Radial Thermal Gradients Research Articles

Numerical study of convection-induced peritectic macro-segregation effect at the directional counter-gravity solidification of Ti–46Al–8Nb alloy

Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace with power-down thermal profile operation. The laboratory refinement of cylindrical ingots growth technique was developed in course of terrestrial preparation experiments in the facility specially designed for a sounding rocket flight. Despite Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically-transformed microstructure was observed in solidified ingots, where Al content locally exceeds 47 at%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat–mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks-up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy obtained confirms the modeling findings.

Simulation of Outside-in Cracking in Boiling Water Reactor Fuel Cladding Tubes under Power Ramp

Abstract In power ramp tests, some high burn-up boiling water reactor (BWR) fuels failed by outside-in cracking. After detailed post irradiation examinations, it was deduced that fracture of radially oriented hydrides precipitated at the outer surface had resulted in the formation of incipient radial cracks that propagated towards the inner surface. Since crack propagation is a key process that dominates the outside-in failure, experimental data of outside-in cracking are obviously required. The objective of the present study is to evaluate the threshold conditions and the velocity of the outside-in cracking during a power ramp to assess the integrity of BWR fuels during a power ramp. In the experiments, by using unirradiated Zry-2 fuel cladding tubes, the outside-in cracking was examined both with and without the thermal gradient. Without the radial thermal gradient, there was no radial delayed hydride cracking in all experimental conditions examined. On the other hand, with the radial thermal gradient, the velocity of the outside-in cracking was significantly higher than that expected from the data obtained under the isothermal condition in our previous study. The threshold depth of the incipient crack was found to be ∼0.1 mm at the hoop stress of 300 MPa. It was suggested that the outside-in cracking during the power ramp is strongly dependent on the distribution of dissolved hydrogen as a result of the thermal diffusion.

Preparation and characterization of carbon/carbon aircraft brake materials with long service life and good frictional properties

A320 series aircraft carbon/carbon brake materials were prepared by processing a needle-punched carbon fiber preform with quasi-three-dimensional integrity. The preform was first impregnated with a resin and heated to 800 °C to convert the resin to carbon. This was followed by chemical vapor infiltration (CVD) using propylene as the source gas and a radial thermal gradient (“external heating, internal cooling” or “internal heating, external cooling”). For the CVD process the performs were separated by graphite pads to allow free flow of propylene along the channels thus formed. The final brake discs were obtained by a further high-temperature treatment from 1600–2400 °C. These carbon brake discs were compared with the advanced discs made abroad in terms of ground test and service performance. The two kinds of carbon brakes have the same friction coefficients under the designed landing energy and overload energy braking condition for A320 airplanes. Especially, under the aborted take-off condition, the friction coefficient of the domestically developed carbon brake disc was 21-48% higher and the static friction coefficient was 28% higher than that of the ones provided by the aircraft manufacturer. The average brake service life exceeds 2700 landings, 24% times larger than the original ones.

Thermal analysis of the effects of thermally induced nonreciprocity in fiber optic gyroscope sensing coils

The response of interferometric fiber optic gyroscope sensing coils to a time-varying, radial thermally induced nonreciprocity is investigated. Sensing coils wind with polarization-maintaining fibers in quadrupolar wind configurations developed in our study examining the effect of radial thermal gradients are used with experimental setup. Induced gyroscope bias drift, measured as a function of temperature and time, is compensated for quadrupolar winding configurations using our model. Our results show that thermally induced nonreciprocity can be calculated with the same coefficient in difference temperature changing conditions using our model.

High-pressure vertical Bridgman growth of lead magnesium niobate–lead titanate single crystal

A modified high-pressure vertical Bridgman (HPVB) method has been used to grow single crystals of lead magnesium niobate–lead titanate (PMN–PT) using self-seeding. The starting charge was prepared close to stoichiometric composition of (1− x)Pb(Mg 1/3Nb 2/3)O 3– xPbTiO 3, ranging from x=0.29 to x=0.32. The ingots grown had a top diameter of ∼114 mm and weighing ∼1.9 kg with a volume of ∼230 cc. Standard form off-the-shelf platinum crucibles from vendors with a wall thickness ∼0.34±0.01 mm were used. The thinner platinum helped to attain a larger radial thermal gradient as well as enhancing the cost effectiveness of the process by reducing usage of precious metals. An oxygen partial pressure of 8000–10 000 ppm was used and controlled during the entire growth process. Self-seeding technique was used in order to attempt to grow large diameter single crystals of PMN–PT. The composition variation in the ingot is dependent on the growth rate, thermal gradients and oxygen partial pressure. The segregation coefficient of Ti was 0.86±0.008 for the starting composition of x=0.32.

Modeling creep in a rotating disc with linear and quadratic composition gradients

PurposeThe purpose of this paper is to investigate the effect of imposing linear and quadratic composition gradients on the steady state creep behavior of a rotating functionally graded Al‐SiCP disc operating under a radial thermal gradient.Design/methodology/approachMathematical model to describe steady state creep behavior in rotating discs made of isotropic aluminum composite containing linear and quadratic distributions of Silicon Carbide (SiCP) in the radial direction has been formulated. The discs are assumed to operate under a radial thermal gradient originating due to braking action as estimated by FEM analysis. The steady state creep behavior of the discs under stresses developing due to rotation has been determined following Sherby's law. Based on the developed model, the distributions of stresses and strain rates have been obtained and compared for various functionally graded material (FGM) discs containing the same average amount (20 vol per cent) of dispersoid. The creep response of a composite disc with uniform SiCP content of 20 vol per cent and operating under a radial thermal gradient has also been computed for comparison with the results obtained for FGM discs.FindingsThe study reveals that the distribution of stresses and strain rates in a rotating composite disc operating under a radial thermal gradient are significantly affected by different particle distributions with in the disc. The creep stresses and steady state creep rates in a rotating FGM disc can be significantly reduced by employing more SiCP particles in the middle compared to the inner and the outer radii.Originality/valueThe study provides an understanding of the required tailoring of composition in order to control creep stresses and creep rates in a rotating FGM disc operating under a radial thermal gradient.

A synaptic DEG/ENaC ion channel mediates learning in C. elegans by facilitating dopamine signalling

An important component of learned behaviour is the ability to forecast positive or negative outcomes based on specific sensory cues. Predictive capacity is typically manifested by appropriate behavioural patterning. However, the molecular mechanisms underlying behavioural plasticity are poorly understood. Caenorhabditis elegans displays experience-dependent behavioural responses by associating distinct environmental signals. We find that ASIC-1, a member of the degenerin/epithelial sodium channel family, which localizes at presynaptic terminals of dopaminergic neurons, is required for associative learning in C. elegans. ASIC-1 functions in these neurons to amplify normal dopaminergic signalling, necessary for associative learning. Our results reveal a novel role of DEG/ENaC ion channels in neuronal communication by enhancing the activity of dopaminergic synapses. Similar mechanisms may facilitate synaptic plasticity in vertebrates.

Grain control using spot cooling in multi-crystalline silicon crystal growth

We report a novel idea for grain control by using spot cooling for multi-crystalline silicon (mc-Si) crystal growth. The method employs a graphite crucible with cooling spots to generate radial thermal gradients of the silicon melt for nucleation during directional solidification. The effect of cooling spots on the grown ingot was studied by minority charge carrier lifetime mapping and electron back scattered diffraction (EBSD) of the grown crystals. It was found that charge carrier lifetime for silicon crystals grown with cooling spots was higher than those grown without cooling spots. The EBSD analysis also showed the formation of dendrite structures during cooling controlled growth. Further, the proposed method was found to be feasible for industrial-scale growth of mc-Si by directional solidification, and some preliminary results are presented.

Physical properties of the 0.12 KLu(WO 4) 2–0.88 K 2W 2O 7 solution and single-crystal growth of KLu(WO 4) 2

We have obtained the solubility curve of KLu(WO 4) 2 in K 2W 2O 7 solvent. From this curve, we have chosen a solution composition, 0.12 KLu(WO 4) 2–0.88 K 2W 2O 7, for subsequent studies of physical properties, prior to choosing the growth conditions for single-crystal growth from a high-temperature solution. Near the saturation temperature, the density of the solution is 3.98×10 3 kg/m 3, the surface tension is 0.108 N/m and the dynamic viscosity is 37.5×10 −3 Pa s, values that are, a priori, suitable for single-crystal growth by Top Seeded Solution Growth-Slow Cooling (TSSG-SC) technique. Taking these results into account, the rotation rate for TSSG-SC was fixed at 40 rpm, while the axial and radial thermal gradients in the solution were 1.5 K/cm. Single crystals of KLu(WO 4) 2, transparent and free from macroscopic defects, have been obtained for cooling rates up to 0.2 K/h.

Thermal gradients in round TMF specimens

The effect of radial thermal gradients in thermomechanical fatigue (TMF) specimens on the number of cycles to failure has been studied. Different radial gradients were obtained by asymmetric placement of the specimen in the coil. And by using different induction heating frequencies. The different induction frequencies give different depths of penetration for the heat. A special thermal gradient specimen was used to assess the gradients through the thickness of the specimen. Measurements performed with different induction heating coil set-ups and different heating/cooling rates show that the internal temperature varies from the external temperature. The minimum difference measured was to about ±10 °C for a specimen cycled using a 400 kHz coil and a heating rate of 2 °C/s and the maximum ±40 °C using a 10 °C/s heating rate. TMF out of phase tests were performed with three different thermal gradients. Specimens tested with high heating rates exhibit more significant compressive creep compared to lower heating rates. The specimens exhibit a stress range that is 10% higher when using a coil with heating frequency of 66 kHz compared to 400 kHz.

Thermal-Lens-Effect Compensation of Nd:YAG Rod Laser Using a Solid Element of Negative Temperature Coefficient of Refractive Index

Experimental results are reported on the compensation of thermal lensing caused by the radial thermal gradient in an optically pumped Nd:YAG laser. Poly(methyl methacrylate) (PMMA) with a higher negative dn/dT, a high damage threshold and low absorbance is very useful for compensating thermal effects in the amplifier. The results revealed that the use of PMMA resulted in the complete reduction of the thermal lens effect by an order of magnitude.

Thermal–solutal flows and segregation and their control by angular vibration in vertical Bridgman crystal growth

Thermal–solutal flows and their induced segregation and supercooling are important in the growth of an alloy crystal. For the vertical Bridgman crystal growth having a stabilized thermal profile, in addition to the thermal convection induced by radial thermal gradients, solute gradients can either induce or suppress the flow depending on the solute density. Such thermal–solutal flows significantly affect the segregation behavior, constitutional supercooling, and thus the morphological instability of the solidification front. A transparent Bridgman growth of succinonitrile containing a lighter (acetone) or heavier (salol) solute was investigated. The evolution of the interface shape as well as morphological instability was visualized and was interpreted through computer simulation. To further control the flow and segregation, angular vibration was applied and its effects on the thermal–solutal flows and morphological instability were investigated.

Effects of the interface curvature on cellular and dendritic microstructures

We study the solidification of succinonitrile-based alloys contained in cylindrical glass crucibles. In such samples, the solid–liquid interface is curved as the latent heat generated upon solidification is mainly evacuated through the crucible wall of higher thermal conductivity. This creates radial thermal gradient which drives some important modifications in solute macrosegregation and of the microstructure array. A facility dedicated to the in situ and real time characterization of solid–liquid interface morphology during directional solidification of bulk samples has been developed by CNES (French Space Agency) in the frame of the DECLIC project. The effects on microstructure dynamics of fluid flow driven by the radial thermal gradient associated to curvature have already been pointed out, but other effects of curvature – function of the type of structure developed – are here presented. For a cellular microstructure, curvature is at the origin of advection that manifests in observation by the gliding of cells from the border to the centre of the interface. For a dendritic microstructure, a strong dependence of dendrite growth direction on the curvature appears that alters the columnar structure.

Modeling of thermal stresses and probability of survival of tubular SOFC

The temperature profile generated by a thermo-electro-chemical model was used to calculate the thermal stress distribution in a tubular solid oxide fuel cell (SOFC). The solid heat balances were calculated separately for each layer of the MEA (membrane electrode assembly) in order to detect the radial thermal gradients more precisely. It appeared that the electrolyte undergoes high tensile stresses at the ends of the cell in limited areas and that the anode is submitted to moderate tensile stresses. A simplified version of the widely used Weibull analysis was used to calculate the global probability of survival for the assessment of the risks related to both operating points and load changes. The cell at room temperature was considered and revealed as critical. As a general trend, the computed probabilities of survival were too low for the typical requirements for a commercial product. A sensitivity analysis showed a strong influence of the thermal expansion mismatch between the layers of the MEA on the probability of survival. The lack of knowledge on mechanical material properties as well as uncertainties about the phenomena occurring in the cell revealed itself as a limiting parameter for the simulation of thermal stresses.

Defect evolution in thermal barrier coating systems under multi-axial thermomechanical loading

In service, gas turbine components with thermal barrier coatings experience high cyclic mechanical and thermal loading. Important, but not yet considered sufficiently, are the multi-axial stresses arising from thermal gradients. In this study, multi-axial stresses were simulated in laboratory experiments using a specially designed test rig. Cyclic thermomechanical fatigue experiments with radial thermal gradients (TGMF) were performed on tubular specimens consisting of a directionally densified super-alloy substrate, a NiCoCrAlY bond coat, and a ceramic thermal barrier coating (TBC). The test setup enables surface temperatures of 1000 °C, temperature differences over the whole wall thickness of the specimen of about 170 °C, and high heating and cooling rates. The resultant defects have specific features consisting of cracks parallel to the bond coat/TBC interface. They are located within the bond coat close to this interface. Weakening thus this interface, the defects enhance TBC spallation. Finite element analyses, calculating stress distributions during the quasi-stationary condition of the TGMF test cycle at high temperature and the transient stress distributions during cooling, were used to discuss the evolution of these specific defects.

Tailoring of dendritic microstructure in solidification processing by crucible vibration

We analyzed the effects of both natural convection and forced flows on solid–liquid interface morphology during upward Bridgman solidification of metallic alloys. Experiments were carried out on Al–3.5 wt% Ni alloy, for a cylindrical sample. The influence of natural convection induced by radial thermal gradient on solidified microstructure was first analyzed as a function of the pulling rate. Then, the influence of axial vibration on solidification microstructure was experimentally investigated by varying vibration parameters (frequency and amplitude). Experimental results demonstrated that vibrations could be used to either attenuate fluid flow in the melt and obtain a uniform dendritic pattern or to promote a fragmented dendritic microstructure. However, no marked effect was observed for cellular growth. This pointed out the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure.

Directional solidification of refined Al–3.5 wt% Ni under natural convection and under a forced flow driven by a travelling magnetic field

A new Bridgman-type furnace has been designed and built. This furnace has two specific items: a device conceived to minimise any residual radial thermal gradient near the solid–liquid interface (induced by the difference in thermal conductivities between liquid and solid), and a travelling magnetic field to drive a forced flow in the liquid above the solidifying interface. We have already performed comparative experiments of directional solidification on refined and non-refined Al-based alloys, under natural convection and under forced flow, controlled by the travelling magnetic field. We will present some characteristics of this new furnace, and the preliminary results obtained by solidifying refined Al–3.5 wt% Ni, with and without the travelling magnetic field.

The rotary kiln: An investigation of bed heat transfer in the transverse plane

Rotary kilns are commonly employed to thermally process granular materials. While kiln rotation promotes particle mixing and heat transfer, it also leads to de-mixing through segregation of finer or denser particles, creating a “kidney” within the bed. Experience and experimental evidence indicate that rotation-induced mixing is insufficient to eliminate radial thermal gradients within the bed, but it is unclear as to whether particle segregation plays a significant role in the development of these gradients. This article presents experimental data obtained by heating sand of varying particle-size distributions within a 0.4-m ID batch rotary kiln to bed temperatures up to 775 °C. The results suggest that segregation has little influence on heat transfer within the bed and that radial thermal gradients are primarily the result of inadequate particle mixing. In order to scale up the experimental results, a mathematical model for heat transfer within the transverse plane was developed. A key variable for the model is the mixing-induced conductivity applied to the active layer, for which an empirical value is derived by fitting model predictions to the experimental data. Based on several assumptions for scaling of mixing conductivity with kiln size, model predictions for radial thermal gradients are presented for industrial-scale kilns in the 2- to 4-m ID range.

Double-diffusive convective modes and induced microstructure localisation during solidification of binary alloys

We analysed the coupling of convection and solid-liquid interface morphology during upward Bridgman solidification of both transparent and metallic alloys in a cylinder. Localised microstructures are observed for typical alloys but not for lead-30 wt% thallium. This difference is quantitatively explained from phase diagrams and from the more general form of constitutional supercooling criterion, extended for non-dilute alloys. Even if the convection induces a lateral concentration gradient, there is not any lateral gradient on the level of morphological instability and there is not any propagation front of morphological instability for lead-30 wt% thallium alloy. In solidification experiments, convection can be generated by either radial thermal gradient or solute axial gradient. The formation and dynamics of double-diffusive convective pattern is analysed during the transient and different thermal radial gradient conditions.

Optimization of the Bridgman crystal growth process

A numerical optimization method of the vertical Bridgman growth configuration is presented and developed. It permits to optimize the furnace temperature field and the pulling rate versus time in order to decrease the radial thermal gradients in the sample. Some constraints are also included in order to insure physically realistic results. The model includes the two classical non-linearities associated to crystal growth processes, the radiative thermal exchange and the release of latent heat at the solid–liquid interface. The mathematical analysis and development of the problem is shortly described. On some examples, it is shown that the method works in a satisfactory way; however the results are dependent on the numerical parameters. Improvements of the optimization model, on the physical and numerical point of view, are suggested.