and can be produced now by a sol-gel process, whereas the MgF2 layers in the layer stacks contains also traces of MgF2-2xOx.
Dielectric, anti-reflective or high reflective systems consist of low and high refractive index layers. Common systems are oxides. The preparation of low refractive index MgF2-films of optical quality by means of an anhydrous low temperature solgel synthesis is presented. The MgF2-sol is prepared by spin-coating on silicon and glass substrates. Various film thicknesses between 20 nm and 435 nm have been deposited. It has been shown that the thickness increase is proportional to the number of coating steps. The deposited MgF2-films consist of 10 nm to 20 nm large nanoparticles and have smooth surfaces with an average roughness (Ra) of (1.7 ± 0.3) nm. The optical constants n and k of the films are in agreement with the literature data of bulk-MgF2.