Starch Consolidation Casting of Cordierite Precursor Mixtures—Rheological Behavior and Green Body Properties

The rheology of suspensions and mechanical properties of green bodies with cordierite composition (raw materials 37 wt% kaolin, 41 wt% talc, 22 wt% alumina, resulting in 46.6 wt% SiO₂, 38.1 wt% Al₂O₃, 13.6 wt% MgO) and two types of starch (corn or potato) are investigated. Rotational viscometry of suspensions with solids loading 50, 60, and 70 wt% without starch showed that all tend to be shear‐thinning with a small degree of thixotropy. Suspensions with a total solids loading of 60 wt% with 25 wt% replaced by starch exhibited higher viscosity and thixotropy, but the viscometric behavior is almost identical for the two starch types (apparent viscosities 130–50 mPa ·s). Oscillatory rheometry shows that for suspensions with potato starch the onset temperature for gelatinization is 61°C–63°C, that is, lower than for corn starch (72°C–73°C). Maximum storage moduli and phase shift values after gelatinization are similar for both systems. The mechanical properties of green disks, measured via diametral compression tests, reveal clear differences between materials prepared with corn and potato starch, with the latter showing higher elastic modulus, higher strength, and higher deformation at fracture, obviously because of incompletely gelatinized starch granules in the green bodies prepared with corn starch .     

Worst case equalizer for noncoherent HIPERLAN receivers

Coherent detection of HIPERLAN Gaussian minimum-shift keying signals calls for complex and expensive receivers. However, when the channel delay spread is limited to at the most 50% of the symbol time and a reliable line-of-sight component of the radiated signal exists (Rician fading model), noncoherent detectors are capable of achieving a good performance. Based on the above motivations, we compare four different demodulation techniques, namely the following: (1) one-bit differential detector; (2) discriminator detector; (3) limiter discriminator detector; and (4) limiter discriminator integrator detector (LDID). The intersymbol interference introduced by these demodulators is nonlinear (with respect to the data symbols) and a decision-feedback equalizer (DFE) based on a mean square-error criterion may not be appropriate. Moreover, at this high speed, a DFE may be very complex to implement. Hence, we propose a new DFE design method that increases the eye-diagram aperture by removing the worst case interference. Performance of the above demodulators in the presence of a simple nonlinear DFE (with feedback part only) is computed in terms of the bit-error rate (BER) by means of the saddle-point approximation. This procedure, for static channels, turns out to be a very general tool with a simple and robust implementation. The same method can be applied, for multipath fading channels, to the BER evaluation as part of a semianalytic approach. The main conclusion from this work is that for LDID demodulators and in the presence of Rician fading channels with an average normalized root mean square delay spread of 0.3 and dual antenna diversity, the new equalizer lowers the outage probability from 60% to 10% at a BER of 10^-4     

Effective thermal conductivity of functionally graded random micro-heterogeneous materials using representative volume element and BEM

This work introduces a numerical methodology for the computation of the effective thermal conductivity (ETC) of random micro-heterogeneous materials using representative volume elements and the Fast Multipole Boundary Element Method (FMBEM). The methodology is applied to solve two-dimensional foam-like materials consisting of random distributions of circular isolated holes. The computed ETC values are successfully used to predict the temperature fields of two materials with functionally graded ETCs. Numerical and analytical results are experimentally validated. The proposed methodology is flexible and versatile, as it is capable to account for both, the geometrical and topological details of the material microstructure.     

Effect of a coalescing aid on the earliest stages of polymer diffusion in poly(butyl acrylate-co-methyl methacrylate) latex films

In this article we use fluorescence resonance energy transfer (FRET) to investigate how a classic coalescing aid, such as 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol?) (TX), acts on the earliest stages of polymer diffusion as the latex film is still drying. In our approach, we temporarily arrest the drying process of a partially wet latex film by sealing it in an airtight chamber previously cooled to near the latex T_g. At these conditions, we are able to effectively stop the drying process and the polymer diffusion. FRET measurements at various locations on such a sample provide us information about the mechanism operating at the initial stages of polymer diffusion as the latex film is still drying. We complete our study with FRET measurements carried out at longer aging times on predried latex films. We analyze our diffusion data in terms of free volume theory and propose a mechanism that can account for the results obtained.     

Spectral analysis in DMT-based transceivers

A general analytical formula is presented for the spectral density evaluation of the discrete-time sequence obtained by IFFT in a DMT-based transceiver     

Performance of digital DECT radio links based on semianalyticalmethods

We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (γ _e) in the threshold detector. Both the DF equalizer coefficient and γ_e are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10^-6 with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel     

钢包引流砂性能的测定

本文利用不同的实验室手段,分忻了一组不同成分的铬铁砂.为了了解操作条件下铬铁砂的烧结性能,在各种温度下对试样进行热处理,得到进一步的微观组织,并测定试样的力学性能.还通过动态热力学分析实验测定了试样的放热情况.实验结果表明,正常钢水温度下,铬铁砂表现出两种烧结性能,主要取决于其石英含量.烧结过程中,含石英的铬铁砂具有较高的致密性,形成的液相多;不含石英的铬铁砂体积变化小,机械强度较高.     

An analysis of the effect of chip timing jitter in MC-CDMA systems

We investigate the effect of chip synchronization error (or chip timing jitter) in a multicarrier-code division multiple access (MC-CDMA) scheme. The chip timing jitter is modeled by a stationary random process with a known statistic, and the error produced by the jitter is reduced to an additive source of noise. This decomposition is useful because it allows for the use of analytical methods for the computation of the error probability. First, the analysis has been applied to the simple Gaussian channel; afterwards, it has been extended to a multipath fading channel typical of large indoor areas. Use of a semi-analytical approach has been the key factor in obtaining reliable results at moderate computational complexity     

Finite element model of interdiffusion and reaction in polydisperse polymer blends

Numerical modelling of polymer blends production and processing, while technologically important, poses a considerable computational challenge when one or more of the polymer species is polydisperse. The mathematical formulation of such problems, where molecular weight distribution may range over several hundreds—which are very frequent in practice—leads to a huge set of coupled differential equations. In this work a numerical technique is presented that reduces the problem to a manageable number of degrees of freedom while keeping the essential physics of the original statement. For this purpose the distributions of molecular degrees of polymerization are approximated by continuous variables, and the set of concentration profiles replaced by a continuous field. The resulting equations are then reexpressed in weak form as a Galerkin integral identity and discretized using finite elements. Mass conservation and interchange chemical reactions as transesterification are treated using Lagrange multipliers. The accuracy of the technique—which can also be applied to the monodisperse case—is assessed through a number of simulations, and comparisons with experimental results published earlier. Copyright © 2000 John Wiley & Sons, Ltd.     

Thermal stress analysis of cordierite materials subjected to thermal shock

Cordierite disks were obtained by slip casting and sintering at 1,450 °C for 2 h. Sintered disks were indented at the center of one flat surface. The fracture strength of indented disks was measured in biaxial flexure. The critical temperature differential was determined by sudden cooling of the center of the indented surface at high temperature, using a high-velocity air jet at room temperature. The initial temperature was successively increased by 10 °C until crack propagation was detected with the naked eye. Temperature and stress distributions during air impinging were calculated by a finite element analysis. The heat transfer coefficient was estimated by fitting experimental temperature profiles. A radial variation of the coefficient that considers its increment beyond the stagnation region and a diminution toward the disk periphery was assumed. The calculated cordierite thermal stresses were analysed in relation to experimental data. Also, the comparison with alumina tested in similar conditions was included.