Solidification Period Research Articles

Improved heat recovery from paraffin-based phase change materials due to the presence of percolating graphene networks

Abstract Presently, engineers are unable to fully utilize the high thermal energy storage capacities of paraffin-based phase change materials (PCMs) in electronics cooling and waste heat recovery applications due to their inherently low thermal conductivities, which result in slow melting and solidification rates. In order to increase the paraffin’s thermal conductivity, several groups have implanted nanoparticles within PCMs. Despite remarkable improvements in their thermal conductivities, however, it is expected that less thermal energy can be harvested during the nanocomposite PCM’s solidification period due to the removal of some PCM mass in favor of the nanoparticles. In this study, a heat exchanger system is used to extract thermal energy that has been stored within paraffin nanocomposites embedded in a thermal containment unit (TCU). We find that the amount of thermal energy that can be harnessed from MWCNT, Al or TiO2 nanocomposite PCMs (at 20 v.% concentrations) is approximately 15–17% lower than the amount that can be extracted from the base paraffin during its period of solidification, as expected. However, the amount of thermal energy that can be harnessed from paraffin in the presence of graphene nanoparticle networks (15 nm thickness, 15 μm diameter, at 20 v.%) is found to be nearly 11% greater than for the base paraffin. Based on these results, it is posited that the paraffin’s alkane molecules are beginning to align across the face of the graphene nanoparticles, resulting in a more crystalline paraffin structure at the graphene–paraffin interface and a higher absolute phase change enthalpy. Consequently, it is expected that this work will open up new avenues of research for the creation of advanced nanocomposite PCMs with exceptionally high thermal conductivities and thermal energy storage capacities.

Analysis of the influence of the gas velocity, particle size and nucleation temperature on the thermal history and microstructure development in the tool steel during atomization

The paper deals with the analysis of the influence of the nitrogen gas velocity, particle size and nucleation temperature on the microstructure development in the tool steel of ledeburite type Ch12MF4 in the process of atomization. Three main types of solidification microstructures were identified using microscopic analysis in rapidly solidified powder particles: dendritic, cellular (grain-refined) and compound, representing the mixture of dendritic and cellular microstructures. It can be supposed that the transition from initially dendritic to compound and cellular or grain-refined microstructure occurs by the mechanism of thermally induced fragmentation of dendrites during quasi-isothermal period of solidification if additionally the recalescence temperature is sufficiently high. The microstructures documented the procedure of dendrites fragmentation and following spheroidization of dendrite fragments in rapidly solidified powder particles from the Ch12MF4 alloy are presented.Applying developed mathematical model of cooling and solidification of a single spherical droplet in atomization process, the influence of nitrogen gas velocity and other parameters as nucleation temperature on the thermal history of a droplet with various diameters during atomization is studied. Based on the comparison of results of mathematical modeling and microstructural observations, the thermophysical conditions for the transition from dendritic to cellular (grain-refined) microstructure development in the rapidly solidified powder particles from the Ch12MF4 steel are predicted.

Semi-quantitative model of the microstructure development in the high-alloyed iron based alloy during atomization

The paper deals with the analysis of microstructure formation in the tool steel of ledeburite type Ch12MF4 with the chemical composition of 2.37% C, 12.06% Cr, 1.2% Mo and 4.0 % V [wt. (%)] in the process of nitrogen gas atomization. Three main types of solidification microstructures were observed in rapidly solidified powder particles: dendritic, compound and cellular. Based on the morphological features of microstructures observed in rapidly solidified particles and mathematical modeling of the thermal history of solidifying spherical droplets, the semi-quantitative model of the microstructure development in the Ch12MF4 steel during atomization was suggested. According to this model, it is supposed that the transition from dendritic to partially dendritic (compound) and nondendritic microstructures results from the thermally induced fragmentation of dendrites by the mechanism of their remelting, morphological changes of dendrite fragments and following spheroidization. The intensity of dendrite fragmentation in solidifying particles of different diameters is controlled mainly by the recalescence temperature and duration of quasi-isothermal period of solidification.

Numerical modeling to understand liquation cracking propensity during laser and laser hybrid welding (I)

Two-dimensional thermal elasto-plastic model was established by finite element method to investigate weld heat-affected zone (HAZ) liquation cracking behavior during laser and laser–gas metal arc (GMA) hybrid welding nickel-based superalloy. Transient temperature, cooling rate, and stress and strain history were used and combined with solidification theory to analyze the mechanism of liquation cracking in the HAZ. Weld cracking mechanical driving force is related to local stress–strain development at the period of solidification, and stress and strain components were correlated along the fusion boundary of weld pool. Strain rate and weld pool geometry were demonstrated during in the stage of solidification process, which provide valuable insight into the quantitative evaluation the tendency of solidification cracking and give well understanding why laser–GMA welding is beneficial for minimizing cracking susceptibility than laser welding.

Influence of the thermal history of a particle during atomization on the morphology of carbides in a hypereutectic iron based alloy

Basic principles and consequences of the rapid solidification processing of melts have been successfully exploited in several progressive technologies of material production. In the paper, the solidification microstructures developed in the hypereutectic iron based alloy with the chemical composition of 3% C–3% Cr–12% V (wt.%) prepared by nitrogen gas atomization are presented and analysed. Several main types of solidification microstructures were identified in the rapidly solidified powder particles. According to the morphological features of carbide phases and computed thermal history of rapidly solidified particles, the microstructures were divided into two groups – microstructures morphologically non-affected during the post-recalescence period of solidification, and microstructures with morphological transitions occurring during the quasi-isothermal period of structure development. Based on the thermokinetic newtonian model of rapid solidification of a spherical droplet in the process of atomization, the thermal history of droplets with diameter from 20μm to 400μm rapidly solidified from different nucleation temperatures was studied. The thermo-physical conditions necessary for the development of variable microstructures in single rapidly solidified powder particles are predicted and discussed. The nucleation temperature, recalescence temperature and duration of quasi-isothermal plateau are supposed to be the most important parameters influencing the microstructure development in the rapidly solidified particles of the investigated alloy.

Immobilization of Bacillus megaterium MTCC 2444 by Ca-alginate entrapment method for enhanced alkaline protease production

Optimization of culture conditions and immobilization parameters for alkaline protease production was carried out by employing Bacillus megaterium MTCC2444. The partially purified enzyme was tested for its stability in the presence of oxidants, surfactants and commercial detergents. The optimum temperature, pH, incubation time and inoculum size were 55 oC, 11, 48 h, 1 %, respectively. Calcium alginate was used as the immobilization matrix and the effects of gel concentration, bead size, age of immobilized cells, solidification period and initial biomass concentration on alkaline protease production and cell leakage were investigated. The results indicated that the immobilization was most effective with 4 % gel concentration, bead size of 3 mm, 24 h aged immobilized cells for a solidification period of 12 h at 1.5 % initial biomass concentration. The enzyme showed good stability in the presence of oxidants, surfactants and commercial detergents.

Effect of steady crucible rotation on the segregation of impurities in vertical Bridgman growth of multi-crystalline silicon

We have performed axisymmetric, transient simulations of the vertical Bridgman growth of multi-crystalline (mc) silicon to study the effect of the steady crucible rotation on the melt flow and impurity segregation. A solute transport model has been applied to predict the final segregation pattern of impurities in a circular ingot. Imposing rotation rates of 1–5 rpm on the system makes radial segregation much worse compared to the non-rotating case. Low rotation rates at 1–2 rpm increase radial segregation in the first half period of solidification, whereas at rotation rates above the effect is insignificantly small. Contrary behavior was observed for the second half period of solidification. Here radial segregation is increased at high rotation rates from 3 to 5 rpm with small impact at 1–2 rpm.

Simulation of mold filling and prediction of gas entrapment on practical high pressure die castings

Element parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio for DFDM (direct finite difference method) were proposed to describe shape and location of free surfaces in casting mold filling processes. A mathematical model of the filling process was proposed specially considering the mass, momentum and heat transfer in the vicinity of free surfaces. Furthermore, a method for gas entrapment was established by tracking flow of entrapped gas. The model and method were applied to practical ADC1 high pressure die castings. The gas entrapment prediction was compared with the fraction and maximum size of porosities in the different casting parts. The comparison shows validity of the proposed model and method. The study indicates that final porosities in high pressure die castings are dependent on both gas entrapment during mold filling process and pressure transfer within solidification period.

Effect of sodium modification on microstructure and mechanical properties of thick-walled AlSi6Cu2.5 rheocast component

The microstructure and tensile properties were investigated in a thick-walled section (approximately 45 mm×43 mm) of a rheocast component produced by the RheoMetal™ process. Due to the long solidification period of such components, it is expected that the Al-Si eutectic formed will be coarse. Therefore, sodium (Na) was used as a modifying agent to reduce the coarseness of the eutectic. Tensile test bars were machined from three different sets of materials: 1) non-modified melt, 2) modified melt cast directly after Na addition, and 3) modified melt cast 30 min after Na addition. The alloy used was a secondary AlSi6Cu2.5 alloy (STENAL Rheo1), specially developed for rheocasting. The material was studied in the as-cast condition as well as after a T6 heat treatment. The results show that the Al-Si eutectic is significantly refined by the Na addition, even after a fading time of 30 min. However, it is observed that the Na modification generally has a detrimental effect on the mechanical properties, despite the structure refinement. This is especially true in the T6 heat treated condition, where the yield strength is reduced by more than 30%. Some possible mechanisms for the degradation of mechanical performance are discussed.

Influence of yttrium additions on the hot tearing susceptibility of magnesium–zinc alloys

The influence of Y additions on the hot tearing behaviour of Mg–Zn alloys was investigated in this study. In permanent mould castings and in direct chill cast ingots, alloying of a few wt.% Y results in a significant reduction of hot tearing susceptibility. The reduced susceptibility is attributed to the effect of Y on the solidification path at the terminal period of solidification: it increases the solidus temperature and thus shortens the solidification path, which in turn reduces the terminal freezing range. Via thermodynamic calculations it is shown that this is caused by the formation of the ternary phase Mg 3YZn 6. Using a simplified version of Clyne and Davies’ model, the influence of the terminal freezing range on hot tearing susceptibility is clearly illustrated.

Letter to the Editor re: Superselective arterial embolisation with a liquid polyvinyl alcohol copolymer in patients with acute gastrointestinal haemorrhage

Sir, We read with great interest the article by Lenhart and colleagues recently published in European Radiology and reporting effectiveness of selective arterial embolisation using Onyx® in patients with acute gastrointestinal bleeding [1]. We have several comments. First of all, we would like to congratulate the authors for their study which represents the first series to date reporting results on arterial embolotherapy with Onyx® as an embolic agent in the gastrointestinal tract. Until now, the only available data on the use of this embolic material for peripheral applications came from case reports. Indeed, Onyx® is a new liquid embolic agent composed of ethylene-vinyl alcohol copolymer dissolved in dimethyl sulphoxide (DMSO), with emerging applications in neurovascular procedures, predominantly embolisation of cerebral aneurysms and arteriovenous malformations [2]. The main advantages of Onyx® are its nonadhesive properties, high radiopacity, and long solidification periods, which, compared with acrylic glues, make the embolisation procedure more controllable and predictable. However, the authors did not bring to attention several important disadvantageous characteristics of the use of Onyx®. First, DMSO can cause severe vasospasm if injected rapidly. This is most likely to occur early in the procedure and can be avoided by using no more than 0.2 ml of DMSO in the first minute of injection. Even if the authors reported a total injection time of DMSO and Onyx® of 10 min on average for their procedure, in our experience, the duration of injection is often much longer and varies depending on the amount of Onyx® used. Secondly, DMSO is volatile and is excreted via respiration and sweat. This has a typical smell not unlike that of diabetic ketoacidosis and may last a few days. The patient and ward staff should be warned to expect this. Lastly, the use of Onyx® has cost implications as it is much more expensive than alternative embolic materials and it requires specific DMSO-compatible microcatheters. The prohibitive costs of using Onyx® have led to its restricted use in neuroradiology in most of the institutions around the world. In conclusion, we think this embolic agent is very promising but the important cost aspect needs to be borne in mind when there are other cheaper alternatives that would be as effective and faster when used on an emergency basis.

Core Melt Solidification Characteristics in PRV Lower Head-Experimental Results From LIVE Tests

Core melt solidification phenomena in the lower plenum of pressurized reactor vessel during external reactor vessel cooling is investigated in late in-vessel phase experiment tests under different external cooling conditions and melt pouring positions. The melt solidification behavior, which has not yet been given sufficient attention, is an important issue since it influences not only the transient but also the steady state of melt pool thermal hydraulics. A noneutectic melt (80 mol %KNO3–20 mol %NaNO3) was used to simulate the core melt. It has been found out that when the vessel is cooled with water during the whole test period (water cooling), the cooling is more effective than the case that the vessel lower head is first cooled with air and flooded by water (air/water cooling). Water cooling at the beginning leads to faster buildup of crust layer on the vessel inner wall and lower crust thermal conductivity compared with air/water cooling. In comparison with the air/water cooling, the water cooling also achieves shorter time period of crust growth. During the solidification period in all tests, the constitutional supercooling condition is fulfilled. Pouring position near the vessel wall results in considerable asymmetry in the heat flux distribution through the vessel wall.

Jews and Jesuits in a Confessional Age: Heinrich Heshusius and the Boundaries of Community in Hildesheim

The early age of orthodoxy in German Lutheranism was a crucial period of transi-tion and solidification between the conciliatory Formula of Concord in 1577 and the work of early seventeenth-century dogmaticians such as Johann Gerhard (1582– 1637). During this “confessional age” Lutheran theologians struggled to protect the “pure teachings of Luther” while consolidating religious reforms and engaging in polemical battles with Roman Catholics, Calvinists, Anabaptists, and Jews. This article sheds new light on early orthodoxy by examining the career of Heinrich Heshusius (1556–97), a Lutheran superintendent in the north German city of Hildesheim. Together with the town council, mayor, and a network of Lutheran pastors, Heshusius fought what he saw as the dangerous inroads of local Jews and Jesuits, using a variety of political and polemical tactics. This article explores the confessional literature related to the controversy, explains attempts at conversion as well as expulsion, and contributes to ongoing discussions about how clergymen shaped and controlled community in urban, parish, and academic settings.

Pelletizing and alkaline leaching of powdery low grade zinc oxide ores

Low grade powdery zinc oxide ores (5.2% Zn, <2 mm) are mixed with cement 5 wt.%, pelletized and solidified. The diameters of the pellets obtained are between 5 mm to 8 mm. When the pellet solidification periods are 3 days, 10 days and 45 days respectively, the alkaline leaching rates of zinc in the pellets are up to 92.2%, 87.3% and 72.9% respectively. Decreasing the solidification time can reduce reaction time, increase dissolution of zinc in pellets and lower the effect of initial zinc concentration on leaching rate. The experiment results show that the minimum solidification time is three days, and the kinetic study indicates that alkaline leaching of the low grade zinc oxide pellets is controlled by the diffusion of the leach liquor through the gangue layer in the whole leach process, and the apparent rate constants are 3.51×10−2 day−1, 8.09×10−3 day−1, 4.74×10−3 day−1 respectively.

The numerical simulation of the water-filling in the water tank with two ingates

Recently, with the development of computer technology and CFD (computational fluid dynamics) methods, the Navier-Stokes equations are directly solved for the simulation of many surface problems including fluid flow in channel, wave breaking and overtopping processes. In this paper, a computational method, which is based on a modified SMAC method, has been developed to analyze two dimensional incompressible viscous flows and to calculate the visualization of fluid flow during the transient filling of liquid water for the solidification period. The flow pattern, filling sequence, and the flow gradient distribution in the water tank are predicted. In this work, the changes of flow velocity and inflow are simulated by the diameter changes of two ingates for 4-Cas es, respectively. The simulation results are expressed in the form of visual information with plots of marker particle configurations and velocity vectors. Due to this information, the water-filling flow pattern in the water tank can be simulated very well. This numerical simulation will also be applied for the design of structures such as an open flume and a porous breakwater.

An estimate of stability of large solidifying droplets in fuel–coolant interaction

An initial period of surface solidification of large opaque particles of molten metal oxides is considered. It is shown that low overheating and very high melting temperature of corium lead to very fast formation of solid crust on the particle surface. The transient problem solution showed that maximal tensile stress in this crust due to pressure drop in expanding steam bubble around the particle is much less than the stress in relatively thin crust layer on the surface of alumina particle. The solidifying corium particle seems to be more stable as compared with the alumina particle of the same size. The latter is treated as one of the reason of the experimental results on relatively low explosivity of corium.

Heat transfer characteristics of lost foam casting process of magnesium alloy

Effects of vacuum, pouring temperature and pattern thickness on the heat transfer of magnesium alloy lost foam casting(LFC) process were explored. The results indicate that without vacuum a positive thermal gradient from the gate to the end of the casting was formed immediately after the mold filling. The average temperature of the casting, the temperature gradient and solidification times increase significantly with pouring temperature and pattern thickness. Vacuum plays a quite different role in the heat transfer during mould filling and solidification periods: it significantly increases the cooling rate of the filling melt, but decreases the cooling rate of the casting during solidification period. The temperature of the liquid metal drops sharply and varies greatly with no apparent mode in the casting after the mold filling. The amplitude of temperature fluctuations in the casting increases with vacuum, pouring temperature and pattern thickness. The average temperature increases with pouring temperature and pattern thickness, but less rapidly than that without vacuum. The effect of vacuum on the solidification times of castings is found to depend on pouring temperature, vacuum makes solidification times increase greatly at high pouring temperature, while decreases slightly at low pouring temperature.

Modelling of the micro-segregation of twin-roll thin strip casting

In this paper, a developed micro-segregation model was used to simulate the micro-segregation of solutes in twin-roll casting of carbon and stainless steels. The relationship between the solidus fraction in solidification and temperature was given, which was used to determine the LIT, ZST and ZDT temperatures in the period of non-equilibrium solidification. Based on the experiment of twin-roll thin strip casting in lab, the cooling rate was determined. The effects of the cooling rate and temperature on the micro-segregation were discussed. When the cooling rate increases the micro-segregation between the dendrites becomes large, and it is small within the dendrite. According to the computational results, the solidification completion temperature in the twin-roll strip casting was then determined, which can provide a basis for dealing with casting process and quality controlling.

Morphological variants of carbides of solidification origin in the rapidly solidified powder particles of hypereutectic iron alloy

Abstract The paper deals with the analysis of the morphological variants of solidification microstructures and vanadium rich M 4 C 3 carbide phases in the rapidly solidified (RS) powder particles from hypereutectic Fe–C–Cr–V alloy prepared by the nitrogen gas atomisation. Five main types of solidification microstructures were identified in RS particles: microstructure with globular carbides, microstructure with globular and star-like carbides, microstructure with primary carbides in the centres of eutectic colonies, microstructure with eutectic colonies without primary carbides and microstructure with eutectic spherulites. Based on the morphological features of carbide phases and the thermal history of RS particles, the microstructures were divided into two groups—microstructures morphologically affected and non-affected during the post-recalescence period of solidification. Thermophysical reasons for the morphologically different M 4 C 3 carbide phases development in the RS powder particles are discussed.

Phase separation and structural evolution of undercooled Fe–Sn monotectic alloy

Abstract Fe–48.8wt.% Sn monotectic alloy was rapidly solidified both in a drop tube and with the glass fluxing method, and its phase separation characteristics and structural evolution are investigated. The regime of cooling rate of different size droplets in the drop tube is calculated from 7.0×102 to 2.8×104 K s−1 with a decrease of droplet diameter from 800 to 100 μm. The effect of Marangoni migration is more significant than that of Stokes motion with the decrease of gravitational acceleration, but it has a weak influence on the dispersion of the second liquid phase. The immiscibility gap is calculated and it shows that spinodal decomposition is difficult to occur because the necessary undercooling is quite large. Nucleation and growth show different characteristics when droplet size varies. Although the Sn-rich liquid phase separates from the undercooled melt, it cannot change significantly the composition of parent liquid phase, and the growth of terminal composition solid is suppressed, which is confirmed by the measured and calculated dendritic growth velocity by bulk undercooling experiments with the glass fluxing technique. Well-branched dendrites of the solid phase are difficult to form during monotectic transformation when droplets solidify in the drop tube owing to the very short solidification period.