Vacuum Induction Melting Furnace Research Articles

Microstructure evolution and mechanical properties of Ti[sbnd]48Al alloy melted by double-scale yttrium oxide crucible

Ti-48Al-xY (x = 0, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0 at. %) alloys were prepared in a vacuum induction melting furnace using the micro-submicron double-scale yttrium oxide crucible. The microstructure and interfacial reaction between the melt and crucible were charactered, and the mechanical properties of the alloy after melting were tested. The results show that the purity of the double-scale yttrium oxide crucible prepared by the slip casting method is 99%, the linear shrinkage of the crucible is 31%, and the internal pores exist in the gain of the crucible after sintering. After melting, there are no new compounds are formed on the surface of the alloy ingots, which indicates that the crucible does not react with the alloy melt. A small part of Y2O3 is dissolved into the melt, resulting in the formation of Y and O. The active alloy melt absorbs O element, and Y element can contribute to reducing the dissolution of Y2O3. The formation heat and activity coefficient of each alloy element were calculated by the Miedema model. The formation heat of AlY and TiAl is negative, while the formation heat of TiY is positive. The activity coefficient of Y is the highest of all. As a result, as the Y content increases, the binding force of the Al atom increases, and the binding force of the Ti atom weakens, resulting in a decrease in the melt activity of Ti-48Al-xY alloys. The microhardness of Ti-48Al-xY alloys by the double-scale yttrium oxide crucible increases with the increase of Y content. The compressive strength of the Ti-48Al-xY alloys by the double-scale yttrium oxide crucible increases initially and decreases subsequently with the addition of Y. The total compressive strength is higher compared to the compressive strength of Ti-48Al-xY alloys by the water-cooled copper crucible. Therefore, the double-scale yttrium oxide crucible is suitable for the melting of TiAl alloys.

Multiphysics modeling and experimental validation of heat and mass transfer for the vacuum induction melting process

Vacuum induction melting is crucial in casting nickel-based superalloy components, ensuring excellent properties for aero-engine applications. Precise melting temperatures are vital for achieving optimal metallurgical quality before casting. Hence, a multiphysics numerical model is developed to simulate the induction melting process for the Inconel 718 superalloy. The proposed holistic model integrates magnetic fields, induced currents, and heat and momentum transfer phenomena in a single coupled model. A moving mesh approach reproduces the magneto-hydrodynamic behavior of the free surface, simulating the oscillations of the melt. The stabilized deformed surface profile is correlated with experimental measurements, reporting a proper correlation. Then, the flow field and recirculation effect are modeled with a Low Reynolds Number turbulence approach and coupled with the melt convective heat transfer, developing a complete magneto-thermo-hydrodynamic model. In a laboratory-scale vacuum induction melting furnace, transient melting operation variables are characterized and introduced to the numerical model to compute the temperature evolution. An accurate reproduction of the transient melt temperature variations is reported with a relative error of less than 5%. The influence of the crucible thermal insulating capacity is assessed, emissivity dependence evaluated, and melt homogenization is reported at different process stages. This comprehensive numerical and experimental approach offers valuable insights for enhancing vacuum induction melting for Ni-based superalloys.

Influence of high strain rate deformation on the microstructure and mechanical behavior of NiCoCrFe high entropy alloy

The paper reported a high-entropy alloy, NiCoCrFe, which was produced in a Vacuum Induction Melting (VIM) furnace. In order to homogenize the microstructure, the as-cast ingot was rolled, annealed and machined to appropriate dimensions. Then the Split Hopkinson Pressure Bar (SHPB) was used to evaluate the specimens under various strain rates. True stress–strain curves of this test showed similar patterns in all strain rates, with two peaks that can be assigned to the competition between strain hardening and thermal softening. Modified Johnson–Cook model applied to characterize flow behavior of the alloy. X-ray diffraction results indicated that the alloy was single-phase with FCC crystal structure at the casting condition, after thermomechanical operation and after high-strain rate tests. The specimens were further examined by EBSD. The results indicated the impressive presence of twins in the samples deformed at strain rate of 1500 s−1, confirms the significant contribution of twinning in the strengthening process; Regarding the low SFE of the mentioned alloy, it can be said that Twin-Induced Plasticity (TWIP) has occurred. By strain rate enhancement to 3000 s−1, finer and elongated grains were observed while twinning decreased because of SFE increase as a result of temperature elevation due to adiabatic heating during deformation. According to KAM map, there was strain gradient in mentioned sample, which results in production of GNDs and GNDs pile-up leading to strain hardening by creating obstacles to further slip. Moreover, in the hat-shaped samples, which used for shear localization resistance evaluations, bundles of nano-twins were observed whose formation largely prevented strain localization and make the above alloy suitable for applications exposed to high-speed impacts.

Influence of copper on the deformation behavior of NiTi shape memory alloys in isothermal compression

NiTi alloys are more commonly employed in engineering and medical applications because of its appealing shape memory and superelastic qualities. Recent efforts have been made to increase their thermal memory, mechanical capabilities, and corrosion resistance by alloying, heat treatment, and other means. NiTiCu alloys are the most often utilised ternary alloy in actuator applications. However, there is a scarcity of literature on the hot deformation properties or deformation process maps of NiTiCu shape memory alloys. To make cylindrical ingots, elemental forms of high purity nickel, titanium, and copper were melted in a vacuum induction melting furnace. These ingots were homogenised at 900 °C, quenched in water to room temperature, then machined to achieve a length-to-diameter ratio of 1.5, which is appropriate for doing hot deformation tests in a Gleeble thermo-mechanical simulator. The samples were exposed to strain rates of 0.01, 0.1, 1, and 10 s−1, respectively, and the data acquired was used to build a deformation processing map. The zones of instability were defined, and the dominating operational mechanisms in various regimes were projected and reported.

Effect of Heat Treatment on Tensile Properties and Microstructure of Co-Free, Low Ni-10 Mo-1.2 Ti Maraging Steel.

Production of high-quality maraging steel is dependent not only on the production technology but also on the alloying design and heat treatment. In this work, cobalt-free, low nickel, molybdenum-containing maraging steel was produced by melting the raw materials in a vacuum induction melting furnace and then refining with a shielding gas electroslag remelting unit. The critical transformation temperatures of the investigated steel samples were determined experimentally by differential scanning calorimetry (DSC) analysis and theoretically aiding Thermo-Calc software. Types and chemical composition plus volume fraction and starting precipitation temperature of suggested constituents calculated with the aid of Thermo-Calc software. The microstructures of forged steel specimens that were heat-treated under several conditions were evaluated by X-ray diffraction (XRD), optical microscopy (OP), scanning electron microscopy (SEM), and electron backscattering (EBSD), in addition to transmission electron microscopy (TEM). The mechanical properties of the investigated steel specimens were evaluated by measuring the tensile strength properties and micro-hardness, furthermore, estimating their fracture surface using scanning electron microscopy at lower magnification. The metallographic results show that the microstructure of steel in aged conditions includes high-alloyed martensite and nickel-rich phase, in addition to the low-alloyed-retained-austenite, intermetallic compounds, and lavas-phase (MoCr). Furthermore, TEM and EBSD studies emphasized that the produced steel has high dislocation density with nano-sized precipitate with an average size of ~19 ± 1 nm. Moreover, the metallographic results show that the mentioned microstructure enhances the tensile properties by precipitation strengthening and the TRIP phenomenon. The tensile strength results show that the n-value of investigated steel passes two stages and is comparable with the n-value of TRIP-steel. Steel characterized by 2100 MPa ultimate tensile strength and uniform elongation of more than 7% can be produced by the investigated production routine and optimum heat treatment conditions.

Hot deformation characteristics of NiTiV shape memory alloy and modeling using constitutive equations and artificial neural networks

In this study, an alloy conforming to the composition Ni50Ti48V2 (at. %)was cast in a vacuum induction melting furnace, and hot deformation tests were performed on the samples using a GLEEBLE 3800 thermomechanical simulator over a range of strain rates (0.01 s−1, 0.1 s−1, 1 s−1, 10 s−1) and temperatures (1073 K, 1173 K, 1273 K, 1373 K). Deformation processing maps, which are a combination of instability map and efficiency map, were derived based on dynamic materials model. The mechanisms of deformation were interpreted based on stress-strain plots, kinetic analysis, process maps and microstructure. Material constants along with activation energy, which was found out to be 205.58 kJ/mol., were calculated. Since strain also plays a major role in this analysis, a Strain-Compensated Arrhenius-Type (SCAT) model was developed. A comparison between strain-compensated Arrhenius type and artificial neural network models (ANNM) was made through the use of relative error, adjusted R2 values and root mean square errors between the predicted and experimental stress values. All three parameters unanimously indicated the use of ANNM for predicting the stresses developed in the material during hot deformation process. Finally, the flow stress required for the onset of dynamic recrystallization (DRX) was calculated using work hardening theory and correlated with the power dissipation efficiency generated from processing maps. Results show that Ni50Ti48V2 exhibits the highest power dissipation efficiency of 43.50% when deformed at 1173 K and 0.01 s−1. The least critical stress of 58.014 MPa was also captured under the same working conditions. Results are elaborately discussed in the paper.

Effect of Ni addition on bainite microstructure of low-carbon special bar quality steels and its influence on CCT diagrams

This work presents the effect of Ni content on the bainite morphology and in the mechanical properties of low-carbon special bar quality steels. Four steel ingots with the same base chemical composition, having only Ni content as their main difference, were melted in a pilot scale vacuum induction melting furnace, hot forged into square bars followed by cooling in still air to room temperature. Samples for Charpy-V notch tests of every steel were tempered to equalize their hardness level. Through dilatometry tests, it was found that Ni slows down the transformation kinetics of ferrite and pearlite phases, and, acts in favor of bainite and martensite formation. In low-carbon bainitic steels, Ni also favors the formation of lower bainite at the expanse of upper bainite. The steels with lower Ni contents, 0.03 % and 0.70 %, have their microstructures mostly composed of upper bainite, with clusters of alloy carbides precipitated in the boundaries of the bainitic ferrite packages. The clusters of hard carbides act as stress raisers, facilitating the propagation of cracks, and, reducing the toughness of the steels. The other two steels studied with higher Ni, 1.40 % and 2.75 %, showed superior hardenability and predominant microstructures of lower bainite and martensite, respectively. The steels composed of lower bainite and martensite have higher toughness than the steels with upper bainite due to their more refined microstructure and homogenous dispersion of the alloy carbides in the steel matrix. It was also observed in the bainitic steels that tempering does not influence the location where the alloy carbides precipitated during bainite transformation. After tempering, these carbides remain at the bainitic ferrite boundaries or dispersed in the bainitic ferrite packages depending on the type of bainite formed.

Effect of carbon addition on microstructure and mechanical properties of as-cast nickel-based heavy density matrix alloy reinforced by high tungsten content

In present work, a novel carbon-doped interstitial strengthening nickel-based heavy density matrix alloy reinforced by high tungsten content (Ni-W-Co) was prepared by conventional casting technology using a laboratory vacuum induction melting (VIM) furnace. The effects of carbon addition on as-cast microstructure and phase constituents were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). And the tensile properties and microhardness were tested as well. Results demonstrated that the present heavy density alloy has a single-phase FCC structure and exhibits typical dendrite characteristics. The carbon content (0–0.021 wt%) presents little influence on as-cast microstructure and phase constituents, but significantly improves the mechanical properties. Both tensile strength and ductility of the matrix alloy were increased synchronously with the addition of interstitial carbon atoms on account of the interstitial strengthening. The tensile fracture morphology revealed the transition from complete ductile dominated features to a mixed fracture mode consisting of quasi cleavage and ductile fractures. Minor carbon addition can trigger an obvious increase for lattice parameters and thus contribute to the strength improvement, and the increase in lattice friction stress enhanced the work-hardening rate and postponed the necking of C-addition alloy compared with C-free alloy.

High-Temperature Oxidation Behavior of AlTiNiCuCox High-Entropy Alloys.

In this study, the high-temperature oxidation behavior of a series of AlTiNiCuCox high-entropy alloys (HEAs) was explored. The AlTiNiCuCox (x = 0.5, 0.75, 1.0, 1.25, 1.5) series HEAs were prepared using a vacuum induction melting furnace, in which three kinds of AlTiNiCuCox (x = 0.5, 1.0, 1.5) alloys with different Co contents were oxidized at 800 °C for 100 h, and their oxidation kinetic curves were determined. The microstructure, morphology, structure, and phase composition of the oxide film surface and cross-sectional layers of AlTiNiCuCox series HEAs were analyzed using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD). The influence of Co content on the high-temperature oxidation resistance of the HEAs was discussed, and the oxidation mechanism was summarized. The results indicate that, at 800 °C, the AlTiNiCuCox (x = 0.5, 1.0, 1.5) series HEAs had dense oxide films and certain high-temperature oxidation resistance. With increasing Co content, the high-temperature oxidation resistance of the alloys also increased. With increasing time at high temperature, there was a significant increase in the contents of oxide species and Ti on the oxide film surface. In the process of high-temperature oxidation of AlTiNiCuCox series HEAs, the interfacial reaction, in which metal elements and oxygen in the alloy form ions through direct contact reaction, initially dominated, then the diffusion process gradually became the dominant oxidation factor as ions diffused and were transported in the oxide film.

Electrochemical properties of as-cast La2-xYxMg16Ni (x = 0, 0.1, 0.2, 0.3, 0.4) alloys

Hydrogen storage alloys of La2-xYxMg16Ni (x = 0, 0.1, 0.2, 0.3, 0.4) were smelted by vacuum induction melting furnace. The microstructure of the cast alloys were observed by scanning electron microscope, and the alloys structure were analyzed by X-ray Diffractometer (XRD) before and after hydrogen absorption. The electrochemical capacity, cyclic stability, high-rate discharge, potentiodynamic polarization curves, and constant potential crossing curves were tested. Besides, the limited current density and diffusion coefficient were calculated. The results show that the main phase of the as-cast alloy is La2Mg17, and a little amount of the second phase Mg2Ni exist. In addition, LaH3 and MgH2 and a small amount of Mg2NiH4 phases were formed in the alloy after saturated hydrogen absorption. With the increasing of La element substituted by Y element in La2Mg16Ni alloy, the discharge specific capacity of alloy shows a decreasing tendency, it's unconspicuous, though. The cyclic stability and high rate discharge of hydrogen storage alloys increase first and then decrease with the increasing of La element substituted by Y element in La2Mg16Ni alloy. When S50 is 34.01%, the cyclic stability of La1.8Y0.2Mg16Ni alloy is best. Both the limited discharge current density and hydrogen diffusion coefficient of the alloy increase first and then decrease with the increasing of the La element of La2Mg16Ni alloy replaced by Y element. The maximum current density of La1.8Y0.2Mg16Ni alloy is IL = 325.11 mA/g, and the maximum diffusion coefficient is D = 1.849 × 10−8 cm2/s.

Bi-objective Optimization of Maraging Steel Produced by Vacuum Induction Melting Using Evolutionary Algorithms

Maraging steel is a special alloy exhibiting excellent combination of ultra-high strength with considerable ductility. Thus, such steel become significant worldwide for strategic sectors (like nuclear, aerospace and defence) where stringent quality standards with respect to chemistry and properties is compulsory. Manufacturing of these steel is extremely difficult, and vacuum induction melting (VIM) furnace plays most significant part as a primary melting unit. There are a large number of processing parameters to manufacture these special alloys and on top of that, these type of vacuum furnaces are connected with various valves, motors, sensors along with safety systems, which lead to involvement of additional interdependent process variables. Accordingly, building logical agent is often difficult because the developer requires possessing the complete and intricate knowledge of all the agents, which is viable only for deterministic environment. In case of nonlinear condition, the data generation and collection in digital form is a very useful resource of valuable information for meaningful process optimization-related investigation by developing intelligent model from databases. In this perspective, data-driven optimization using evolutionary algorithms is effective tool in order to optimize parameters related to melting time of manufacturing maraging steel through critical analysis of the data. Data of 130 heats collected from VIM 6.5 Tonne furnace and the considered parameters are weight of carbon added, leak rate of system, complete meltdown time, carbon–oxygen–nitrogen pct in opening sample, carbon–oxygen–nitrogen pct in final stages of melting, refining time, leak rate during melting, tapping temperature, melt duration and tap to start time between previous and present melting and lining life. Out of aforementioned fifteen parameters, minimization of refining time and melt duration is the objectives of the present investigation. Evolutionary neural network has been used as a primary optimization algorithm for present investigation which is coupled with predator–prey genetic algorithm (PPGA). Optimization using PPGA has been achieved successfully and obtained the trade-off between refining time and melt duration. The analyses of all input parameters reveal that the complete meltdown time and carbon addition at start are the two main parameters which can affect the melting and refining time greatly. Putting extra care on these two parameters will greatly affect the melting time of maraging steel in VIM furnace.

Enhancing Mechanical Properties of Hot Wrought Steel by Microalloying and Optimizing Heat Treatments

Novel methods to increase performance of steels without increasing production costs are always sought after. In this study, a chemistry–process–structure–property paradigm was employed to modify rolled homogeneous armor (RHA) steels that contain no nickel (Ni) or chromium (Cr) and instead, microalloying with niobium (Nb). Characterization of 0.02 and 0.05 wt.% Nb additions was targeted in tandem with optimized heat treatments to improve performance without detrimentally increasing carbon equivalence. Designed alloys were cast in a vacuum induction melting furnace and thermo-mechanically processed. Optimal heat treatment conditions were determined with computational simulation software, JMatPro, for materials properties, which resulted in an exploration of three tempering temperatures, 400, 500, and 600 °C. Microstructures were investigated by optical and electron microscopy, where Nb additions were determined to reduce average grain area of ferrite, bainite, and martensite by as much as 75, 59, and 58%, respectively. Hardenability was characterized by Jominy end-quench testing and showed a slight increase in average hardness for Nb-bearing alloys due to the precipitation of NbC. Compression and tension tests revealed minimal strain rate sensitivity and a nearly 30% increase in strength in comparison with reference RHA materials. Impact tests showed a moderate increase in energy absorption of up 61%. Fractography of the failed specimens highlighted MnS precipitates as well as NbC and Mo2C carbides. Overall, results showed small (≤ 0.05 wt.%) additions of Nb accompanied by an optimized heat treatment resulted in a modified, less-expensive RHA with similar performance to typical Ni/Cr-RHA.

In-situ TiC reinforced Al-4Cu matrix composite: Processing, microstructure and mechanical properties

Al-4Cu alloy matrix composites reinforced in-situ by TiC particulates were prepared by microwave heating (MH) and conventional heating (CH). The samples were heated up to 1273 K in vacuum induction melting furnace and vacuum microwave heating furnace, respectively. The thermodynamics and dynamics associated with the two systems were discussed and the activation energy related to two heating methods was also calculated. The influence of microwave heating on chemical reaction was analyzed. The results showed that Al reacted with Ti to form TiAl3 and carbon atom then displaced Al in TiAl3 to form TiC. The activation energy of TiC reaction generated by MH and CH were 41.4 kJ/mol and 270.0 kJ/mol, respectively. The ultimate tensile strength increased with the increase of TiC volume fraction. In addition, the ultimate tensile strength of the composite prepared by MH was higher than that of the sample prepared by CH.

The effects of homogenization time on the crystal structure and hardness of NiMnGaMo alloy

This study is based on X-ray diffraction (XRD) and DSC analysis, Scanning Electron Microscopy (SEM) and Optical Microscope observations and Vickers Micro Hardness Test results with structural effect and phase transformations on NiMnGa shape memory alloys with different composition. The addition of different elements to the shape-memory NiMnGa alloys and the heat treatment of the alloys are extremely useful for improving the shape memory, magnetic, mechanical, chemical and other physical properties. The NiMnGaMo alloys were successfully obtained using a vacuum induction melting furnace and then homogenized by heat treatment at 950 °C for 2, 24 and 96 h, respectively. After homogenization, all samples were cooled in ice water. Crystallographic results calculated from XRD data were evaluated by comparison with the data obtained by DSC analysis, Scanning Electron Microscope (SEM), Optical Microscope and Vickers Micro Hardness Test Device. The results show that the alloy with a higher Mo amount has a smaller crystallite size and the L21 lattice volume increases with heat treatment time. The change on microhardness values were discussed with the change on the obtained lattice constants.

Evaluating the Risk of Corona Discharge in Superalloy Vacuum Induction Melting Furnace Applications

Corona discharge, or gas breakdown, is undesired in superalloy vacuum induction melting (VIM) applications as it can damage insulation, produce ozone and subsequently oxidize and contaminate the melt. Gas breakdown also introduces uncontrolled variations to the casting process and increases the risk of needing to scrap the end product. To reduce the risk of corona, a rule-of-thumb of 150 to 1000 micron (150 mTorr to 1 Torr) was developed in the past as a No-Go Pressure Zone. Operating within this zone has been observed to create an unwanted, readily ionizable environment. An analytical model is developed within this work to evaluate the historical rule-of-thumb. The analytical model incorporates the following elements: Paschen’s law, gas compressibility, gas and insulation permittivity, and electric field non-uniformity. The model is then applied to a sampling of VIM furnace designs to evaluate the shifts of the No-Go Pressure Zone due to variations in configuration and other operating conditions. In the end, the objective is to improve VIM furnace design and operation to reduce the risk of corona discharge—thereby improving the casting process, practice of alloy melting and quality of the end product.

Studies on Intergranular Corrosion Characteristics of Cast Duplex Stainless Steel Stabilized with Niobium

Abstract To study intergranular corrosion (IGC) characteristics, two cast grades of duplex stainless steels, 5A and 5A stabilized with niobium, were produced in a vacuum induction melting furnace. The samples were tested under three different heat-treatment conditions that were selected using a property diagram generated through the TCFE8 database of the Thermo-Calc® software (Thermo-Calc Software, Solna, Sweden). A Feritscope FMP30® (Fischer Technology, Inc., Windsor, CT) was used to measure the ferrite phase content in the samples and compared with that specified in the property diagram. Metallurgical characteristics were analyzed through optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry. Each heat-treated sample possessed a distinct texture due to the formation of various intermetallic phases in the alloy. The corrosion behavior of the samples was studied using an oxalic acid etch test. Double-loop electrochemical potentiokinetic reactivation and sulfuric acid tests were conducted on the niobium-stabilized samples, and the results demonstrated that the samples possess superior resistance to corrosion compared to those not stabilized with niobium. The addition of niobium produced a mixture of niobium compounds that suppressed the chromium depletion and reduced the degree of sensitization. This study demonstrated the potential of using niobium as a stabilizer in the duplex stainless steel alloy to considerably reduce IGC.

Effect of dwelling time in VIM furnace on chemical composition and mechanical properties of a Ni–Fe–Cr alloy

The mechanical properties and changes in chemical compositions and microstructure of a Ni–Fe–Cr alloy in different dwelling time in a vacuum induction melting (VIM) furnace were studied. The chemical analysis shows that the dissipations of Al, Ti and Si increase with increasing the dwelling time in the VIM furnace. With the decrease in Si content due to increasing the dwelling time in the VIM furnace, the segregation of niobium is reduced, resulting in lower incoherent δ phase in the grain boundaries. This incoherent δ phase promotes intergranular fracture in the specimen with the lowest dwelling time. The dissipations of Al and Ti with increasing the dwelling time in the VIM furnace lead to decrease the hardness, yield and ultimate tensile strengths of the alloy, while the elongation increases owing to the lower number of inclusions and strengthening phase (γ″).

Effect of the Combined Addition of Y and Ti on the Second Phase and Mechanical Properties of China Low-Activation Martensitic Steel

In this study, approximately 0.35% Ti and two different Y contents were added to China low-activation martensitic (CLAM) steel during melting in a vacuum induction melting furnace. Scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile tests, and Charpy impact tests were used to investigate the effects of the combined addition of Y and Ti on the second phase and mechanical properties. The results indicated that Y and Fe formed the large intermetallic compound Fe-Y; the compound easily aggregated in the grain boundaries and exhibited the strength of CLAM steel. Ti did not combine with Y to form the Y-Ti-O phase; however, it could combine with Ta and W to form MC precipitates, which were generally in the 20-50 nm size range. The CLAM steel with a higher Y content exhibited lower yield and tensile strengths at room temperature, with both steels yielding almost identical strengths at 600 °C.

Enhanced catalytic properties of La-doped CeO 2 nanopowders synthesized by hydrolyzing and oxidizing Ce 46 La 5 C 49 alloys

The Ce46La5C49 alloy was first prepared in a 25 kg vacuum induction melting furnace. The La-doped CeO2 nanopowders were then prepared by hydrolysis and oxidation of Ce46La5C49 at room temperature. These nanopowders were calcinated at different temperatures in order to improve their catalytic activities. The lanthanum ions were used to partially replace the cerium ions in the CeO2 lattice, forming a solid solution of cerium lanthanum. Compared to the pure CeO2, the thermal stability of La-doped CeO2 was increased due to the lanthanum doping. The La-doped CeO2 nanopowders show enhanced CH4 catalytic performance.

Nanocrystalline Ce1−x La x O2−δ Solid Solutions Synthesized by Hydrolyzing and Oxidizing

We undertook a novel batch production approach for the synthesis of CeO2 nanopowders doped with rare earth elements. Solid solution nanopowders of Ce1−xLaxO2−δ (x = 0.15) were successfully synthesized in a large-scale and low-cost production by hydrolyzing and oxidizing Ce-La-C alloys at room temperature and subsequent calcining of their powders at different temperatures (873–1073 K) for 1 h. The Ce-La-C alloys were prepared in a vacuum induction melting furnace. The final products were characterized by x-ray diffraction, transmission electron microscopy, Brunner–Emmet–Teller (BET) surface area analyzer, and Raman spectroscopy. The calculated lattice parameters of the cubic fluorite-type phase of CeO2 tended to increase when La3+ was incorporated into CeO2. The F2g band shift and the absence of a peak corresponding to La2O3 in the Raman spectra consistently confirmed the incorporation of the La3+ ion into CeO2, and the formation of Ce1−xLaxO2−δ solid solutions as manifested by increased oxygen vacancy defects. High-quality Ce1−xLaxO2−δ nanopowders of ~10–15 nm diameter with a high BET surface area of ~77 m2 g−1 were obtained. The average crystallite size of Ce1−xLaxO2−δ was found to be smaller than that of CeO2 for the same calcination temperature of 1073 K, demonstrating that the introduction of La3+ into CeO2 can stabilize the host lattice and refine the grain size at high temperatures.