Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia

Yttria-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

Analytic approximations for the broadening of the spectral lines of hydrogen-like ions

Broadband approximate expressions for calculating the broadening of the spectral lines of hydrogenlike ions in a multicomponent plasma are derived taking into account both the influence of the interaction between plasma particles on the distribution function of the plasma microfield and the effect of the microfield dynamics on the broadening of the central component of the spectral line. With the approximate expressions proposed, the calculation of the shape of a given spectral line of a certain ion in a plasma with a given ion composition requires only a few seconds of computer time. The approximate expressions provide a good computational accuracy not only for the central component of the spectral line but also for the spectral line wings

A Closed-Form Solution of the Multi-Period Portfolio Choice Problem for a Quadratic Utility Function

In the present paper, we derive a closed-form solution of the multi-period portfolio choice problem for a quadratic utility function with and without a riskless asset. All results are derived under weak conditions on the asset returns. No assumption on the correlation structure between different time points is needed and no assumption on the distribution is imposed. All expressions are presented in terms of the conditional mean vectors and the conditional covariance matrices. If the multivariate process of the asset returns is independent it is shown that in the case without a riskless asset the solution is presented as a sequence of optimal portfolio weights obtained by solving the single-period Markowitz optimization problem. The process dynamics are included only in the shape parameter of the utility function. If a riskless asset is present then the multi-period optimal portfolio weights are proportional to the single-period solutions multiplied by time-varying constants which are depending on the process dynamics. Remarkably, in the case of a portfolio selection with the tangency portfolio the multi-period solution coincides with the sequence of the simple-period solutions. Finally, we compare the suggested strategies with existing multi-period portfolio allocation methods for real data.Comment: 38 pages, 9 figures, 3 tables, changes: VAR(1)-CCC-GARCH(1,1) process dynamics and the analysis of increasing horizon are included in the simulation study, under revision in Annals of Operations Researc

Numerical modeling of the thermal contact in metal forming processes

Heat flow across the interface of solid bodies in contact is an important aspect in several engineering applications. This work presents a finite element model for the analysis of thermal contact, which takes into account the effect of contact pressure and gap dimension in the heat flow across the interface between two bodies. Additionally, the frictional heat generation is also addressed, which is dictated by the contact forces predicted by the mechanical problem. The frictional contact problem and thermal problem are formulated in the frame of the finite element method. A new law is proposed to define the interfacial heat transfer coefficient (IHTC) as a function of the contact pressure and gap distance, enabling a smooth transition between two contact status (gap and contact). The staggered scheme used as coupling strategy to solve the thermomechanical problem is briefly presented. Four numerical examples are presented to validate the finite element model and highlight the importance of the proposed law on the predicted temperature.The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) under the project PTDC/EMS-TEC/1805/2012 and by FEDER funds through the program COMPETE Programa Operacional Factores de Competitividade, under the project CENTRO-07-0224- FEDER-002001 (MT4MOBI). The second author is also grateful to the FCT for the postdoctoral grant SFRH/BPD/101334/2014. The authors would like to thank Prof. A. Andrade-Campos for helpful contributions on the development of the finite element code presented in this work.info:eu-repo/semantics/publishedVersio

Power sequence definition under woodworking milling on contour-milling machines

Introduction. The analysis of the research papers shows the relevance of this topic, since there are no publications devoted to the determination of the spectral components of the power sequence of the milling process which makes it difficult to analyze models of the noise and vibration excitation. The noise exposure limits excess due to the high tool speed presents a critical problem. The highest speed is observed when the copy milling machines are operated. In the cutting conditions standards, only the power action amplitude is specified for woodworking which makes it difficult to determine the noise spectra. This study is devoted to determining parameters of the power sequence arising under the circular moulding. Materials and Methods. Methods of the cutting theory, spectral analysis, and statistical processing are used in the course of this study. Techniques of the noise and vibration levels analysis are largely determined by specifying the power action as a time function. Research Results. As a result of the conducted research, the mechanism of the force generation under milling; rules of the forces distribution over the projections; and patterns of variation in the cut-off allowance under milling are determined and specified. Discussion and Conclusions. The constructed mathematical model of the milling process dynamics and the software developed on its basis make it possible to evaluate the sound pressure levels created by both the cutting tool and the spindle case. This sound pressure actually determines the excess of noise levels over the maximum permissible values

Experimental studies on noise and vibration spectra of copy milling machines

The research objective is to study the development of the noise spectra patterns in the operating area of copy mill operators. The experimental studies have shown that among harmful factors specific to these machines, there is an octave sound pressure level increase up to 15 dB in the idle mode and up to 15 dB under woodworking. These data suggest that the main noise source creating excess over the maximum permissible values is a spindle unit, and in particular, the spindle case. The vibration measurements on the machine carrier system have confirmed the results of the noise monitoring. The main source of the increased vibration levels for these machine tools is spindle rolling-contact bearings

Numerical modelling of transient liquid phase bonding and other diffusion controlled phase changes

Diffusion in material of inhomogeneous composition can induce phase changes, even at a constant temperature. A transient liquid phase (TLP), in which a liquid layer is formed and subsequently solidifies, is one example of such an isothermal phase change. This phenomenon is exploited industrially in TLP bonding and sintering processes. Successful processing requires an understanding of the behaviour of the transient liquid layer in terms of both diffusion-controlled phase boundary migration and capillarity-driven flow. In this paper, a numerical model is presented for the simulation of diffusion-controlled dissolution and solidification in one dimension. The width of a liquid layer and time to solidification are studied for various bonding conditions. A novel approach is proposed, which generates results of a high precision even with coarse meshes and high interface velocities. The model is validated using experimental data from a variety of systems, including solid/solid diffusion couples