Influence of magnetic field on the aragonite precipitation

Many laboratory investigations of magnetic water treatment (MWT) for scale control explained the formation of less compact scales by raised portion of aragonite, which is needle-like and less adhesive than rhombohedral calcite crystals, while our experiment was conducted with tap water, which contains Mg⁺² and Fe⁺² ions in concentrations exceeding thresholds for calcite inhibition, common for major tap waters. MWT efficiency was evaluated by amounts of scale precipitated in boilers and pipes during three-weeks run of two parallel experimental lines—one with and another without magnetic treatment. All scales were identified by X-ray diffractometer to be aragonite, but in the case of magnetic treatment, scales occurred in much smaller amounts: the scale on heating copper-pipe spiral was 2.5-times thinner due to MWT and in zinc-coated steel pipe occurred as very thin powder-like coating, while in the line without the treatment abundant hard lining was formed. The scales’ morphology was observed by scanning electron microscope: the husks from both lines consisted of parallel distributed needles, but these crystals were about four times thinner in the case of MWT. The present work demonstrates that hard scale deposits can form even under conditions where aragonite precipitates predominantly, and that MWT can also affect the crystallization of this polymorph in a manner conducive to scaling control.     

Interpretation of calcite growth data using the two-step crystal growth model

The crystal growth rates of calcite were evaluated in a dense fluidized-bed crystallizer using a constant-composition method. Several operation variables related to solution properties that affected growth rate, including supersaturation, pH, ionic strength and activity ratio, were systematically investigated. Then the crystal growth-rate data were analyzed by the two-step crystal growth model and thus the mass-transfer and surface-reaction coefficients were obtained. The effects of the solution properties on the two individual coefficients were observed. Good explanations on these effects are needed.     

The effects of temperature and Ce-dopant concentration on the synthesis of zirconolite glass-ceramic

Zirconolite based glass-ceramic has been deemed as a promising waste form for high-level waste (HLW). In this study, the zirconolite (nominally CaZrTi₂O₇) glass-ceramic based on a SiO₂-B₂O₃-Na₂O-CaO-ZrO₂-TiO₂-Al₂O₃ system was sought to be synthesized by two-step thermal treatment. The temperature effects such as nucleation temperature and cooling rate were studied. The influence of the Ce-dopant concentration on crystallization behavior was also investigated. For characterization, powder X-ray diffraction (XRD), thermogravimetry-differential scanning calorimeter (TG-DSC) and scanning electron microscopy (SEM) were applied. Our results illustrate that homogeneous zirconolite with the particle size of 1.8?μm can be the target phase when the glass matrix underwent a rapid cooling process and the nucleation temperature was controlled at around 860?°C. Besides, the results also indicate that the distribution of the zirconolite along with its lattice parameters change regularly as the increase of CeO₂ concentration. A larger crystal size of zirconolite phase and some trace amounts of precipitated phases are observed when the CeO₂ concentration exceed 3?wt%. This work emphasizes the synthesis of zirconolite glass-ceramic in a SiO₂-B₂O₃-Na₂O-CaO-ZrO₂-TiO₂-Al₂O₃ system, which is expected to provide basic technical data for underground disposal of HLW.     

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.     

Studies on impurity ion and radiofrequency electric field effects on calcium carbonate deposition

The influences of impurity ions (Mg²⁺, Zn²⁺, Fe²⁺, Fe³⁺, and Al³⁺ ) and radiofrequency (44 MHz) electric field (RF) on calcium carbonate deposition in glass capillaries were studied. Calcium carbonate was precipitated from 0.1 M CaCl₂ and Na₂CO₃ solutions (100+ 100 ml) just before the inlet to the capillary. It was found that except for Al ³⁺ ions at 0.001 M concentration, the rest of the ions studied decreased CaC0₃ deposition. A minor influence of the RF field on the deposition was found in some of the systems tested, but it was in the range of the standard deviation of the results. An increase in deposition was observed in systems without impurity ions when equimolar volumes (500 + 500 ml) of 0.0025 M solutions of CaCl₂ and Na₂CO₃ were used for the experiment.     

Effect of magnetic water treatment on calcium carbonate precipitation: Influence of the pipe material

The main purpose of this work was to investigate the influence of the material of the pipe, through which scalant water (4 mM of CaCO₃) was magnetically treated, on the calcium carbonate precipitation process. Tested solutions were exposed to a magnetic field (MF) of 0.16 T for 15 min with different flow rates (0.54–0.94 L min⁻¹) and pH (6–7.5). Magnetic water treatment and pipe material (PTFE, Tygon, PVC, copper, and stainless steel) effects on CaCO₃ crystallization were assessed by means of a precipitation test based on the dissolved-CO₂ degasification method. It was shown that magnetic treatment (MT) affects calcium carbonate crystallization by increasing the total precipitate quantity and by favouring its formation in the bulk solution instead of its incrustation on the walls. This was observed for all used pipe materials but, it is strongly dependent on their physico-chemical properties. It was found that the homogeneous and total precipitation ratios were significantly influenced when MF was applied through non-conductive materials. It was also found that for the same pipe material, the surface roughness plays an important role on the effect of MT on CaCO₃ crystallization. In addition, it was shown that the pipe material strongly influences the nucleation process of CaCO₃ even in the absence of MF.     

The role of diffusional resistance on crystal growth: Interpretation of dissolution and growth rate data

Experimental results on crystal growth and dissolutions kinetics were taken from literature to test two theoretical models describing the crystal growth from solutions. One of the models, designated by two-step model (TSM), is being used for many years to describe the relation between crystal growth rates with supersaturation, temperature and hydrodynamic conditions. The other model was presented in a previous work and will be called concurrent-step model (CSM). The chosen literature data allowed calculating the mass transfer coefficients during crystal growth, for different hydrodynamic conditions. The obtained results are interpreted taking into consideration well-established mass transfer theories. According to the TSM, the measured crystal growth kinetics can only be explained by means of an unrealistic variation of the mass transfer coefficient with the relative crystal-solution velocity. Conversely, mass transfer coefficients obtained by the CSM were confirmed by appropriate semi-theoretical correlations, both in their order of magnitude and in their behaviour. In addition, crystal growth and dissolution experiments of sucrose were carried out at in a batch crystallizer for different agitation speeds. The resulting kinetics are used to test the CSM in a system that is significantly different from the inorganic salts used in the analysed literature works. As predicted by this model, the existence of an adsorbed layer in the crystal surrounding is likely to have affected the solute molecular diffusivity in the medium. Based on this premise, the results obtained with sucrose are well described by the CSM.     

磁场调控磁性纳米流体流动和传热研究进展

磁性纳米流体在实现能量高效和可控传递领域极具发展潜力。本文综述了磁场作用下磁性纳米流体对流换热及沸腾换热的最新进展,主要包括强制对流换热、混合对流换热、自然对流换热、池沸腾换热及管内沸腾换热等方面的实验研究,分析了磁场类型、强度、梯度、频率、方向及磁铁位置等对磁性纳米流体流动和热传输特性的影响,指出可通过改变外加磁场来实现对磁性纳米流体流动和传热的控制,并探讨了磁性纳米流体流-磁耦合作用下的传热机理以及目前所面临的挑战。在此基础上,提出了未来磁场调控磁性纳米流体对流换热和沸腾换热的主要发展方向：制备稳定的磁性纳米流体,建立系统有效的流动和传热理论模型,并从微介观尺度诠释热-流-磁耦合协同换热机理。     

磁场对碳酸钙结晶影响的探讨

磁处理技术在世界各地水处理系统中的广泛应用，证实了磁处理确实对抑制水垢的生成具有显著的效果。碳酸钙是各种水垢中最为常见的组成成分，为探索磁处理水的防垢机理，各国的研究人员对其进行了大量的实验研究，得到了许多实验结论。文章结合试验结论，采用“洛仑兹力”模型理论就磁场对碳酸钙结晶的影响进行了探讨，并且对某些实验结论不一致的现象提出了个人的看法。     

Influence of a magnetic field on the electrodeposition of nickel-iron alloys

The electrodeposition of nickel-iron alloys is studied under the influence of a superimposed external static magnetic field. It is shown that the direction of the magnetic field with regard to the electric field affects the electrodeposition process (current efficiency, composition and morphology of the layers). Furthermore, the influence of the simultaneous action of natural and magnetically driven convection is discussed in function of the orientation of the working electrode. The electrochemical quartz crystal microbalance (EQCM) technique is used to determine the partial current due to the hydrogen evolution reaction (HER). It is observed that the magnetic field applied perpendicular to the electric field lines increases the rate of the HER, while no significant variation is observed in a magnetic field parallel to the electric field.     

Bi electrodeposition under magnetic field

Bi was galvanostatically electrodeposited in a hydrochloric acid solution in the presence and absence of a 0.5 T field. The effects of magnetohydrodynamics (MHD) convection were focused on the concentration overpotential as well as the current efficiency. The morphological and microstructural variation of electrodeposited Bi thin film was also investigated. Dendritic growth enhanced at higher current density was considerably suppressed by superimposition of a 0.5 T field, while the effect on the crystal microstructure was not confirmed.     

Robust temperature coefficient of resistance of polycrystalline La0.6Ca0.4MnO3 under magnetic fields at room temperature

In the presence of magnetic field, reducing the loss in temperature coefficient of resistance (TCR) and increasing metal−insulator transition point (T_MI) to room temperature are the most important concerns for the application of perovskite manganites in high−precision thermistors. Based on emerging evidence, relevant factor to address these problems lies in the interaction between Jahn−Teller (JT) distortion and magnetic field, which results in spin−orbital coupling (SOC) effect and significantly influences TCR and T_MI. In this work, we studied the magnetic field induced SOC effect in polycrystalline La_1−xCa_xMnO₃ (x = 0.225–0.45) materials synthesized via sol−gel technique. Compounds undergo the JT distortions with increasing Ca dopant content, and the most pronounced distortion of 0.0144 at x = 0.40 is correlated with a basal−plane distortion mode. All samples exhibit T_MI values between 262 K and 288 K, indicating the enhancement of doping−induced double−exchange interaction. In the magnetic field of 1 T, SOC effect sufficiently suppresses the deterioration of TCR caused by deficient magnetization in La_0.6Ca_0.4MnO₃ to a value of 2.9%·K⁻¹ at room temperature (287 K).     

Near field and magnetic field generator for thermal assisted magnetic recording

We fabricated a generator that produces optical near field and magnetic filed in a nanometer area for achievement of thermally assisted magnetic recording. The generator consists of an embedded wire with a bottleneck structure on a SiO₂ substrate. The magnetic field is mainly generated around the bottleneck structure by feeding current through the wire. The near field is produced on an edge of the narrow wire by focusing a laser beam on the bottleneck structure through the backside of the substrate. The generator is anticipated as application to control ordering, chirality, and phase transition of diamagnetic materials in a nano-area. We confirmed the three-dimentional localization of near field in the nanometer size around the bottleneck structure by means of a near field scanning optical microscope.     

Particle growth kinetics of calcium fluoride in a fluidized bed reactor

Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L₀, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.     

Study of effect of temperature gradient on solid dissolution process under action of transverse rotating magnetic field

The study of effect of the transverse rotating magnetic field on the dissolution process of rock‐salt sample is the main purpose of this work. Moreover, the experimental study of the influence of the temperature gradient between the surface of sample and the solvent temperature on this process is presented in this article. The results of investigations are worked out by means of the novel type dimensionless equations including standard and magnetic numbers. The obtained results are compared with the data given in the previous literature. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Calcite particles formation, in the presence of soluble polyvinyl-alcohol matrix

The kinetics of calcite particles formation in the presence of soluble polyvinyl-alcohol matrix was investigated by the constant composition method. The presence of the above-mentioned polymer, induces the precipitation of this calcium carbonate polymorph at room temperature possibly through the -OH moieties. The apparent order for the crystallization process was found to be 2.25 ± 0.16 indicative of a surface nucleation mechanism. The number of ions forming the 3D critical nucleus was found to be p = 9. The surface energy of the growing phase was 28 mJ m^− 2 relatively small because of imperfections, in the form of vacant sites.     

层流冷却传热系数对金属物料温度场的影响

层流冷却的冷却强度决定着金属物料微观组织结构,从而影响产品的性能,而传热系数直接影响着冷却强度的大小。本工作提出了一种获得层流冷却传热系数的方法,通过工艺分析得到其分布形式为以喷头为中心由半波正弦和直线构成的分段函数,给出了通过结构参数、运行参数和实验数据获得函数特征参数的方法。以实验台验证了该方法的准确性,同时分析了特征参数变化对金属物料温度场的影响规律。结果表明,振幅增加1 W/(m2?K),金属物料表面温度相对下降量在最后一个区域达到最大值,为?0.36 m2?K2/W,断面温差最大相对变化量为0.07 m2?K2/W;周期减小1 s,金属物料表面温度相对增加量在最后一个区域达到最大值,为13.24 K/s,断面温差呈现周期性变化,变化范围为?2.5~0 K/s。     

温度场对硅酸铝纤维使用寿命的影响

利用实验室气氛电炉模拟陶瓷窑炉实际工况 ,使硅酸铝纤维在不同的温度场下暴露加热 ,探讨了硅酸铝纤维粉化机理及抗粉化方法。该研究对延长陶瓷窑炉使用寿命及窑炉结构的优化意义深远     

Interpreting the effects of operating variables on the induction period of CaCl2-Na2CO3 system by a cluster coagulation model

In this article the effects of several operating variables, including supersaturation, temperature, and the presence of additive and seeds on the induction period of CaCO₃ were studied experimentally and theoretically. In experimental, the induction period was measured by a conductivity method so that the induction period was well identified. The results show that the induction period increases with decreasing supersaturation and temperature. The presence of Mg²⁺ in solution prolongs the induction period and the addition of seeds reduces the induction period. In theoretical, a cluster coagulation model, which brings together the current nucleation models and the theories describing the behavior of colloidal suspensions, was applied to estimate the induction period under various operating variables. A comparison between the two results shows that the present model is a suitable one to interpret the effects of various operating variables on the induction period of aqueous CaCl₂-Na₂CO₃ solution.