G.N. Angelopoulos

Wettability and interfacial energies in SiC-liquid metal systems

The sessile drop technique is used to measure the contact angles of molten Si, Sn, Cu and Ni in contact with mono- and polycrystalline α-SiC as well as CVD β-SiC in purified argon atmosphere and at various temperatures. The contact angle of silicon, near its melting point, is about 38° on a mono- as well as polycrystalline α-SiC substrate and about 41.5° on β-SiC. Tin does not wet the SiC. Using data from the available literature, the work of adhesion and the interfacial energy between SiC and Si or Sn were calculated. In the α-SiC-Sn system, both quantities are linearly dependent on temperature in the investigated temperature range 523–1073 K. The metals copper and nickel react with silicon carbide. The silicon content of the copper drop depends on the annealing temperature. The nickel drop after cooling forms the compound Ni3Si2. The interferometric measured groove angle of SiC (thermal etching) in vacuum at 2020 K gives a mean value of 157.6±5.8°.

Formation of aluminide coatings on nickel by a fluidised bed CVD process

Abstract The present work concerns the formation and modelling of aluminide diffusion coatings onto nickel and nickel alloys by means of a fluidised bed chemical vapour deposition (FBCVD) process. In this process, the treated object is suspended within the FB reactor, which contains the treating agent. The latter is a mixture of powders of the donor of the coating-forming element (FexAly), and of a filler material (Al2O3). As an activator of the process, a halide compound is used (NH4Cl). The bed is fluidised by inert gas (Ar). The precursor vapours of the element to be deposited are formed in situ, by the reaction of the donor with the activator. The produced coatings are characterised by optical microscopy, SEM and EPMA. The modelling of the nickel–aluminide coating growth on nickel, has been based on the diffusion equations of aluminium in pure nickel, considering the intermetallic compounds predicted by the Ni–Al phase diagram. The model predicts the aluminium composition profiles as functions of the time and the concentration of Al in the donor, for the aluminisation conditions under consideration. The model is in satisfactory agreement with experimental results.

Formation and modelling of aluminide coatings on iron by a fluidised bed CVD process

Abstract Aluminide coatings have been deposited on ARMCO iron and an iron–chromium alloy (15 at.% Cr) by a fluidised-bed chemical vapour deposition (FBCVD) process at 1000 °C with the use of FeAl donor powder and an NH 4 Cl activator. Examination of the coated samples by means of optical microscopy, SEM and microanalysis has shown the formation of dense and homogeneous coatings consisting of an FeAl outer phase and an α-solid solution inner phase. A simplified process model based on the diffusion equations of aluminium in pure iron, considering the intermetallic compounds according to the FeAl phase diagram, has been derived. The model yields the aluminium composition profiles as a function of time for particular process conditions and can be used for evaluation of the coating growth rate and morphology during aluminisation.

Valorisation of electric arc furnace steel slag as raw material for low energy belite cements.

In this paper, the valorisation of electric arc furnace steel slag (EAFS) in the production of low energy belite cements is studied. Three types of clinkers were prepared with 0 wt.% (BC), 5 wt.% (BC5) and 10 wt.% (BC10) EAFS, respectively. The design of the raw mixes was based on the compositional indices lime saturation factor (LSF), alumina ratio (AR) and silica ratio (SR). The clinkering temperature was studied for the range 1280-1400°C; firing was performed at 1380°C based on the results regarding free lime and the evolution of microstructure. In order to activate the belite, clinkers were cooled fast by blown air and concurrent crushing. The results demonstrate that the microstructure of the produced clinkers is dominated by belite and alite crystals, with tricalcium aluminate and tetracalcium-alumino-ferrite present as micro-crystalline interstitial phases. The prepared cements presented low early strength development as expected for belite-rich compositions; however the 28-day results were 47.5 MPa, 46.6 MPa and 42.8 MPa for BC, BC5 and BC10, respectively. These values are comparable with OPC CEMI 32.5 N (32.5-52.5 MPa) according to EN 197-1. A fast setting behaviour was also observed, particularly in the case of BC10, whereas soundness did not exceed 1mm.

Formation of TiC coatings on steels by a fluidized bed chemical vapour deposition process

Abstract The use of a fluidized bed process for the production of hard wear- and corrosion-resistant coatings of carbides, nitrides and carbonitrides of cementing metals is promising since it can lead to a flexible surface treatment technology combining adaptability with relatively low capital and operational costs. In this paper the feasibility of a fluidized bed process for the formation of titanium carbide coatings using chemical-vapour-deposition-based reactions is discussed. The method involves a treating agent consisting of a powder mixture of Al 2 O 3 with FeTi which is fluidized at elevated temperature with Ar. As an activator of the process NH 4 Cl was used. The use of explosive hydrogen is avoided by the formation of the vapours of the materials to be coated in situ in the reactor. With this method, adherent and dense coatings of TiC were applied on various steel grades.

Effect of firing atmosphere and soaking time on heavy clay ceramics with addition of Bayer's process bauxite residue

Abstract Ceramics were developed using Bayers process bauxite residue and a clay mixture in a weight ratio 1 : 1. Firing was performed in a propane firing kiln at 1000°C for 1 or 4 h soaking. For oxidising atmosphere, water absorption varies between 18⋅9 and 19⋅5% and bending strength between 25⋅9 and 27⋅9 MPa. The crystalline phases identified are haematite Fe2O3, quartz SiO2, plagioclase [anorthite (Ca,Na)(Al,Si)2Si2O8], clinopyroxene [augite Ca(Mg,Fe+3,Al)(Si,Al)2O6] and corundum Al2O3. The microstructure is comprised of Si rich vitrified areas that contain quartz and have closed, round pores and an assemblage of particles creating a mass with open and interlinked pores and no signs of glassy phase. For reducing atmosphere and 1 or 4 h soaking, water absorption varies between 18⋅2 and 18⋅7% and bending strength is ∼29⋅5 MPa. The crystalline phases identified are qualitatively the same as for the oxidising atmosphere. The microstructure is comprised of two concentric zones due to the reduced gas permeability in the core of the body. The outer shell is highly vitrified with closed and rounded pores. The microstructure in the core of the body resembles that of samples fired in oxidising atmosphere; however, the average pore size is higher and the vitrified regions are more extensively formed. The build-up of compressive stresses in the outer shell may also account for the increase in the bending strength; however, cracks were developed in few occasions. In all cases, quartz participates in the development of liquid phase and dissolves in the melt. Increase in the soaking time results in a decrease in the quartz content in the body and an increase in the vitreous phase.

Valorisation of different types of boron-containing wastes for the production of lightweight aggregates.

Four boron-containing wastes (BW), named as Sieve (SBW), Dewatering (DBW), Thickener (TBW) and Mixture (MBW) waste, from Kirka Boron plant in west Turkey were investigated for the formation of artificial lightweight aggregates (LWA). The characterisation involved chemical, mineralogical and thermal analyses as well as testing of their bloating behaviour by means of heating microscopy. It was found that SBW and DBW present bloating behaviour whereas TBW and MBW do not. Following the above results two mixtures M1 and M2 were prepared with (in wt.%): 20 clay mixture, 40 SBW, 40 DBW and 20 clay mixture, 35 SBW, 35 DBW, 10 quartz sand, respectively. Two different firing modes were applied: (a) from room temperature till 760 °C and (b) abrupt heating at 760 °C. The obtained bulk density for M1 and M2 pellets is 1.2g/cm(3) and 0.9 g/cm(3), respectively. The analysis of microstructure with electron microscopy revealed a glassy phase matrix and an extended formation of both interconnected and isolated, closed pores. The results indicate that SBW and DBW boron-containing wastes combined with a clay mixture and quartz sand can be valorised for the manufacturing of lightweight aggregates.

The role of nano-perovskite in the negligible thorium release in seawater from Greek bauxite residue (red mud)

We present new data about the chemical and structural characteristics of bauxite residue (BR) from Greek Al industry, using a combination of microscopic, analytical, and spectroscopic techniques. SEM-EDS indicated a homogeneous dominant “Al-Fe-Ca-Ti-Si-Na-Cr matrix”, appearing at the microscale. The bulk chemical analyses showed considerable levels of Th (111 μg g−1), along with minor U (15 μg g−1), which are responsible for radioactivity (355 and 133 Bq kg−1 for 232Th and 238U, respectively) with a total dose rate of 295 nGy h−1. Leaching experiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece, indicated significant release of V, depending on S/L ratio, and negligible release of Th at least after 12 months leaching. STEM-EDS/EELS & HR-STEM-HAADF study of the leached BR at the nanoscale revealed that the significant immobility of Th4+ is due to its incorporation into an insoluble perovskite-type phase with major composition of Ca0.8Na0.2TiO3 and crystallites observed in nanoscale. The Th LIII-edge EXAFS spectra demonstrated that Th4+ ions, which are hosted in this novel nano-perovskite of BR, occupy Ca2+ sites, rather than Ti4+ sites. That is most likely the reason of no Th release in Mediterranean seawater.

Thermochemical aspects on the formation of Ti-bearing coatings in a fluidized bed CVD process

Abstract A thermodynamic approach of a fluidized bed chemical vapour deposition process for the formation of Ti-bearing coatings on steels is attempted. For the thermochemical analyses, a system consisting of Al 2 O 3 or SiO 2 , Fe 2 Ti, Fe and C as well as NH 4 Cl or NH 4 I as activators of the process was considered. The partial pressures of the equilibrium gaseous species and the condensed phases of the above closed system have been calculated for amounts of the above constituents analogous to those used in the experimental practice and for different amounts of NH 4 Cl activator. The analyses indicate, that in all cases, the Ti-halide vapours, which are necessary for the coating formation, exist in the examined temperature range 773–1473 K. Different modes for the coating deposition are discussed. With NH 4 Cl as activator higher deposition rates of the coatings can be expected, in comparison to the NH 4 I. SiO 2 is not a good candidate as filler oxide as it reacts with the fluidizing medium. Although the process is feasible without hydrogen, addition of 10% of the latter is beneficial for the process as it increases the partial pressures of the Ti-chlorides.

Modelling of titanium carbide coating growth on carbon steels by conventional and fluidized bed CVD

The present work is concerned with modelling titanium carbide coating growth on carbon containing steels by CVD without the presence of hydrocarbons in the gas phase. The model is based on a simultaneous solution of the mass balance and diffusion equations of the system, allowing solutions for variable carbon concentration in steel. The obtained numerical results have been compared to experimental coating growth measurements of TiC coatings produced on steels from previous studies using conventional CVD, as well as the fluidized bed CVD (FBCVD) process. The model results are in a good agreement with the experimental results in both cases.