112 research outputs found
Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work
No full textA review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive, and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved
A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate
No full textThe sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured
A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings
No full textA sintering model is presented for prediction of changes in the microstructure and dimensions of free-standing, plasma-sprayed (PS) thermal barrier coatings (TBCs). It is based on the variational principle. It incorporates the main microstructural features of PS TBCs and simulates the effects of surface diffusion, grain boundary diffusion and grain growth. The model is validated by comparison with experimental data for shrinkage, surface area reduction and porosity reduction. Predicted microstructural changes are also used as input data for a previously developed thermal conductivity model. Good agreement is observed between prediction and measurement for all these characteristics. The model allows separation of the effects of coating microstructure and material properties, and captures the coupling between densifying and non-densifying mechanisms. A sensitivity analysis is presented, which highlights the importance of the initial pore architecture. Predictions indicate that the microstructural changes which give rise to (undesirable) increases in thermal conductivity and stiffness are very sensitive to surface diffusion
Properties and Performance of High-Purity Thermal Barrier Coatings
No full textIt was found that the content of impurity oxides in 7YSZ, such as SiO2 and Al2O3, has a significant effect on coating sintering resistance and phase stability of 7YSZ thermal barrier coatings (TBCs). The reduction of impurity content will significantly improve sintering resistance and phase stability of 7YSZ TBCs and thus allow the 7YSZ TBCs to be used at higher temperatures
Historia de la Educación en el País Vasco : la Ikastola. Datos para la biografía de Elvira Cipitria : la Ikastola en San Sebastián 1942-1969
No full textRecoger datos que puedan ser utlizados un día en la realización de la historia del fenómeno de la Ikastola. Historia de la Educación. En la primera parte de este estudio se realiza un esbozo de la historia de la Ikastola en San Sebastián entre los años 1942 y 1969. La segunda parte se refiere a la labor desempeñada por la Andereño Elvira Cipitria entre los años 1942 y 1969 en San Sebastián, realizada mediante entrevistas -efectuadas en Euskera- y documentos acreditativos de aquello que se reseña. Entrevistas de carácter oral constituidas por las vivencias y los recuerdos de personas que conocieron de cerca a Elvira Cipitria. Documentos Oficiales. Análisis teórico.ES
Sintering Kinetics of Plasma-Sprayed Zirconia TBCs
No full textA model of the sintering exhibited by EB-PVD TBCs, based on principles of free energy minimization, was recently published by Hutchinson et al. In the current paper, this approach is applied to sintering of plasma-sprayed TBCs and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with experimental data obtained by dilatometry and BET analysis respectively. The sensitivity of the predictions to initial pore architecture and material properties are assessed. The model can be used to predict the evolution of contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously-developed analytical model
Replication data for: The role of perceptual salience in L2 morphology acquisition: Attention, awareness, and intake
No full text Dataset description : The dataset includes 68 participants reading a total of 254 sentences (8 learn-practice, 140 learn, 6 test-practice, 80 test) in Englishti, an English-based semi-artificial language incorporating high- and low-salient morphemes reflecting the meaning of possessive determiners (Simoens et al., 2017). Eye-tracking data focuses on two specific Interest Areas (IA): IA7, which contains the target morpheme, and IA5-8, which covers a broader area surrounding the morpheme. In addition to the eye-tracking data, the dataset includes basic demographic data (age, gender), participants' awareness interview scores, Working Memory scores as assessed by the Reading Span Task (van den Noort et al., 2008), and implicit learning ability as reflected by the Serial Reaction Time task (Kaufman et al., 2010). Abstract of the publication : Acquiring morphology poses a considerable challenge in second language acquisition (SLA), highlighting the need to explore methods that facilitate this task for L2 learners. One potential facilitator is salience, which is theorized to aid language acquisition by directing learners’ attention to certain linguistic elements (Goldschneider & DeKeyser, 2001). To empirically investigate the impact of one type of salience, perceptual salience, a text-based eye-tracking experiment was conducted with 68 L1 Dutch speakers who read 240 sentences in Englishti, an English-based semi-artificial language featuring perceptually high-salient (-ulp) and low-salient (-o) morphemes according to length (Simoens et al., 2017). Within an implicit learning paradigm, participants were assigned to intentional or incidental learning contexts. The task consisted of two phases: a learning phase involving input flooding of the target morphemes followed by content-related questions, and a testing phase where participants completed a grammaticality judgment task on Englishti sentences, half of which were familiar from the learning phase and half of which were new. The results revealed a significant influence of salience, mediated by learning context and English proficiency, on fixation durations, thus empirically confirming the effect of perceptual salience on attention allocation in L2 morphology acquisition
Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia
No full textYttria-stabilized zirconia powders, containing different levels of SiO2 and Al2O3, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<_50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes
Effect of Heat Treatment on Pore Architecture and Associated Property Charges in Plasma Sprayed TBCs
No full textPlasma sprayed TBCs exhibit many interlamellar pores, voids and microcracks. These microstructural features are primarily responsible for the low global stiffnesses and the low thermal conductivities commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400C, for times of up to 100 hours) and coating purity on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and BJH Analysis. While the overall porosity level (measured by densitometry) remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (inter-connected) specific surface area, although these changes occurred less rapidly with coatings produced using high purity powders. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (>~1300C). These changes are thought to be associated with improved inter-splat bonding and increased contact area, leading to disappearance of much of the very fine inter-splat porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Measured thermal conductivity data are compared with predictions from a recently-developed analytical model [1], using the deduced inter-splat contact area results as input parameters. Good agreement is obtained, suggesting that the model captures the main geometrical effects and the porosity architecture measurements reflect the most significant microstructural changes. REF.1. Golosnoy, IO, Tsipas, SA and Clyne, TW, An Analytical Model For Simulation Of Heat Flow In Plasma Sprayed Thermal Barrier Coating, J. Thermal Spray Techn., 14 (2005) 205-214
Design, fabrication and characterization of PCL electrospun\ud scaffolds—a review
No full textThe expanding interest in electrospinning fibers for bioengineering includes a significant use of polyesters, including poly(3-caprolactone) (PCL). This review summarizes literature on PCL and selected blends, and provides extensive descriptions of the broad range of parameters used in manufacturing such electrospun fibers. Furthermore the chemical, physical and biological approaches for characterizing the electrospun material are described and opinions offered on important information to include in future publications with this electrospun material
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