Activation energy of mullite crystallisation from precursor fibres determined by differential scanning calorimetry

Abstract

Mullite fibres were prepared by the sol–gel method using aluminium lactate and tetraethylorthosilicate. The aluminium lactate was prepared by mixing aluminium nitrate and lactic acid with a molar ratio of 1∶3. Thermogravimetry–differential scanning calorimetry, X-ray diffraction and scanning electron microscopy analysis were used to characterise the properties of the gel and ceramic fibres. The gel fibres completely transformed to mullite fibres at 1200°C, with a uniform diameter and dense microstructure. The active energy of mullite crystallisation was 1121·7 kJ mol^?1 by the Kissinger equation, which was consistent with most data reported by other authors.     

Transient and thermal contact analysis for the elastic behavior of functionally graded brake disks due to mechanical and thermal loads

In this paper, the transient and contact analysis of functionally graded (FG) brake disk is presented. The analysis was carried out using ANSYS parametric design language (APDL). The FG brake disk is made of metal–ceramic material. The material properties vary in radial direction with the values from full-metal at the inner radius to that of full-ceramic at the outer radius. In the analysis, FG brake disk is in contact with one pure pad disk and coulomb contact friction is considered as heat source. The non-dimensional results are obtained for specific value of grading index (n = 1) by considering different material property divisions of 25, 50, 100 and 200. The results presented are for the pressure distribution, total stress, pad penetration, friction stress, heat flux and temperature during contact, for different values of contact stiffness factor, F_kn, which depends on the property gradation of FG brake disk with 200 material property divisions. The results show that the contact pressure and contact total stress increase with increasing values of F_kn, and hence it can be concluded that gradation of the metal–ceramic has significant effect in the thermomechanical response of FG brake disks.     

Simulation of the effect of material properties and interface roughness on the stress distribution in thermal barrier coatings using finite element method

A Finite Element Model (FEM) was developed to evaluate the stresses induced by the thermal cycling in a typical plasma-sprayed thermal barrier coating system (TBCs). The thermo-mechanical model of this multi-layer system takes into account the effects of thermal and mechanical properties, morphology of the top-coat/bond-coat interface and oxidation on the local stresses that are responsible for the micro-crack nucleation during cooling, especially near the metal/ceramic interface.Two top-coat/bond-coat geometries corresponding to different interfacial asperity morphologies (semicircle or sinusoidal) are modeled considering a two dimensional and periodic geometry. The effect of the geometry and the amplitude of asperities on stress distribution are examined to study the cause of the subsequent delamination of the TBCs system. Moreover, the effect of the creep in all layers and plastic deformation in the bond-coat as well as the oxidation in the perpendicular direction of the top-coat/bond-coat interface are examined toward the stress development and critical sites with respect to possible crack paths. In addition, crack initiation and propagation at the system was predicted.     

Effect of Fe ions on the thermal and optical properties of the ceramic coating grown in-situ on AZ31 Mg Alloy

This research investigates the effect of Fe₂(SO₄)₃ on the thermal and optical properties of the ceramic coatings formed on AZ31 Mg alloy. The different ceramic coatings were obtained by plasma electrolytic oxidation (PEO) in electrolytes that contain varied concentrations of Fe₂(SO₄)₃. The microstructure, element distribution, composition as well as the thermal and optical properties of the coatings were studied with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD), UV–VIS–NIR spectrophotometer and infrared reflectometer. The results show that all of the coatings prepared were mainly composed with MgO, with trace-amount of Fe₃O₄ presents and Fe seems entered into the MgO crystal structure. With the increasing of the concentration of Fe₂(SO₄)₃, the solar absorptance and infrared emittance increased initially but then remain stable. We found that at the concentrations 8 g L⁻¹, the coating has the highest solar absorptance (0.94) and infrared emittance (0.83). Our results show that coatings formed with this method could be useful as a thermal control coating in a variety of applications, such as in the spacecraft.     

Determination and analysis of crack growth resistance in plasma-sprayed thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) are increasingly applied to enhance the performance of advanced gas turbine engines. However, the delamination cracks initiated in these coatings limit their applications. In this research, a sandwiched four-point bend specimen is used to evaluate the crack growth resistance in plasma-sprayed TBCs. Well controlled, stable and measurable crack extension is obtained. A rising crack growth resistance curve is found. The steady state strain energy release rate is obtained to be about 170 J/m². The delamination crack evolution behavior is in situ observed and simulated by the finite element analysis based on a crack bridging model.     

Thermal conductivity of Nd3+ and Yb3+ doped laser materials measured by using the thermal lens technique

Thermal conductivity measurements in various well-known Yb³⁺ and Nd³⁺ doped laser materials are performed by using an all-optical pump–probe approach based on the pump-induced thermal lens phenomenon. The derived values agree well with the existing data and open the way to more extensive and more complete investigations.     

Modelling of a thermal insulation system based on the coldest temperature conditions by using artificial neural networks to determine performance of building for wall types in Turkey

In formation of building external envelope, as two important criteria, climatic data and wall types must be taken into consideration. In the selection of wall type, the thickness of thermal insulation layer (d_i) must be calculated. As a new approach, this study proposes determining the thermal insulation layer by using artificial neural network (ANN) technique. In this technique five different wall types in four different climatic regions in Turkey have been selected. The ANN was trained and tested by using MATLAB toolbox on a personal computer. As ANN input parameters, U_w, T_e,Met, T_e,TSE, R_wt, and q_TSE were used, while d_i was the output parameter. It was found that the maximum mean absolute percentage error (MRE, %) is less than 7.658%. R² (%) for the training data were found ranging about from 99.68 to 99.98 and R² for the testing data varied between 97.55 and 99.96. These results show that ANN model can be used as a reliable modeling method of d_i studies.     

Optimization and improvement of thermal energy harvesting by using pyroelectric materials

We deal with the thermal energy which is one of the ambient energy sources surely exploitable, but it has not been much interest as the mechanical energy. In this paper, our aim is to use thermal energy and show that it’s an important source for producing the electrical energy through pyroelectric effect which is the property of some dielectric materials to show a spontaneous electrical polarization versus temperature. In this context, we present a concept to harvest a thermal energy using infrared rays and pyroelectric effect.The pyroelectric material used in this work can generate an electrical voltage when it subjected to a temperature change which will be ensured by the use of infrared lamp. Our experimental results show that the electrical voltage, current and harvested power increased significantly when increasing the area of the pyroelectric element. The experimental results show also that with this simple concept we harvested a heavy amount value of power which will certainly be useful in an extensive range of applications, including sensors and infrared detection. These results shed light on the thermoelectric energy conversion by Ceramic lead zirconate titanate (PZT) buzzer having the pyroelectric property.     

Improvement in thermal stability of B-C-N thin films fabricated by magnetron sputtering using graphite and BN co-target

Thermal stability of amorphous boron-carbon-nitride (B-C-N) films fabricated by magnetron sputtering using a graphite and BN co-target was studied by X-ray photoelectron spectroscopy, infrared absorption spectroscopy, Raman spectroscopy and scanning electron microscopy. The boron-rich B-C-N films showed the higher thermal stability. These results suggest that an incorporation of B atoms into the amorphous CN networks improves the thermal stability. This improvement can be explained in terms of the creation of boron-nitrogen bonds and/or boron-carbon bonds.     

Heterogeneous and anisotropic long-term concrete damage of the dez arch dam using thermal inverse analysis

In this paper, an assumption used in the recent work of the author and his contributors considers the long-term concrete damage of the Dez arch dam as a homogeneous and isotropic process, was investigated in more detail and was adjusted. To this end, the vertical dam sections were divided into nine and six subsections along the thickness and height directions of the dam, respectively. In each subsection, a transversely isotropic damaged elastic constitutive law was considered for diagnostic analyses. Following the previous authors’ mentioned work, an optimization procedure (minimization of a certain error function) accompanied with an inverse thermal analysis was carried out. That analysis was performed within the framework of finite element (FE) method. Mentioned error function was defined as the sum of the squared residuals. Residuals were set as difference between nodal recorded displacements with inverse pendulums of the dam and the corresponding computed ones with the proposed model. Parameters considered as unknowns in the present inverse analysis, which have contributions in that above-mentioned error function, were the five independent elastic moduli presented in the formulation of the earlier mentioned transversely isotropic damaged elastic constitutive law: two Young’s moduli: one in the planes parallel to the up and down stream surfaces of the dam, called in-plane and the other in the perpendicular planes to those planes, named out-of-plane Young’s modulus; two Poisson’s ratios (in-plane and out-of-plane); and one shear modulus (out-of-plane). These parameters were identified performing numerous thermal inverse analyses. Obtained results revealed that the long-term damage of concrete of the Dez dam is a heterogeneous and anisotropic phenomenon because that the magnitude of the mentioned error function was obtained smaller than the corresponding value in the previous study which had been performed before based on the homogeneous and isotropic damage evolution assumption.     

Analysis and simulation of cracking patterns in coating under biaxial tensile or thermal stress using analysis/FEM strain-accommodation method

The cracking patterns in coatings under biaxial tensile or thermal stress are analyzed by the “analysis/FEM strain-accommodation method” that combines the strain of the substrate with a coating obtained from thermo-elastic analysis with the strain of the substrate calculated by a finite element method. The simulation using this method is effective not only for expressing the cracking patterns observed in punch press tests of disk specimens with WC-Co cermet and Al₂O₃-TiO₂ ceramic coatings but also predicting the cracking process for the coating deposited on a part with a complex shape under thermal stress.     

Crack propagation modeling on the interfaces of thermal barrier coating system with different thickness of the oxide layer and different interface morphologies

A finite element model (FEM) is developed to simulate the crack development in a typical plasma sprayed thermal barrier coatings system in consequence of the stresses induced by thermal cycling, the growth of the oxide layer and different interface morphologies. The thermo-mechanical model is designed to takes into account a non-homogenous temperature distribution and the effects of the residual stress generated during coating process.     

Alloying and thermal behaviour of CoCrFeNiMn0.5Ti0.5 high-entropy alloy synthesised by mechanical alloying

An inequi-atomic CoCrFeNiMn_0.5Ti_0.5 high-entropy alloy (HEA) was synthesised by mechanical alloying. The structural and morphological evolution of the alloy powder during the mechanical alloying process and the thermal behaviour of 60?h ball-milled HEA powder were investigated systematically. A simple body-centred cubic solid solution HEA structure was obtained when the blended powder was ball-milled longer than 36?h. A 60?h ball-milled powder had an average particle size of 3?μm and consisted of hard agglomerated crystalline particles with a crystal size of <?20?nm. The body-centred cubic phase transformed into a face-centred cubic phase when the powder was annealed for 1?h at a temperature of 700°C; the liquidus point of the face-centred cubic phases was 1402.8°C.     

Determination of uronic acids in isolated hemicelluloses from kenaf using diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and the curve-fitting deconvolution method

Hemicellulose samples were isolated from kenaf (Hibiscus cannabinus L.). Hemicellulosic fractions usually contain a variable percentage of uronic acids. The uronic acid content (expressed in polygalacturonic acid) of the isolated hemicelluloses was determined by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the curve-fitting deconvolution method. A linear relationship between uronic acids content and the sum of the peak areas at 1745, 1715, and 1600 cm(-1) was established with a high correlation coefficient (0.98). The deconvolution analysis using the curve-fitting method allowed the elimination of spectral interferences from other cell wall components. The above method was compared with an established spectrophotometric method and was found equivalent for accuracy and repeatability (t-test, F-test). This method is applicable in analysis of natural or synthetic mixtures and/or crude substances. The proposed method is simple, rapid, and nondestructive for the samples.     

Structure-function relationships in high-density docosylsilane bonded stationary phases by Raman spectroscopy and comparison to octadecylsilane bonded stationary phases: effects of common solvents

The effects of common solvents on alkyl chain conformational order in a series of high-density C22 stationary phases with surface coverage ranging from 3.61 to 6.97 micromol/m2 are investigated by Raman spectroscopy. Conformational order is evaluated using the intensity ratio of the antisymmetric and symmetric nu(CH2) modes as well as the frequencies at which these Raman bands are observed. Solvents studied include methanol-d4, acetonitrile-d3, water-d2, toluene-d8, chloroform-d. and benzene-d6. Alkyl chain conformational order and, hence, solvation of the stationary phase, is dependent on the Gibbs free energy change for these molecules at infinite dilution in hexadecane (DeltaG(o)HD), as well as stationary-phase properties (polymerization method and surface coverage). In general, polar solvents increase slightly the conformational order of these C22 stationary phases, while nonpolar solvents decrease conformational order. A comparison is made between C22 and C18 bonded-phase systems to further understand the role of alkyl chain length on solvent-stationary phase interactions. The change in alkyl chain conformational order induced by solvent is also compared to that induced by temperature, which provides insight into the effect of chromatographic conditions on stationary-phase shape selectivity, an important application of these materials.     

Ablative and mechanical evaluation of CNT/phenolic composites by thermal and microstructural analyses

Highly ablation resistant carbon nanotube (CNT)/phenolic composites were fabricated by the addition of low concentrations of CNTs. Tensile and compressive mechanical properties as well as ablation resistance were significantly improved by the addition of only 0.1 and 0.3 wt% of uniformly dispersed CNTs. An oxygen–kerosene-flame torch and a scanning electron microscope (SEM) were used to evaluate the ablative properties and microstructures. Thermal gravimetric analysis (TGA) revealed that the ablation rate was lower for the 0.3 wt% CNT/phenolic composites than for neat phenolic or the composite with 0.1 wt% CNTs. Ablation mechanisms for all three materials were investigated using TGA in conjunction with microstructural studies using a SEM. The microstructural studies revealed that CNTs acted as an ablation resistant phase at high temperatures, and that the uniformity of the CNT dispersion played an important role in this ablation resistance.     

Determination of the Cd-bearing phases in municipal solid waste and biomass single fly ash particles using SR-microXRF spectroscopy

By using an excitation energy of 27.0 keV, synchrotron radiation-induced micro-X-ray fluorescence (SR-microXRF) is employed to extract information regarding the composition and distribution of Cd-bearing phases in municipal solid waste (MSW) and biomass fly ashes. Significance of observation is based on statistics of totally more than 100 individual MSW and biomass fly ash particles from a fluidized bed combustion (FBC) plant. Cd concentrations in the parts-per-million range are determined. In general, although previous leaching studies have indicated Cd to be predominant in the smaller-size ash particles, in the present study Cd is more evenly distributed throughout all the particle sizes. For MSW fly ashes, results indicate the presence of Cd mainly as CdBr2 hot-spots, whereas for biomass fly ashes, which exhibit lower CdX2 concentration, a thin Cd layer on/in the particles is reported. For both ashes, Ca-containing matrixes are found to be the main Cd-bearing phases. Support for this observation is found from independent first-principles periodic density functional theory calculations. The observations are condensed into a schematic mechanism for Cd adsorption on the fly ash particles.     

Study of space charge in gallium nitride by the thermal step method

In this paper, we report the electric investigation of thin nitride gallium films by the capacitance voltage technique and the thermal step method (TSM). The C–V analysis at 1 MHz of Au/GaN diode reveals MOS behaviour and shows strong capacitance hysteresis.

This may be due to the presence of trapped charge in this structure. The space charge dynamics is studied by thermal step method at different applied voltages. The TS currents are reverted from negative ones to positive ones above inversion threshold of +0.2 V. This change corresponds to charge modulation from accumulation to the inversion one, in good agreement with the C–V characteristics. The stored charge in this sample is related to the nature of gallium nitride and to the manufacturing processes. The results confirm the possibility to apply the TSM for the measurement of the space charge in the semiconductor materials.     

Growing polystyrene chains from the surface of graphene layers via RAFT polymerization and the influence on their thermal properties

The polystyrene (PS) macromolecular chains were grafted on the surface of graphene layers by reversible addition-fragmentation chain transfer (RAFT) polymerization. In this procedure, a RAFT agent, 4-Cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid, was used to functionalize the thermal reduced graphene oxide (TRGO) to obtain the precursor (TRGO-RAFT). It can be calculated that the grafting density of PS/graphene (PRG) composites was about 0.18 chains per 100 carbons. Successful in-plain attachment of RAFT agent to TRGO and PS chain to TRGO-RAFT was shown an influence on the thermal property of the PRG composites. The thermal conductivity (λ) improved from 0.150 W m⁻¹ K⁻¹ of neat PS to 0.250 W m⁻¹ K⁻¹ of PRG composites with 10 wt% graphene sheets loading. The thermal property of PRG composites increased due to the homogeneous dispersion and ordered arrangement of graphene sheets in PS matrix and the formation of PRG composites.