Soldering of Mg Joints Using Zn-Al Solders

Magnesium has applications in the automotive and aerospace industries that can significantly contribute to greater fuel economy and environmental conservation. The Mg alloys used in the automotive industry could reduce mass by up to 70 pct, providing energy savings. However, alongside the advantages there are limitations and technological barriers to use Mg alloys. One of the advantages concerns phenomena occurring at the interface when joining materials investigated in this study, in regard to the effect of temperature and soldering time for pure Mg joints. Eutectic Zn-Al and Zn-Al alloys with 0.05 (wt pct) Li and 0.2 (wt pct) Na were used in the soldering process. The process was performed for 3, 5, and 8 minutes of contact, at temperatures of 425 °C, 450 °C, 475 °C, and 500 °C. Selected, solidified solder-substrate couples were cross-sectioned, and their interfacial microstructures were investigated by scanning electron microscopy. The experiment was designed to demonstrate the effect of time, temperature, and the addition of Li and Na on the kinetics of the dissolving Mg substrate. The addition of Li and Na to eutectic Zn-Al caused to improve mechanical properties. Higher temperatures led to reduced joint strength, which is caused by increased interfacial reaction.     

Influence of macromolecular structure of novel 2- and 4-armed polylactides on their physicochemical properties and in vitro degradation process

The objective of this study was to analyse how macromolecular structure of polylactides influences their properties and degradation rate. To achieve this, novel 2- and 4-armed PDLLA and PLLA (noted as 2b and 4b) were synthesized by ring-opening method. 1,4-butanediol and pentaerythritol were used as initiators and stannous octoate was used as catalyst. The obtained polymers were investigated in terms of molecular weight by size exclusion chromatography, thermal properties by differential scanning calorimetry and thermogravimetry, and hydrophilicity by the contact angle measurements. The in vitro degradation test was carried out in PBS solution at 37 °C by means of the mass loss, water uptake, molecular weight and thermal properties changes. The branched polylactides including 2bPDLLA, 4bPDLLA, 2bPLLA and 4bPLLA were successfully synthesized and the average molecular weights were around 40-45 kDa. The numbers of arms in each polymer just slightly influenced the thermal properties and the contact angle. The crystallinity of 4bPLLA was 23 %, whereas for 2bPLLA it was 41 %. The degradation rates of both 2b and 4bPLLA were similar and the degradation process was similar only during first 7 weeks. After this period, the degradation rate of 4bPDLLA increased. Consequently, thermal properties and degradation profiles of the branched polymers would depend on plural factors, such as chain length and crystallinity in branched structure.     

Modeling of the degradation kinetics of biodegradable scaffolds: The effects of the environmental conditions

The rate of hydrolytic degradation of tissue‐engineered scaffolds made from bioresorbable polyesters is dependent on several factors. Some are related to the properties of the degrading polymeric material, but others are related to the geometry of the porous structure and the operating environment. It is well known that the rate of hydrolytic degradation of a given object, porous or nonporous, is lower when it is exposed to dynamic conditions, a flowing medium, than when it operates in static conditions. The most likely reason is the more efficient removal of the acidic degradation products from the vicinity of the polymeric material when it is operating in a flowing medium. In this article, we present a new phenomenological reaction–diffusion model of aliphatic polymer degradation. The model can be used to predict the significance of various factors in in vitro degradation tests, with particular reference to the flow of the degradation medium, and the frequency of medium replacement in the case of static conditions. The developed model was used to simulate the degradation of poly(dl ‐lactide‐co‐glycolide) scaffolds with different porosities subjected to static and dynamic testing conditions. The results confirm that the porosity of the scaffold had a significant influence on the degradation rate. It was shown that the combination of dynamic conditions and high porosity effectively reduced the mass loss and molecular weight loss of the degrading polymer. However, the effect of changes in the velocity of the flowing medium had a negligible effect on the rate of degradation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40280.     

Sensitivity Analysis as a Tool for Estimating Numerical Modeling Results

Modeling of real physical processes by numerical methods is highly time-consuming and requires significant computational capacity. In some cases, tens or even hundreds of hours of high-power computing are needed to virtually model a real process that lasts one second. Processes that take many hours, such as drying, pose an even greater challenge. This problem can be solved in two ways: by using faster computers (such as computing clusters) or by significantly simplifying the modeled process (its geometry, physical phenomena, or the impact of individual factors). For this reason, all methods which speed up or minimize the number of simulations required to achieve the research objective should be analyzed. This article focuses on the latter approach, and it proposes a simple method for predicting the responses of a numerical model (values of any output parameter) to changes in input values (values of any input parameter). This method requires a base model, such as a numerical model which is qualitatively and quantitatively consistent with experimental observations, and a sensitivity analysis. This article discusses the mathematical and logical premises for the discussed model, and it proposes two methods for predicting numerical simulation results. Those methods are illustrated with examples which analyze the behavior of the Eulerian Multiphase Model and describe phase interactions based on Gidaspow's approach. The discussed example relies on data from a series of articles published by the authors in Drying Technology. This article was inspired by the observations made during a time-consuming process of modeling a spouted bed grain dryer, which was described in the above publications. The objective of this study was to discuss the advantages and possibilities created by sensitivity analyses of numerical models and to encourage their practical application.     

Determination of thermal radiative properties of packed-bed media containing a mixture of polydispersed particles

Thermal radiative characteristics of packed beds containing a mixture of polydispersed SiO₂, ZnO, and C particles are determined numerically by employing the Monte Carlo technique, which is validated with the experimentally measured overall transmittance. A radiative heat transfer model is formulated for a pseudo-continuum multi-component medium of Mie-scattering particles. Good agreement is achieved by incorporating approximate phase functions that reproduce the experimentally observed preference for forward scattering.     

The Numerical Comparison of Heat Transfer in a Coated and Fixed Bed of an Adsorption Chiller

Ecological adsorption technology is becoming a focus of attention by industry due to the utilization of low grade thermal energy sources for cooling production. It can be a promising part of sustainable development concept of the global economy. Therefore, research aiming at improving their performance i.e. Coefficient of Performance (COP) by optimizing the construction of sorption beds with a built in heat exchanger system is crucial. The heat transfer characteristics between the bed of porous media (sorbent) and surface of the heat exchanger system determine the heating power of an adsorption chiller. The HP increase can be obtained by heat transfer intensification due to the increase in the thermal conductivity of the sorbent layer in the vicinity of the heat exchanger’s surface. The novel modification of the sorbent layer structure is proposed in the paper in order to improve the heat transfer processes in the heat exchanger boundary layer. The analysis of desorption process conditions in the parametric model of a coated and fixed adsorption bed design is presented in the paper. The computational fluid dynamics (CFD) with conjugate heat transfer analysis is used to determine the crucial input parameters (temperature distribution in the sorbent bed) for further analytical calculations. The commercial code Ansys Fluent was used to perform numerical simulations. The developed computational model consisted of three subdomains representing heating water, heat exchanger material (copper) and sorbent (silica gel). The comparison of a novel coated design and a conventional fixed bed is discussed in the paper. The numerical analysis is based on experimental thermal conductivity measurements of the sorbent layer in different configurations, which were performed using Laser Flash Method.     

N‐Oligo(3‐hydroxybutyrate)‐functionalized polypyrroles: towards bio‐erodible conducting copolymers

New copolymer materials have been prepared by chemical grafting of oligomeric 3‐hydroxybutyric acid (OHB) onto polypyrrole (PPy) derivatives. The influence of grafting density and molecular weight of OHB brushes on the physicochemical properties of prepared copolymers was investigated. PPy substrates were prepared by FeCl₃‐driven oxidative homopolymerization of N‐(2‐carboxyethyl)pyrrole or its copolymerization with pyrrole. The grafting method employed involved controlled anionic polymerization of β‐butyrolactone on pyrrole‐tethered potassium carboxylate active sites. Obtained PPy‐g‐OHB copolymers of varying grafting density and pendant polyester chain length were characterized and the observed structure–property relationships discussed. The impact of real time exposure to phosphate‐buffered saline environment was investigated and the residue products were characterized. Cross‐correlation of spectroscopic, thermal, electrical and elemental analysis data afforded comprehensive evaluation of the structure of prepared materials and their behaviour in hydrolytic medium. Erosion and degradation pathways have been identified, indicating ways to consciously tailor the physicochemical properties of these new biomimetic materials. © 2016 Society of Chemical Industry     

Stripping of copper coatings from steel in Cr(VI)-free commercial bath

In this work the electrochemical characteristics of copper and steel in chromate, cyanide, and phosphate baths as well as in a commercial bath (ENSTRIP S-180), in the absence of chromium and cyanides were determined. Average rates of copper coatings stripping from steel in the above mentioned baths and the baths influence on the morphology of steel surfaces were described. It was found that the commercial bath ENSTRIP S-180 could be successfully used for stripping of copper coatings from steel elements.     

Control of linear extended nD systems with minimized sensitivity to parameter uncertainties

A new view on uncertain system parameters is presented considering them in the same way as other independent variables, e.g., time or space variables. After re-interpreting the well known equations for the sensitivities of a system to parameter changes, we consider the problem of optimal control that takes into account not only the quality of control itself, but also a reduction in the influence of parameter changes. Firstly, we re-derive and elucidate known results for systems described by linear ordinary differential equations in the state-space form. Then, it is shown how to extend the well known theory of designing optimal controllers with quadratic criterion so as to cover the reduction of uncertainties in systems described by a class of linear partial differential equations. As a result, we obtain a controller that has a new modal structure in space. Furthermore, the controller incorporates additional sensitivity signals for each mode.