Preparation and characterization of ceramic thin film thermocouples

Indium tin oxide (ITO), alumina doped zinc oxide (ZnO) and NiCrCoAlY/alumina nanocomposites were systematically investigated as thermoelements. These ceramic thermoelements were initially tested relative to a platinum reference electrode and the resulting thermoelectric properties were evaluated. Bi-ceramic junctions comprised of the most stable and responsive ceramic thermoelements, i.e. those thermoelements with the largest and most stable Seebeck coefficients relative to platinum, were fabricated and tested. A bi-ceramic junction based on nitrogen-doped ITO:oxygen-doped ITO exhibited excellent high temperature stability and reproducibility, however, this thermocouple pair had a relatively low Seebeck coefficient (6 μV/°C). Alumina doped ZnO:ITO thermocouples generated a very large electromotive force at low temperatures but lacked high temperature stability. When nitrogen-doped ITO was combined with a NiCoCrAlY/alumina nanocomposite, a very large and stable Seebeck coefficient (375 μV/°C) was realized. Ceramic thermocouples based on several candidate materials were demonstrated at temperatures up to 1200 °C and the potential of using these materials in other thermoelectric devices including those for energy harvesting is discussed.     

Conditions for matrix crack deflection at an interface in ceramic matrix composites

This paper describes a numerical approach developed to simulate the mechanism of matrix crack deflection at the fibre/matrix interface in brittle matrix composites. For this purpose, the fracture behaviour of a unit cell (microcomposite) consisting of a single fibre surrounded by a cylindrical tube of matrix was studied with the help of a finite element model. A fracture mechanics approach was used to design a criterion for deflection at the fibre/matrix interface of an annular crack present in the matrix. The analysis of the fracture behaviour of SiC/SiC and SiC/glass ceramics microcomposites shows that the introduction of a low modulus and low toughness interfacial layer at the fibre/matrix interface (e.g. a carbon coating) greatly favours matrix crack deflection at the interphase/fibre interface.     

Advanced Ceramic Materials for High Temperature Applications

The increasing interest in ecological aspects related to the reduction of harmful emissions to the atmosphere and, at the same time, the need to achieve higher efficiencies of energy production are the driving forces that justify the current development of advanced ceramic materials for high temperature applications, namely those associated to energy and transportation industries. Ceramic matrix composites (CMCs), thermal barrier coatings (TBCs), environmental barrier coatings (EBCs) and solid oxide fuel cells (SOFCs) are increasingly used to work under the new demanding conditions. In this review, the recent progress and trends in the research and development of CMCs, TBCs, EBCs and SOFCs based on ceramic materials for high temperature applications are highlighted.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Fracture in angle-ply ceramic matrix composites

An analytical/computational fracture mechanics scheme, combined with finite element computations, is presented to explain observed fracture patterns in angle ply fiber-reinforced ceramic composite laminates. Unlike polymeric composites, where microcracks are channeled parallel to the fiber directions, cracks in ceramic composites initiate perpendicularly to the load axis. The results presented herein provide rational explanations for the experimental observations and confirm the advantages bestowed by multi-directional reinforcements. Such reinforcements would still allow the development of multitudes of matrix cracks, bridged by intact fibers, that lead to the desirable gradual and `graceful' failure of the composite. At the same time, multidirectional reinforcement prevents premature failures by constraining the laminate against interfacial fiber/matrix debondings. Such debondings, which may otherwise grow rapidly along the abovementioned weak interfaces, may readily occur in unidirectionally reinforced off-axis laminates, as noted in a preceding article.     

Tensile mastercurve of ceramic matrix composites: significance and implications for modelling

The concept of a tensile mastercurve which uniquely represents the mechanical response to time-independent tensile loading is presented. It is shown how the mastercurve can be determined from individual tensile tests with unloading–reloading cycles, and how it provides a rationalisation of both the temperature dependence and the scatter in tensile behaviour. Implications for modelling of the mechanical behaviour are outlined.     

Viscoelastic analysis of mismatch stresses in ceramic matrix composites under high-temperature neutron irradiation

Fiber-reinforced ceramic matrix composites, such as SiC---SiC, are proposed for structural applications in future fusion energy systems. In a fusion nuclear reactor environment time-dependent inelastic effects are induced by irradiation and hence mismatch stresses are expected to change in time. The time evolution of the internal mismatch stresses in ceramic fiber composites under high-temperature neutron irradiation is presented, with application to SiC---SiC composite structures. The effects of thermal creep, irradiation-induced creep and dimensional changes on the build-up and relaxation of the interface pressure and residual stresses in fibers and matrix are investigated. Residual stresses are determined as functions of temperature and neutron exposure time. It is shown that initial mismatch stresses are relaxed within hours because of irradiation creep. It is also demonstrated that fibers with low density, such as Nicalon, will debond from the matrix due to excessive irradiation-induced densification.     

A model for the transverse strain response of unidirectional ceramic matrix composites during tensile testing

A comprehensive micromechanical model relating the longitudinal stress and transverse strain of unidirectional fibre toughened ceramic matrix composites (CMCs) is presented. The model uses different cylindrical unit-cells to describe the composite throughout a tensile test and considers all relevant damage mechanisms. The proposed model takes into account the Poisson contraction of fibre and matrix, the relief of thermal residual stresses upon damage development, and the build-up of compressive radial stresses at the interface due to mismatch between fibre and matrix after debonding and sliding. Thus the modelled transverse strain response depends on a wide range of microstructural and micromechanical parameters. The approach is checked by comparing the experimentally observed and simulated response of a unidirectional SiC/CAS composite of which all constituent properties were determined experimentally. The agreement between experiment and theory is excellent.     

Ceramic matrix composites based on Mg-PSZ with Cr-Ni-steel-additions with improved thermo-mechanical properties

The application of ceramic materials is limited due to their inherent brittleness. In the past years attempts have been made to improve the fracture toughness of structural ceramics by adding a secondary phase. In the present paper the influence of metastable austenitic TRIP-steel powder on the thermo-mechanical properties of magnesia partially stabilised zirconia has been investigated. Ceramic matrix composites have been prepared using slip casting technology. The sintering was performed in different argon atmospheres. The incorporation of the metastable metallic phase led to the successful generation of composite materials with advanced mechanical properties, especially after thermal shock attack.     

High-temperature oxidation of ceramic matrix composites dispersed with metallic particles

Oxidation behavior of ceramic matrix composites dispersed with metallic particles is discussed to establish materials design for high-temperature applications. Oxidation kinetics of ceramic matrix composites dispersed with metallic particles is understood from the viewpoint of the diffusion properties and defect chemistry of matrix oxides. High-temperature oxidation of Ni(p)/partially stabilized zirconia, Ni(p)/Al₂O₃ and Ni(p)/MgO was described as examples.     

On the mechanical behaviour under cyclic loading of ceramic matrix composites

In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.     

Thermal cycling response behavior of ceramic matrix composites under load and displacement constraints

Thermal cycling response of a two-dimensional carbon fiber reinforced SiC matrix composite (2D C/SiC) to load constraint (LC) and to displacement constraint (DC) in an oxidizing environment was investigated. During thermal cycling between 700 and 1200 °C, a constraint strain with a 0.208% range and a constraint stress with a 180 MPa range were, respectively, generated on the composites in LC and DC. It was found that with increasing cycles, the constraint strain increased in LC and the constraint stress decreased in DC. After 50 cycles, in contrast to the as-received composite materials, the as-cycled composites suffered greater loss in mechanical properties: the residual strength and failure strain are 204 MPa and 0.49% for the LC tested samples, and 223 MPa and 0.64% for the DC tested samples, respectively. Microstructural observations indicated that the LC could develop thermal microcracks and assist in oxidizing the internal fibers, whereas the DC reduced crack propagations and fiber oxidation because of decreasing tensile and increasing compressive stresses.     

Defects in ceramic matrix composites and their impact on elastic properties

Defects created during the manufacture of an oxide/oxide and two non-oxide (SiC/SiNC and MI SiC/SiC) ceramic matrix composites (CMCs) were categorized as follows: (1) Intra-yarn defects such as dry fibers, (2) Inter-yarn defects such as those at crossover points, matrix voids, shrinkage cracks and interlaminar separation, and (3) Architectural defects such as layer misalignment. Their impact on elastic properties was analytically investigated using a stiffness averaging approach considering the defects to have volumetric and directional influences. In-plane tensile and shear moduli as well as the through-thickness compressive modulus were experimentally evaluated. Results of analytical model were around 7% on average from the mean value of the experimental data. It was observed that interlaminar separation drastically reduced the through-thickness modulus by about 63% for the SiC/SiNC, 40% for the MI SiC/SiC and around 32% for the oxide/oxide composites. Shrinkage cracks in oxide/oxide composite reduced the in-plane tensile and shear moduli by 14% and 8.8%, respectively.     

Zinc oxide-based thin film functional layers for chemiresistive sensors

Sol-gel wet-chemical techniques were used to prepare ZnO, Al-ZnO (Al:Zn = 1:10 mol/mol) and Cu-ZnO (Cu:Zn = 1:10 mol/mol) thin films for characterization as functional layers for chemiresistive oxygen sensors. Cu and Al minor components influence the ZnO films' topography and their thermally induced chemical and structural evolution. As prepared (room temperature) films have the structure of layered basic zinc acetate, a lamellar ZnO precursor. Upon annealing at temperatures through 973 K, the films display similar chemical evolution patterns—temperatures above 773 K are needed to completely desorb solvents and decompose precursors. Cu facilitates c-axis orientation of the film as its structure matures, while Al slows its crystallization. Chemiresistive sensors, fabricated by coating thin film functional layers onto interdigitated electrode (IDE) transducers, were evaluated for their responses to oxygen at operating temperatures through 873 K. A ZnO/IDE sensor displays high sensitivity for O₂ at an intermediate temperature, 673 K, reflecting an optimal balance between surface O₂ coverage and carrier availability. At 1:10 mol/mol Cu:Zn and Al:Zn, the developing ZnO structure cannot accommodate all minor component atoms. Surplus atoms accumulate in independent phases at grain boundaries, contributing to both high base resistances (in N₂) and low sensitivity to oxygen.     

The yarn size dependence of tensile and in-plane shear properties of three-dimensional needled textile reinforced ceramic matrix composites

The yarn size scaling of tensile and in-plane shear properties is examined for three-dimensional needled textile reinforced ceramic matrix composites (3DN CMC) fabricated by chemical vapor infiltration. The results showed that large yarn size would cause the nonwoven yarn of 3DN CMC crimp and lower composite density, resulting in decrease of tensile and in-plane shear properties. The “modified lamina modeling” was presented to predict the tensile and shear elastic moduli of 3DN CMC with different yarn size. Other two methods were also proposed to evaluate the tensile and in-plane shear strengths of 3DN CMC with different yarn size, respectively. All predicted results showed consistent well with the experimental results.     

Investigations on the structure, composition and performance of nanocrystalline thin film thermocouples deposited using anodic vacuum arc

This paper deals with the performance study of nanocrystalline thin film thermocouples (TFTCs) fabricated using anodic vacuum arc plasma aided deposition technique. Various single junction single elemental metal-metal pairs, elemental metal-metal alloy pairs, and metal alloy-metal alloy pairs were developed on glass substrates Elemental metal films were annealed at 10^− 4 Pa for 4 h while metal alloy films were annealed for 5 h. Their thermoelectric response has been studied in ambient air up to a maximum temperature difference of 300 °C between hot junction and cold junction. The phase purity, microstructure and composition of individual layer films were extensively studied. Elemental metal pairs agree well with their wire thermocouple equivalents. Thermoelectric power (TEP) of Cu-Ni and Fe-Ni TFTCs were found to be 17.81 μV/°C and 27.94 μV/°C at 300 °C, respectively. Among metal alloy-metal alloy TFTCs, a TEP of 32.87 μV/°C at 300 °C was obtained for Chromel-Alumel TFTCs which agree fairly well with its wire counterpart. However, Constantan based TFTCs deviated considerably from their wire counterparts. Cu-Constantan, Fe-Constantan and Chromel-Constantan showed a TEP of 26.48 μV/°C, 35.76 μV/°C and 37.41 μV/°C at 300 °C respectively. This deviation in thermoelectric power of Constantan based TFTCs with their wire counterparts were due to the fractionation of the Constantan arm. This fractionation leads to decrease of Ni content in the film which in turn reduces their TEP.     

Influence of fibre properties on the mechanical behaviour of unidirectionally-reinforced ceramic matrix composites

Tensile tests on unidirectionally-reinforced ceramic matrix composites have been simulated using a micromechanical approach based on a shear-lag model and the chain-of-bundles theory. The model presented takes the probabilistic nature of the constituent properties into account. The results of the simulation have been compared with experimental data from literature in order to obtain information about the in-situ properties of the constituents and the influence of the fibre properties on the composite behaviour.     

Aluminium nitride-molybdenum ceramic matrix composites possessing high thermal shock resistance

Aluminium nitride-molybdenum ceramic matrix composites were produced by hotpressing a mixture of AlN and Mo powders. Thermal shock resistance of a composite (AM25) which contained 25 vol.% of metallic phase dispersed in the AlN matrix, was studied in order to verify the influence of metallic phase additional on the thermal properties. Results showed that AM25 possess a critical temperature difference (ΔT_c) for thermal shock of about 550°C compared to a mean value of 350°C measured in the case of pure hot pressed AlN. This improvement in thermal shock resistance was attributed to better mechanical properties and higher thermal conductivity of AM25 as compared to pure AlN.     

Natively textured surface aluminum-doped zinc oxide transparent conductive layers for thin film solar cells via pulsed direct-current reactive magnetron sputtering

Natively textured surface aluminum-doped zinc oxide (ZnO:Al) layers for thin film solar cells were directly deposited without any surface treatments via pulsed direct-current reactive magnetron sputtering on glass substrates. Such an in-situ texturing method for sputtered ZnO:Al thin films has the advantages of efficiently reducing production costs and dramatically saving time in photovoltaic industrial processing. High purity metallic Zn-Al (purity: 99.999%, Al 2.0 wt.%) target and oxygen (purity: 99.999%) were used as source materials. During the reactive sputtering process, the oxygen gas flow rate was controlled using plasma emission monitoring. The performance of the textured surface ZnO:Al transparent conductive oxides (TCOs) thin films can be modified by changing the number of deposition rounds (i.e. thin-film thicknesses). The initially milky ZnO:Al TCO thin films deposited at a substrate temperature of ~ 553 K exhibit rough crater-like surface morphology with high transparencies (T ~ 80-85% in visible range) and excellent electrical properties (ρ ~ 3.4 × 10^− 4 Ω cm). Finally, the textured-surface ZnO:Al TCO thin films were preliminarily applied in pin-type silicon thin film solar cells.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.