Gradient Furnace Research Articles

X-radiography front tracking gradient furnace for directional solidification of bulk Al-alloys

A unique gradient furnace for directional solidification experiments with bulk Al-alloy samples developed at German Aerospace Center is presented. It allows for in situ process control in solidifying samples by using x-radiography, and further insight into the solidification process is gained in combination with x-ray computational tomography on the solidified samples. Tracking of interfaces during directional solidification of bulk samples via in situ x-radiography (TIREX) enables the investigation of the melting process and observation of the movement of the entire mushy zone through the sample, tracing the solid–liquid interface during directional solidification and correlating the observations with the microstructure of the samples. Monitoring the temperature profile inside the sample by in situ observation of the length of the mushy zone is particularly important because the temperature gradient G and the rate of interfacial growth v determine the microstructure of solidification. The x-radiography setup offers temporal and spatial resolutions of 0.5 s and 70 μm, respectively, with a field of view of 10 × 50 mm2. Constant solidification velocities of up to 0.15 mm s−1 at a temperature gradient of up to 8 K mm−1 can be achieved in a temperature range of 537–1373 K. A flat solid–liquid interface inside a rod-like sample with 5 mm diameter is achieved by surrounding the sample by thermal isolating graphite foam. Performance tests with hypoeutectic Al–10 wt. % Cu alloy samples show the functionality of the furnace facility.

One-Step Crystallization of Gahnite Glass-Ceramics in a Wide Thermal Gradient

The glass crystallization regime plays a crucial role in the fabrication of glass ceramics: it affects both phase composition and microstructure, and thus the properties of the final product. In the search for new glass-ceramic materials, the development of a proper heat-treatment schedule involves the utilization of numerous glass samples that need to be thermally treated and then investigated to determine the values of the target characteristics. In this study, we evaluated the effect of crystallization temperature on the glass structure, phase composition, and hardness of glass ceramics in the ZnO-MgO-Al2O3-SiO2 system containing TiO2 and ZrO2 as nucleators. To maximize the number of heat treatments, we performed polythermal crystallization of the glass in a wide temperature range with the help of a gradient furnace. Using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we showed the precipitation of gahnite nanocrystals as the main phase in the bulk of a single glass sample and observed a gradual change in its microstructure, transparency, and hardness. The dependence of Vickers hardness values on heat treatment temperature was found to follow a non-linear trend, revealing the optimal thermal range for glass crystallization.

Investigation of Al-20Sn-10Cu alloy directional solidification by laboratory X-radiography

Al-based alloys with a soft phase such as Sn are extensively used for bearing components due to their self-lubricating properties. Al-Sn alloys lack the ability to support heavy loads so the alloying with Cu as a third element provides solution strengthening of the aluminium matrix. A crucial issue in the manufacturing of Al-Sn-Cu alloys is the miscibility gap in the phase diagram of the system. Liquid immiscibility is responsible for severe segregation during the solidification process, due to the large density difference between the Al-rich and Sn-rich liquids, which limits their utilization in industry. It is therefore both scientifically and technically important to accurately understand their solidification path. In the present work, the solidification of a ternary Al-20wt.%Sn-10wt.%Cu alloys was investigated in-situ by using X-radiography. Directional solidification experiments were performed on sheet-like samples in the laboratory device SFINX (Solidification Furnace with IN-situ X-radiography), which consists of a Bridgman-type gradient furnace and an X-radiography system. The solidification sequence was determined based on the observation of the recorded images, enlightening the successive steps of the solidification path. These observations were compared to predictions obtained from thermodynamic calculations. Complementary post-mortem microscopic analyses showed that the dendrite primary trunk and secondary arms developed along <110> crystallographic axes instead of the usually expected <100>.

In situ observation of the cryophile migration of hydrogen bubbles in Al-alloys during directional melting and the impact of surface tension

During metallurgical processing, gas bubbles nucleate and interact with the microstructure, which can result in severe defects. The research presented in this article focuses on in situ observation of bubble movement in semi-solid aluminum alloys (Al-Ge and Al-Cu) during a melting cycle. X-radiography is used to monitor the migration of H-bubbles in the semi-solid mush. For both alloys, H-bubbles are observed to burrow through the mushy zone towards the colder side of a horizontal gradient furnace leaving behind solute-enriched channels. This stands in contrast to the commonly observed bubble movement towards the hot side of the material caused by Marangoni flow-fields surrounding the gas pocket induced by gradients in the bubble’s surface tension. Different dependencies of the surface tension on temperature and concentration for the two alloys are presented as explanation for the observed atypical phenomenon. These results suggest different dominant effects acting on the bubbles, temperature for Al-Ge and concentration for Al-Cu, and explain the cryophile behavior observed.

Dechlorination Apparatus for Treating Chloride Salt Wastes: System Evaluation and Scale-Up.

This paper describes an apparatus used to remove chlorine from chloride salt-based nuclear wastes from electrochemical reprocessing and/or chloride-based molten salt reactors (MSRs) through dechlorination by reacting the salts with ammonium dihydrogen phosphate (NH4H2PO4 or ADP) at temperatures up to 600 °C to produce NH4Cl as a byproduct. The benefits of removing the Cl from these salts include 37Cl recovery from Cl-based MSR salts, formation of UCl3 from the NH4Cl, as well as removal of Cl from the salts and conversion of the salt cations to oxides to allow for immobilization in a chemically durable iron phosphate waste form. This generation-2 system is an improvement over the generation-1 system and provides a means for scaling up salt throughput as well as NH4Cl recovery. The generation-2 system includes a five-zone furnace so the temperature of the four-zone gradient furnace can be tailored to control the location of NH4Cl condensation on a four-piece fused quartz off-gas system. Both ADP and NH4Cl decomposition reactions include the production of NH3 and acids (i.e., H3PO4 and HCl, respectively), so careful temperature control is needed during the ADP-salt reactions to maximize the NH4Cl production and minimize NH4Cl decomposition. In two sets of experiments run in the generation-1 and generation-2 apparatuses, NH4Cl yields were ≥5.5-fold higher for the new system compared to the original prototype system and the batch sizes can be ≥2.5-fold higher. In addition, some thermodynamic experiments evaluating the reactions of ADP + KCl as well as decomposition of pure NH4Cl were performed to assess the temperatures of the reactions and identify off-gas products.

Modeling of nonlinear processes of nucleation and growth of GaSxSe1–x(0 ≤ х≤ 1) solid solutions

The results on the study of modeling and physico-chemical study of the kinetics ofnucleation and growth of GaSxSe1–x(0 ≤ х≤ 1) solid solution. The nucleation heterogeneous process and growth of GaSxSe1–xcrystals have been studied and simulated taking into account nonlinear equations considering the kinetic behavior of crystallizing phases.GaSxSe1–xsingle and nanocrystals were grown from solution, melt, and by chemical transport reaction through steam. GaSxSe1–xcrystals were grownbychemical transport reaction in a two-tem-perature gradient furnace in a sealed quartz ampoule. Iodine was used as a transporting additive. Using the Fokker–Planck equation, the evolution of the distribution function of crystals of solid solutions of the GaS–GaSe system by size at the nucleation time is studied by a numerical method. For the convenience of comparing theory with experimental data, we used the GaS1–xSex(x= 0.7 molar fraction of GaSe) composition of the solid solution. The Monte Carlo method is used to approximate the time evolution of the nucleation of two types of particles for the GaS0.3Se0.7 solid solution, simulated by a constant nucleus size. The results of modeling non-linear crystallization processes are consistent with experimental data.

Bismuth Polycations Revisited: Alternative Synthesis and Electronic Structure of Bi6Br7, and Bonding in Main-Group Polyatomic Ions from a Direct Space Perspective

A bismuth subbromide, Bi6Br7, was synthesized in the form of single crystals using the reaction between Bi and Hg2Br2 in a gradient furnace. Its crystal structure was reinvestigated by low-temperature single-crystal X-ray diffraction (Pnnm, a = 15.4996(6) Å, b = 23.6435(7) Å, c = 9.0231(2) Å, Z = 8, R1 = 0.041, wRall = 0.087). Based on the diffraction data, the structure description was revised as containing Bi95+ cluster polycations and 1∞[Bi3Br145−] ladder-like anions. DFT calculations of band structure showed the compound to be a narrow-gap semiconductor with a band gap of ca. 1.3 eV, with the nature of the compound as ionic salt confirmed by charge density analysis. Direct-space bonding analysis based on the ELF topology and QTAIM partitioning, performed for all known homoatomic bismuth polycations, as well as isoelectronic main-group metal ions, shows patterns of localized pairwise and three-center bonding forming the frameworks of the clusters. In addition to obtaining new data, the use of highly augmented basis sets allowed us to revise and amend several previously made conclusions regarding bonding in such species.

Characterizing Drift Behavior in Type S Thermocouples to Predict In-use Temperature Errors

The Type S thermocouple continues to be used as a primary reference for industrial applications and as a secondary reference in many national metrology institutes (NMI). The Type S thermocouple, in a well annealed state, can yield temperature measurements accurate to better than ± 0.2 °C up to approximately 1100 °C. Metallurgical changes, even in the best cared for thermocouples, quickly lead to drift in use. The main causes of drift are reversible processes occurring in the Pt/Rh alloy thermoelement. Most NMIs will periodically apply a high-temperature (1100 °C) anneal to restore the thermocouple to a state close to that of the original. But, for many second-tier laboratories and industrial users, this annealing procedure is not available or is impracticable. Therefore, these users are faced with measurements that are subject to accumulated or continuous drift errors. Fortunately, recent research has shown that the Pt-10%Rh thermoelement changes with temperature in a predictable way, enabling forecasting of drift as a function of exposure time and temperature. This study aggregates drift results for several Type S thermocouples from different manufacturers after exposure to MSL’s gradient furnace in the range 100 °C to approximately 900 °C. Numerical techniques are then used on these data to anticipate the likely in-use temperature errors for other Type S thermocouples. The method can provide improved confidence for end-users of Type S thermocouples where regular annealing is either difficult or impossible.

A nearly zero temperature gradient furnace system for high pressure multi-anvil experiments

ABSTRACTA new furnace system with an almost zero temperature gradient throughout the sample area was designed for multi-anvil high pressure experiments. Test experiments of the new design were performed using 18/11 and 25/15 cell assemblies at 4 GPa, 1400°C and 1500°C, respectively. The temperature field within the sample capsules appeared to be very homogenous as indicated by Mg2Si2O6–MgCaSi2O6 two-pyroxene thermometry, by direct temperature measurements using two thermocouples within the same assembly, and by distribution of solid and liquid phases in the sample capsule. The temperature gradient is estimated to be <2.4°C/mm over an area of 4 × 5 mm2 within the furnace. It is significantly lower than standard multi-anvil experiments with straight or stepped furnace systems, which are at the levels of 20–200°C/mm.

Indigenously developed vertical semi transparent Bridgman setup for the growth of single crystals

An effective and economically viable vertical semi transparent Bridgman setup has been designed and fabricated by employing a furnace made up of a single Nichrome coil. We made an effort with a new approach by adapting a translation mechanism for the ampoule coupled with a stop clock. This set up is a suitable device for growing organic crystal from melt as it makes use of ampoule translation assembly. Organic nonlinear materials of acenaphthene and benzophenone were grown using this set up. Transparent single crystals were successfully grown as a result of suitable thermal gradient achieved by means of thermal gradient furnace assembly. The grown single crystals were rationalized using X-ray diffraction, spectroscopic and thermal analysis.

Thermal Recovery from Cold-Working in Type K Bare-Wire Thermocouples

Cold-working of most thermocouples has a significant, direct impact on the Seebeck coefficient which can lead to regions of thermoelectric inhomogeneity and accelerated drift. Cold-working can occur during the wire swaging process, when winding the wire onto a bobbin, or during handling by the end user—either accidentally or deliberately. Swaging-induced cold-work in thermocouples, if uniformly applied, may result in a high level of homogeneity. However, on exposure to elevated temperatures, the subsequent recovery process from the cold-working can then result in significant drift, and this can in turn lead to erroneous temperature measurements, often in excess of the specified manufacturer tolerances. Several studies have investigated the effects of cold-work in Type K thermocouples usually by bending, or swaging. However, the amount of cold-work applied to the thermocouple is often difficult to quantify, as the mechanisms for applying the strains are typically nonlinear when applied in this fashion. A repeatable level of cold-working is applied to the different wires using a tensional loading apparatus to apply a known yield displacement to the thermoelements. The effects of thermal recovery from cold-working can then be accurately quantified as a function of temperature, using a linear gradient furnace and a high-resolution homogeneity scanner. Variation in these effects due to differing alloy compositions in Type K wire is also explored, which is obtained by sourcing wire from a selection of manufacturers. The information gathered in this way will inform users of Type K thermocouples about the potential consequences of varying levels of cold-working and its impact on the Seebeck coefficient at a range of temperatures between $$\sim 70\,^\circ $$ C and $$600\,^\circ $$ C. This study will also guide users on the temperatures required to rapidly alleviate the effects of cold-working using thermal annealing treatments.

Seebeck Changes Due to Residual Cold-Work and Reversible Effects in Type K Bare-Wire Thermocouples

Type K thermocouples are the most commonly used thermocouple for industrial measurements because of their low cost, wide temperature range, and durability. As with all base-metal thermocouples, Type K is made to match a mathematical temperature-to-emf relationship and not a prescribed alloy formulation. Because different manufacturers use varying alloy formulations and manufacturing techniques, different Type K thermocouples exhibit a range of drift and hysteresis characteristics, largely due to ordering effects in the positive (K+) thermoelement. In this study, these effects are assessed in detail for temperatures below $$700\, {^{\circ }}\hbox {C}$$ in the Type K wires from nine manufacturers. A linear gradient furnace and a high-resolution homogeneity scanner combined with the judicious use of annealing processes allow measurements that separately identify the effects of cold-work, ordering, and oxidation to be made. The results show most K+ alloys develop significant errors, but the magnitudes of the contributions of each process vary substantially between the different K+ wires. In practical applications, the measurement uncertainties achievable with Type K therefore depend not only on the wire formulation but also on the temperature, period of exposure, and, most importantly, the thermal treatments prior to use.

An Evaluation of Marine Sediments in Terms of their usability in the Brick Industry: Case Study Port of Koper

A dredging process is essential for the development of harbours and ports, allowing the functional daily operation of the port. The management of dredged material represents a worldwide challenge, especially considering unwanted deposition of material. Because of their chemical, petrographic, mineralogical and homogeneity composition, marine sediments could represent an appropriate raw material for the brick industry, particularly for the production of clay blocks, roofing and ceramic tiles. In this study dredged material from Port of Koper was tested for such use, thus various analyses were carried out in order to determine if the dredged material is environmentally friendly and suitable for use in the brick industry. These included: chemical analysis, mineralogical analysis, particle size analysis, chloride content analysis and tests of firing in a gradient furnace. Furthermore, tests of mechanical properties, as well as tests of the frost-resistance of the samples were carried out. On the basis of the primary analyses and samples prepared in a laboratory, it was confirmed that marine sediments from the Port of Koper, without any additives are only conditionally suitable as a source material for producing brick products, because without additives they exhibit too much shrinkage on drying and firing, as well as high water absorption, this can be improved by the incorporation of suitable additives, for example, virgin clay or other suitable waste material. The approach described herewith can also be followed to assess sediments from other ports and rivers regarding its potential use in clay brick sector.

Drift as a Function of Temperature in Platinum–Rhodium-Alloyed Thermoelements

Platinum–rhodium-alloyed thermocouples are the most commonly used high-temperature reference thermometer in national measurement institutes, second tier laboratories and industry. Despite the common use of these thermocouples, there is still a great deal that is not known about the drift processes that occur in them. Drift processes in these alloys are known to be made up of three main components: crystallographic changes, rhodium oxidation and migration, and contamination. Through careful use, contamination can be largely avoided; however, the other two processes often cannot. Research on drift in the different platinum–rhodium alloys is important because the largest uncertainty component during calibration of these thermocouple types is due to inhomogeneity, and the same mechanisms responsible for inhomogeneity are responsible for the drift. This study investigates the drift processes as a function of temperature and time for the 5 %, 13 %, 17 %, 20 %, 30 % and 40 % Rh alloys when paired with pure platinum. The use of a linear gradient furnace and high-resolution homogeneity scanner has enabled identification of drift characteristics in the temperature range $$100 \,^{\circ }$$ C to $$950 \,^{\circ }$$ C, where the bulk of reversible drift occurs. The experiments have quantified the drift rates and magnitude for thermoelements given two commonly used annealing procedures: the high-temperature quench anneal and the low-temperature vacancy anneal. Consequently, this study provides users of platinum–rhodium thermoelements with guidance on what levels of drift they should expect and exposure times before re-annealing is required. It also shows that a Pt–Rh alloy of 20 % Rh is by far the most stable and has properties comparable to the Pt–Pd thermocouple.

Thermal study of the influence of chemical composition on CRT glass crystallization

The influence of alumina oxide on the crystallization of silica–strontium–barium glass from cathode ray tube (CRT) was studied by means of thermal analysis. Thermal characteristics of glasses such as the transition temperature T g, the crystallization temperature T c and thermal stability parameter were determined by DTA/DSC method. The crystallization ability of the obtained glasses was also determined independently by the gradient furnace technique. The crystalline phase was determined by the X-ray diffractometry. The microstructure of the samples was studied by SEM technique. The linear expansion coefficients of glasses as a function of temperature were measured. Analysis of the local atomic interactions in the structure of glasses was used to explain the course of the crystallization. The effect of Al2O3 content on the CRT glass composition was studied by FTIR and solid-state 19F MAS–NMR spectroscopy.

In situ X-ray observations of gas porosity interactions with dendritic microstructures during solidification of Al-based alloys

In situ X-radiography solidification experiments were performed on Al-based alloys, using both synchrotron and laboratory-based X-ray sources, in conjunction with a gradient furnace and a newly developed isothermal furnace, respectively. The effect of gas porosity nucleation and growth within the semi-solid mush during both columnar and equiaxed solidification was thereby observed. In all experimental cases examined, gas porosity was observed to nucleate and grow within the field-of-view (FOV) causing various levels of distortion to the semi-solid mush, and thereafter disappearing from the sample leaving no permanent voids within the solidified microstructure. During columnar growth, a single bubble caused severe remelting and destruction of primary trunks leading to secondary fragmentation and evidence of blocking of the columnar front. Equiaxed solidification was performed under microgravity-like conditions with restricted grain motion in the FOV. The degree to which the nucleated gas bubbles affected the surrounding grain structure increased with increasing solid fraction. However, bubble sphericity remained unaffected by apparent solid fraction or grain coherency.

Formation of intermetallic phases in AlSi7Fe1 alloy processed under microgravity and forced fluid flow conditions and their influence on the permeability

Ternary Al-6.5wt.%Si-0.93wt.%Fe alloy samples were directionally solidified on-board of the International Space Station ISS in the ESA payload Materials Science Laboratory (MSL) equipped with Low Gradient Furnace (LGF) under both purely diffusive and stimulated convective conditions induced by a rotating magnetic field. Using different analysis techniques the shape and distribution of the intermetallic phase β-Al5SiFe in the dendritic microstructure was investigated, to study the influence of solidification velocity and fluid flow on the size and spatial arrangement of intermetallics. Deep etching as well as 3-dimensional computer tomography measurements characterized the size and the shape of β-Al5SiFe platelets: Diffusive growth results in a rather homogeneous distribution of intermetallic phases, whereas forced flow promotes an increase in the amount and the size of β-Al5SiFe platelets in the centre region of the samples. The β-Al5SiFe intermetallics can form not only simple platelets, but also be curved, branched, crossed, interacting with dendrites and porosity located. This leads to formation of large and complex groups of Fe-rich intermetallics, which reduce the melt flow between dendrites leading to lower permeability of the mushy zone and might significantly decrease feeding ability in castings.

Liquidus temperatures of calcium magnesium alumosilicate glass-forming compositions determined via gradient furnace and from the melting peak by differential thermal analysis

The liquidus temperatures (TL) of good glass forming systems within the CaO–SiO2–MgO–Al2O3 system (CMAS) were evaluated. The determination of TL via differential thermal analysis (DTA) was studied in terms of reproducibility and was compared with results from the gradient furnace method. Seventeen different glass compositions within the CaO–SiO2–MgO–Al2O3 system were melted from technical raw materials. Since those glasses are reluctant to crystallize, a prior heat treatment was necessary to induce crystallization. The onset and the endpoint of the melting peaks of pre-crystallized materials were measured with a heating rate of 2K/min and compared with the liquidus temperatures obtained by the gradient furnace method (following the ASTM standard procedure C 829-81 [1]). The endpoint temperatures of the melting peaks were closest to the results determined via the ASTM procedure. Most endpoint results via DTA differ by not more than 10K compared to the liquidus temperatures. Because of the simultaneous crystallization pretreatment of many different compositions, the small amount of glass needed, and the speed of the DTA, this technique may be a valuable option to estimate the liquidus temperature of complex multicomponent glasses.

Novel glass compositions for fiber drawing

Strengthening concrete by fiber reinforcement is a new trend nowadays. Glass is one of the materials that have the highest chemical durability in concrete. Chemically durable glass compositions in alkaline environment have been studied for 25 years by our group. SMZS (SrO–MgO–ZrO2–SiO2) system glass compositions possess high chemical durability in alkaline solutions. Further investigations show that recently discovered SMFMZS (SrO–MgO–FeO–MnO–ZrO2–SiO2) glasses exhibit better chemical durability when compared to the former one containing more SrO in the composition as a modifying oxide.The main purpose of the present study was to produce glass fibers made of our own SMZS and SMFMZS glass systems compositions inhibiting better alkali durability compared to the commercial E-glass fiber. Further study on the SMFMZS glass samples in terms of fiber drawing revealed that magnesia-silicate related many crystalline phases were formed during the process. Determining possible crystallization temperatures of SMFMZS system glasses were carried out by the gradient furnace. Additionally, those formed crystals were fully characterized by SEM and EDX. To prevent surface and bulk crystallization during fiber drawing, the composition changes were made. Consequently, successful composition was achieved.

Physicochemical Properties of Vitrified Forms for LILW Generated From Korean Nuclear Power Plant

ABSTRACTSince 1994, the KHNP has developed a vitrification technology to treat the LILW generated from Korean nuclear power plant. To vitrify the LILW including combustible Dry Active Waste (DAW) and Ion Exchange Resin (IER) containing Zeolite, two borosilicate glasses are formulated. One of the formulated glass, DG2, is for the DAW vitrification solely and the other one, AG8W1, is for the blended wastes (DAW &amp; IER) vitrification in a commercial vitrification facility in HanUl (former Ulchin) nuclear power plant. The physicochemical properties of the two glasses have been evaluated. To evaluate the processability of the glasses, the viscosities and electrical conductivities of the glass melts were measured in the laboratory within a temperature range between 950 and 1,350 degrees C, respectively. The liquidus temperatures of the glasses were evaluated using a gradient furnace for DG2 and data from heat treatment for AG8W1. The Mössbauer spectroscopy for AG8W1 was employed to evaluate the relations between the redox equilibria of iron. In addition, to verify the waste acceptance criteria for the final disposal of the vitrified forms, the compressive strengths of the vitrified forms were tested after an immersion test, a thermal cycling test, and an irradiation test. To verify the chemical durability of the glasses, several tests such as PCT, ISO, VHT, Soxhlet, MCC-1, and ANS16.1 were carried out. The PCT showed leach rates of B, Na, Li and Si were much less than those of the benchmark glass. The ISO test was performed at 90 degrees C for 1,022 days and Cumulative Fraction Leached of all elements in the glasses were analyzed. According to the VHT, the glasses had an outstanding chemical resistance under humid environment at 200 degrees C for 7 days. The Soxhlet leaching was performed on rectangular glass samples at 98 degrees C for 30 days. To analyze the forward dissolution rates of major glass elements, the MCC-1 was conducted at temperatures of 40, 70, and 90 degrees C for three weeks in pH buffer solutions ranging from pH 4 to 11. The processability of the glasses was in the desired ranges. And the product quality of the glasses met all regulatory guidelines. Using two glasses, the CCIM commissioning tests in the UVF were successfully performed and they showed good workability.