Sn Melt Research Articles

Microheater Chips with Carbon Nanotube Resistors.

The specific and excellent properties of the low-dimensional nanomaterials have made them promising building blocks to be integrated into microelectromechanical systems with high performances. Here, we present a new microheater chip for in situ TEM, in which a cross-stacked superaligned carbon nanotube (CNT) film resistor is located on a suspended SiNx membrane via van der Waals (vdW) interactions. The CNT microheater has a fast high-temperature response and low power consumption, thanks to the micro/nanostructure of the CNT materials. Moreover, the membrane bulging amplitude is significantly reduced to only ∼100 nm at 800 °C for the vdW interaction between the CNTs and the SiNx membrane. An in situ observation of the Sn melting process is successfully conducted with the assistance of a customized flexible temperature control system. The uniform wafer-scaled CNT films enable a high level of consistency and cost-effective mass production of such chips. The as-developed in situ chips, as well as the related techniques, hold great promise in nanoscience, materials science, and electrochemistry.

(Poster Award - Honorable Mention) Ta2O5 Catalyst Support for Polymer Electrolyte Fuel Cells

Pt/C is widely used as polymer electrolyte fuel cells (PEFC) electrocatalysts. The cathodic environment of PEFC is prone to high acidity and high potential, which can degrade cell performance due to carbon corrosion 1. Therefore, electrocatalysts with oxide support such as SnO2 supported on carbon-based materials have been developed, achieving high durability. Further improvement of their catalytic performance is desirable, using a Pt-based alloy catalyst and a mesoporous carbon (MC) 2,3. However, studies using SnO2 as a support have suggested that the low melting point of Sn caused alloying of Pt and Sn and/or melting of metallic Sn, causing degradation of catalytic activity 3. From these reasons, here, we focus on tantalum (Ta) with much higher melting point to fabricate Ta2O5/MC electrocatalysts support, which can support Pt alloy catalysts to achieve both high durability and high activity. Ta2O5 was prepared by a sol-gel method using Ta-ethoxide, and Pt-Co alloy nano-particles were deposited via the acetylacetonate method 4. The amount of Ta2O5 on the MC was set to be 30 and 60 wt.%, and the durability was evaluated by varying the amount of Ta2O5. The molar ratio of Pt to Co was set at 3:1. The microstructure of the electrocatalyst was observed by field-emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM), and the alloy was evaluated by XRD measurements. Half-cell tests were made to evaluate electrochemical performance and durability (load cycle and start-stop cycle). From the XRD measurements, the Pt peaks were shifted and the Ta2O5 peaks were confirmed, indicating that Pt-Co alloys were formed without forming the alloy of Pt and Ta. FESEM observation revealed that Ta2O5 particles were impregnated with a diameter of about 5 nm, and Pt-Co alloy particles were impregnated with a diameter of 2 to 3 nm. Some of catalyst particles were found to be impregnated on Ta2O5, but no significant selectivity to Ta2O5 was observed. Both Pt effective surface area (ECSA) and oxygen reduction reaction (ORR) activity of the prepared catalysts were higher than of Pt/C, confirming the positive effect of Pt-Co alloy and MC on activity enhancement. Figure 1 shows that the load potential cycle durability was improved using MC, and the start-stop cycle durability was improved by using Ta2O5. In particular, the electrocatalysts with a lower loading of Ta2O5 exhibited higher durability under load potential cycling and subsequent higher ECSA compared to the Pt-Co/C electrocatalyst. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References Ohma, K. Shinohara, A. Iiyama, T. Yoshida, and A. Daimaru, ECS Trans, 41(1), 775 (2011).Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K.Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018).Inoue, M. Yasutake, Z. Noda, S. M. Lyth, M. Nishihara, A. Hayashi, J. Matsuda, and K. Sasaki, ECS Trans., 109 (9), 413 (2022).Hayashi, H. Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochem. Acta., 53, 6117 (2008). Figure 1

An in situ elemental migration on a nanosize Cu–Sn interface in a microscope

Elemental migration between different elements is intrinsic process for diffusion. Most of the study focused on the presentative shape or size on of Kirkendall pores, yet on the intermediate migration process. A dynamic elemental transition trajectory recording provides realtime diffusion process on a Cu–Sn nanoscale interface is thus necessary to disclose intermediated reaction on diffusion process. In this work, elemental dynamic diffusion processes were recorded and analysed on a microsize Cu–Sn couple lamella by in situ transmission electron microscopy up to Sn melting point. Factors of atomic quantity, contact condition and temperatures were studied on its diffusion behaviours. The work shed new light on conventional metallographically interfaces diffusion and provide intrinsic clues for solder design.

Study on the dissolution diffusion behavior of Al in Sn melt

The Al-Sn alloys with a large melting point difference and large density are widely used in sliding bearings in the aerospace and automobile industries. But the problem of insufficient alloying easily occurs during the preparation process due to the physical properties. This paper investigates the dissolution and diffusion behavior of Al in Sn Melt. The dissolution rates of Al in Sn melts at different temperatures were calculated. The research results show that the Al/Sn diffusion is dominated by the diffusion of Sn into Al, the Sn melt penetrate into the solid Al to form diffusion channel leading to the exfoliation and aggregation of the unmelted Al particles. Then, an electromagnetic field treatment technology was introduced to improve sufficient alloying by regulating the flow field of the metal melt. It can accelerate the diffusion and redistribution of elements at the Al/Sn solid-liquid interface, as well as change the dissolution mechanism of Al/Sn interface.

Purification of High-Purity Tin via Vertical Zone Refining

The present investigation delves into the potential of vertical zone refining as an effective purification technique for achieving high-purity tin (Sn) metal. The utilization of vertical zone refining offers distinct advantages over traditional horizontal zone refining, as it allows for enhanced control over the molten zone and solid–liquid interface, ultimately leading to superior impurity separation efficiency. The present study reveals that the solute partition coefficients (k0) of various impurity elements, such as Zn, Ag, Al, Mg, Ca, Ni, In, Co, Cu, As, Pb, Fe, and Bi, during the vertical zone refining process consistently demonstrate values below one. Notably, the partition coefficient of Sb deviates slightly from the others, being greater than one but approaching one. The authors achieve exceptional levels of purity in both the bottom and middle regions of the rod by subjecting the Sn melt to nine passes of vertical zone refining at a heating temperature of 405 °C and a downward pulling rate of 10 µm/s, resulting in purities exceeding 6N4. Furthermore, by evaluating the effective partition coefficients (keff), it was determined that impurity elements, such as Cu and Bi, closely approach their equilibrium partition coefficients, reaching values of approximately 0.492 and 0.327, respectively. To further enhance the purity of Sn metal and maximize product yield, we propose the utilization of electrolytic refining and vacuum distillation, with particular emphasis on the efficient separation of five specific elements, including Cu, Fe, As, Pb, and Sb. By elucidating these findings, this study not only contributes valuable insights into the efficacy of vertical zone refining as a purification technique for high-purity tin metal, but also offers important recommendations for refining strategies and impurity element separation.

Mechanistic Insights into the Formation of Thermoelectric TiNiSn from In Situ Neutron Powder Diffraction.

Half-Heusler alloys are leading contenders for application in thermoelectric generators. However, reproducible synthesis of these materials remains challenging. Here, we have used in situ neutron powder diffraction to monitor the synthesis of TiNiSn from elemental powders, including the impact of intentional excess Ni. This reveals a complex sequence of reactions with an important role for molten phases. The first reaction occurs upon melting of Sn (232 °C), when Ni3Sn4, Ni3Sn2, and Ni3Sn phases form upon heating. Ti remains inert with formation of Ti2Ni and small amounts of half-Heusler TiNi1+ySn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNi2y'Sn phases. Heusler phase formation is greatly accelerated by a second melting event near 750-800 °C. During annealing at 900 °C, full-Heusler TiNi2y'Sn reacts with TiNi and molten Ti2Sn3 and Sn to form half-Heusler TiNi1+ySn on a timescale of 3-5 h. Increasing the nominal Ni excess leads to increased concentrations of Ni interstitials in the half-Heusler phase and an increased fraction of full-Heusler. The final amount of interstitial Ni is controlled by defect chemistry thermodynamics. In contrast to melt processing, no crystalline Ti-Sn binaries are observed, confirming that the powder route proceeds via a different pathway. This work provides important new fundamental insights in the complex formation mechanism of TiNiSn that can be used for future targeted synthetic design. Analysis of the impact of interstitial Ni on the thermoelectric transport data is also presented.

Enhanced separation of bulk silicon from Si-Sn melt via electromagnetic directional solidification

Developing new methods for solar photovoltaic power generation is an effective way to solve future energy shortages. Solvent refining is one of the most promising methods to achieve the low-cost mass production of solar-grade Si. In this work, a novel method for separating bulk silicon from a Si-Sn melt under intensification by an electromagnetic stirring is proposed. The growth and enrichment of Si crystals were controlled by the upward and downward directional solidification of Si-50at%Sn melt at different moving rates. Compared with traditional directional solidification, electromagnetic stirring yielded a higher growth rate of Si. A lower moving rate improved the enrichment percentage of Si, and a large area of bulk Si with an enrichment percentage of up to 88.16% was obtained by performing downward directional solidification at a rate of 3 µm/s. Moreover, the downward movement improved the Si enrichment, because Si grew along the bottom and side walls of the crucible. These results provide a new scientific perspective and experimental evidence for the separation of primary Si crystals from Si-based multicomponent solvents.

The CPS's pre-heating effect on the capability to withstand extreme plasma loads

This work presents experimental studies of plasma surface interactions (PSIs) during QSPA plasma impacts on the Sn Capillary Porous Structures (CPSs) in conditions, simulating disruption and ELM-like loads. The 3D tungsten target filled by Sn was exposed to the plasma streams with an energy density of up to 3 MJ/m2 which resulted in tungsten melting and evaporation. The initial temperature of a sample between plasma pulses had two options: remained at a room value for one sample, and reached 300 C which is close to Sn melting point for another one. The number of plasma irradiations reached 100 pulses. It is shown that the particle ejection was observed during PSI for pre-heated as well as cold CPS targets. The difference in values of the number of particles ejected from the targets, their velocities, and the start-time during exposition is discussed. Particles leave the target surface during all expositional pulses for both CPS targets. The threshold character of the ejection is more pronounced for the cold target. After the 100 pulses reduced Sn pool is observed. Moreover, areas without Sn occur on the exposed targets. Nevertheless, the severe damage of the W base of the CPS targets is not revealed during high-cycle powerful plasma loads.

Graphene Microheater Chips for In Situ TEM.

Low-dimensional materials are bringing significant innovations to in situ TEM characterization. Here a new graphene microheater chip for TEM was developed by stacking graphene on a suspended SiNx membrane as the Joule heating element. It could be heated up to 800 °C within 26.31 ms with a low power consumption of 0.025 mW/1000 μm2. The bulging was only ∼50 nm at 650 °C, which is 2 orders of magnitude smaller than those of conventional MEMS heaters at similar temperatures. The performances benefit from the employment of graphene, since its monolayer structure greatly reduces the heat capacity, and the vdW contact significantly reduces the interfacial interaction. The TEM observation on the Sn melting process verifies its great potential in resolving thermodynamic processes. Moreover, more multifunctional in situ chips could be developed by integrating other stimuli to such chips. This work opens a new frontier for both graphene and in situ characterization techniques.

Microstructure and Corrosion Behavior of Sn-Zn Alloys.

In the present work, the microstructure, phase constitution, and corrosion behavior of binary Sn–xZn alloys (x = 5, 9 and 15 wt.%) were investigated. The alloys were prepared by induction melting of Sn and Zn lumps in argon. After melting, the alloys were solidified to form cast cylinders. The Sn–9Zn alloy had a eutectic microstructure. The Sn–5Zn and Sn–15Zn alloys were composed of dendritic (Sn) or (Zn) and eutectic. The corrosion behavior of the Sn–Zn alloys was studied in aqueous HCl (1 wt.%) and NaCl (3.5 wt.%) solutions at room temperature. Corrosion potentials and corrosion rates in HCl were significantly higher compared to NaCl. The corrosion of the binary Sn–Zn alloys was found to take place by a galvanic mechanism. The chemical composition of the corrosion products formed on the Sn–Zn alloys changed with the Zn weight fraction. Alloys with a higher concentration of Zn (Sn–9Zn, Sn–15Zn) formed corrosion products rich in Zn. The Zn-rich corrosion products were prone to spallation. The corrosion rate in the HCl solution decreased with decreasing weight fraction of Zn. The Sn–5Zn alloy had the lowest corrosion rate. The corrosion resistance in HCl could be considerably improved by reducing the proportion of zinc in Sn–Zn alloys.

Determination of Al2O3 Powder Thermal Conductivity Using DSC at NIS-Egypt

The mission of the thermal metrology laboratory is to maintain, disseminate, develop and realize the International Temperature Scale 1990 (ITS-90). One of the services that the laboratory has introduced is the routine calibration to all industrial sectors that covers almost all fields inside and outside Egypt. This paper explains how to use Differential Scanning Calorimeter (DSC) to determine the thermal conductivity of unknown powders. The paper is based on the ability to determine the thermal conductivity (k) of powders from the melting peak of the reference metal. This technique assists for the first time to establish a new activity at the National Institute for Standardization to test the thermal conductivity of powders, which had been requested by many clients. Experiments were carried out using alumina powder Al2O3 and metal reference beads of Sn and Zn which have diameter ranges from 1.9 mm up to 4.12 mm in an atmosphere of nitrogen during Sn and Zn melting. The results show good performance. The results showed confidence that will enable to do more studies at NIS to see if this technology can be applied to a wider spectrum of materials.

New insights into the petrogenesis of the Bozhushan W-Sn mineralization-associated granites, Yunnan province, SW China: Evidence of microgranitoid enclaves

The Bozhushan granites, located in the southeastern Yunnan Province W–Sn polymetallic metallogenic belt in SW China, have considerable W-Sn endowment. However, their petrogenesis remains uncertain, as both crustal and mantle origins have been proposed. Numerous microgranular enclaves (MEs) are found in the Bozhushan granites, and the origin of these MEs can give insights into the petrogenesis of these ore-forming granites. Here we present an integrated study of the major and trace elements, whole-rock Sr–Nd isotopes, and zircon Hf–O isotopes of the MEs. The MEs from the Bozhushan granites are commonly composed of monzodiorite and granodiorite, and they have lower SiO2 (51.7–66.4 wt%) and higher TiO2 (0.83–1.67 wt%), MgO (1.68–4.49 wt%), and CaO (1.74–4.26 wt%) contents than those of their hosts (SiO2 = 64.3–76.9 wt%; TiO2 = 0.4–0.67 wt%; MgO = 0.86–1.33 wt%; CaO = 2.08–3.25 wt%). Furthermore, the chemical compositions of the MEs and host granites are discontinuous and do not show linear trends in element variation diagrams, as might be expected if the MEs represented restites, thus implying disparate genesis. The observed near-linear element variation most probably reflects mixing of mafic and felsic magma end-members in various proportions. In addition, the MEs have Sr–Nd–Hf–O isotopic characteristics (87Sr/86Sri = 0.7117–0.7138, εNd = − 12.59 to − 9.26, εHf(t) = − 13.1 to − 6.1, and δ18O = 8.39‰–9.19‰) and zircon U–Pb ages (87.4 ± 1.0 Ma to 86.5 ± 0.4 Ma) that are similar to those of their host granites. We conclude that the similarity of trace-element contents and Sr–Nd isotopic compositions between the MEs and host granites was caused by diffusion and partial re-equilibration. However, if the magma temperature of the MEs was 850 °C, 950 °C, or 1050 °C, the required Zr contents for zircon saturation would be 238–854 ppm, 497–1785 ppm, or 928–3334 ppm, respectively, which are substantially higher than those measured for the bulk rocks (44–436 ppm). Furthermore, zircons from the MEs have Ti-in-zircon temperatures (Tzr; MEs, 669–884 °C; host, 790–842 °C) and Th/U ratios (MEs, 3.43–8.85; host, 2.68–11.86) that are indistinguishable from those of their host granites. Thus, on the basis of our new data, we propose that the MEs of the Bozhushan granites might have been derived from mantle-derived mafic magma. Zircons from the MEs did not crystallize directly within the MEs but rather were xenocrysts that formed during the early stage of magmatic evolution at the bottom of the granitic magma chamber and were subsequently incorporated into the MEs when mafic magma was injected into the granitic magma chamber. The Hf–O isotopes of zircons in this case cannot be used to constrain the primary composition of the magma that formed the MEs. During the Late Cretaceous, owing to lithospheric extension and asthenospheric underplating, the lower crust was partially melted to form high-temperature (790–842 °C) granitic magma in the Bozhushan area. During the formation of the W–Sn mineralization associated with the Bozhushan granites, 10%–20% underplated mafic magma mixed with granitic magma and contributed both heat and mantle-derived material. Therefore, we consider that the Bozhushan granites are clearly characteristic of A-type granites, but the W–Sn mineralization was formed as a result of high-temperature partial melting of Sn–W-rich metasedimentary sources triggered by additional heat provided by mantle-sourced mafic magma.

Effect of Process Parameters on Laser Powder Bed Fusion of Al-Sn Miscibility Gap Alloy

Al-Sn binary system is a miscibility gap alloy consisting of an Al-rich phase and a Sn-rich phase. This system is traditionally applied in bearings and more recently found application as form-stable phase change material (PCM) exploiting solid-liquid phase transition of Sn. A careful choice of production process is required to avoid macro-segregation of the two phases, which have different densities and melting temperatures. In the present study, the additive manufacturing process known as laser powder bed fusion (LPBF) was applied to an Al-Sn alloy with 20% volume of Sn, as a rapid solidification process. The effect of process parameters on microstructure and hardness was evaluated. Moreover, feasibility and stability with thermal cycles of a lattice structure of the same alloy were experimentally investigated. An Al-Sn lattice structure could be used as container for a lower melting organic PCM (e.g., a paraffin or a fatty acid), providing high thermal diffusivity thanks to the metallic network and a “safety system” reducing thermal diffusivity if the system temperature overcomes Sn melting temperature. Even if focused on Al-Sn to be applied in thermal management systems, the study offers a contribution in view of the optimization of manufacturing processes locally involving high solidification rates and reheat cycles in other miscibility gap alloys (e.g., Fe-Cu) with similar thermal or structural applications.

Separation mechanism of bulk Si from Si–Sn melt via upwards directional solidification based on the density difference

There is no systematic study on the separation mechanism of bulk Si from Si–Sn melt by directional solidification, although the system has many outstanding advantages. In this work, the influence of alloy composition and cooling rate on the quality of grown bulk Si were studied. The length of the mushy zone increased with the composition, and the stable convex interface only occurred at the composition of Si higher than 50 at.%; As for the composition of Si–12.5 at.% Sn, the convex interface became flatter. The separation effect deteriorated rapidly with the cooling rate. The evolution of the crystal growth process and the transfer of the interface were simulated, which predicts the shape of the interface and explains the residual Sn trapped in bulk Si.

Synthesis of Ti3SnC2–Al2O3 composite by mechanical alloying and subsequent heat treatment

A Ti 3 SnC 2 –Al 2 O 3 composite was produced by mechanical alloying and subsequent differential thermal analysis (DTA) of 3TiO 2 /4Al/1Sn/2C powder mixture. The starting powder was milled up to 40 h. To determine the formation mechanism, a 40 h milled powder was evaluated by DTA. X-ray diffraction was used to evaluate powder samples. Later, morphological and microstructural characteristics of the samples were obtained by high-resolution scanning electron microscopy. Followed by milling, the patterns of X-ray diffraction showed a homogeneous distribution of fine reactant powders without new phases that after 40 h of milling. According to the DTA, several thermal events occurred during the 40 h of milling and Ti 3 SnC 2 /Al 2 O 3 was formed, consequently. The formation mechanism of Ti 3 SnC 2 /Al 2 O 3 composite consisted of Sn melting, Al melting, TiO 2 reduction, Ti 5 Sn 3 and TiC formation, and reaction among Ti, C, Ti 5 Sn 3 and TiC, as well as Ti 3 SnC 2 formation.

Wettability and spreading behavior of Sn–Cr alloys on SiC

Knowing of the wetting of filler alloy on substrate is significant for brazing. Here, the isothermal wetting behavior of Sn-xCr alloys (x = 1, 2, 3, 4 and 5 wt%) on SiC substrates was studied by a modified sessile drop method in the temperature range of 1173 K–1323 K. The interfacial microstructures were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) and its evolution was analyzed thermodynamically. Also, the spreading kinetics is calculated and the formation of precursor film is discussed. The spreading of Sn melt on SiC consists of rapid-spreading and sluggish-spreading stages. A better wettability is obtained at higher Cr concentration and isothermal temperature. The direct reaction between Cr and SiC forming metallics Cr–C compounds (Cr3C2, Cr7C3 and Cr23C6) at the interface accounts for the improving wettability of Sn melt on SiC. More metallic Cr–C compound occupies a higher proportion of interfacial layer inducing a lower final contact angle. Temperature has a violent influence on the spreading kinetic constant Kr. The spreading activity energy is calculated to be 312 kJ/mol.

Microstructure, melting behavior and thermal conductivity of the Sn–Zn alloys

Four hypereutectic Sn-Zn alloys with 69.5, 48.3, 28.1 and 14.1 at% of Zn were produced by mixing and melting of pure metals Sn and Zn followed by cooling in air to room temperature. Microstructure and chemical compositions of prepared samples were analyzed by using scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS). It was found that morphology of primary (Zn) phase changes from rounded and equiaxed dendritic structure to plate-like grains and finally to needle-like shape of grains with decreasing Zn content. Formation of characteristic broken-lamellar type of eutectic microstructure was observed in all investigated alloys. Melting behavior of the alloys was studied using differential scanning calorimetry (DSC). Phase transition temperatures and corresponding heat effects were experimentally determined and compared with the results of thermodynamic and phase equilibria calculations using the CALPHAD (CALculation of PHAse Diagrams) method and optimized thermodynamic parameters from literature. Thermal diffusivity of the studied alloys was measured using xenon flash method in the temperature range from 25 to 150 °C. Based on the measured values of thermal diffusivity and calculated specific heat capacity data, thermal conductivities of the solid alloys were obtained. It was found that thermal conductivity decreases monotonously with increasing temperature and Sn content. The results of thermal conductivity measurements were compared with literature data and with the data obtained from the Wiedemann-Franz law and measured electrical conductivities.

Effects of La2O3 Addition into CaO-SiO2 Slag: Structural Evolution and Impurity Separation from Si-Sn Alloy

Boron (B) and phosphorus (P) are the most problematic impurities to be removed in the production of solar-grade silicon by the metallurgical process. In this work, the distribution of B and P between CaO-(La2O3)-SiO2 slags and Si-10 mass pct Sn melt was experimentally studied. B distribution coefficient increased from 2.93 in binary CaO-SiO2 slag to 3.33 and 3.65 with 2 and 10 mass pct La2O3 additions, respectively. In the followed acid-leaching experiments, the slag-treated Si-Sn alloys exhibited higher B and P removal than that of the initial alloy without slag treatment. Molecular dynamics simulations were performed to study the effect of La2O3 addition on the slag structural and transport properties. A novel oxygen classification method was proposed to distinguish the different structural roles of La and Ca in the CaO-La2O3-SiO2 system. It was found that La3+ prefers to stay in the depolymerized region, mostly connects with 6-7 non-bridging oxygen, and requires a weak charge compensation with Ca2+. Possible silicothermic reduction was evaluated to discuss the slag chemistry and the mass transfer between slag and metal phase. A thermodynamic model was derived to theoretically study the alloying effect on impurity distribution in slag refining where positive interaction coefficient and high alloying concentration were found most beneficial to improve the impurity removal.

Development of a Thin Film Gas Microsensor of Low Power Consumption for Monitoring of Environmental Pollutants

In this work the development and microfabrication of a gas sensor is described. The microsensor consists of a thin film of SnO2 deposited on a micro-machined silicon substrate [1] . This film is heated by a microheater integrated in the microsensor to obtain a temperature between 100 and 400 °C (microhot plates). With the aim to analyze the proposed design, simulations of the mechanical (structural rigidity, maximum and minimum sizes of the structures) and thermal behaviors (distribution of temperatures and electrical powers involved) were performed [2].The manufacturing process carried out in a clean-room area involved a series of steps that include: the design and mastering of masks, photolithography operations, sputtering metal deposition, chemical attacks with dry (RIE) and wet etching (with KOH), dicing of the silicon wafer in individual sensors of 3mm x 3mm and finally, the encapsulation of the sensors by means of wire bonding (see Fig.1)It is important to note that the silicon substrate is micromachined in order to reduce its thermal inertia and the power necessary to heat the sensor film. Such a film is deposited on a silicon nitride insulating membrane that is only 250 nm thick, which significantly reduces the consumption in terms of electrical power, with respect to the known commercial sensors.The SnO2 thin films were fabricated using the technique Rheotaxial Growth and Thermal Oxidation technique (RGTO) on a microhot plate[3]. RGTO is a technique composed of two steps: depositionof a Sn thin film on the substrate maintained at the temperature slightly above the Sn melting point and oxidation at a temperature up to 400 oC. In the first step of the RGTO technique tin was deposited by sputtering DC on the central part of the membrane by means of a shadow mask. During the deposition, substrates were heated at 310 oC . Thickness of the deposited tin was 100 nm. The second step, involving the thermal oxidation of Sn thin layer, was performed in two sub-steps: 2 h at 250 oC and 60 h at 450 oC in wet air. The final result is a granular film with quasi-spherical grain shape. The microstructure was characterized by X-ray diffraction analysis and scanning electron microscopy (SEM).The characterization of the sensors with gases at different concentrations was done. In the case of ammonia in air, a linear response between 50ppm (maximum permissible limit value) and 6ppm were obtained, the latter being its detection limit (Fig 2). Also H2S were tested (Fig. 3) . The measurements were made by applying a voltage of 5 V to the heater, obtaining dissipated power between 40 and 50 mW. The operating temperature of the sensor film under these conditions was approximately 180°C. On the other hand, the sensor film is connected to a potential of 1 V. The sensor response is then measured as the variation of the electrical resistance as a function of the response time.Additionally, electronic control and signal processing was developed, suitable for incorporating sensors into portable instruments. [4]The results of this development also allow us to have a platform for the microfabrication of gas sensors at the request of other public institutions or the private sector. References 1] Mirzaei, A., Leonardi, S.G., Neri, G. Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review. Ceramics International 42, 15119–15141(2016).[2] G.Korotcenkov, B.K. Cho, Metal oxide composites in conductometric gas sensors: Achievements and challenges . Sensors and Actuators B, Vol. 244, 182–210 (2017).[3] G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli , A. Camanz. A new technique for growing large surface area SnO2 thin film (RGTO technique) . Semicond Sci. Technol. 5:1231–1233 (1990)[4] J.Vorobioff , D. F. Rodriguez, N.G. Boggio , C.A. Rinaldi . Development of an Electronic Nose for Determining the Freshness of Fish by the Desorption Constants of Sensors. Sensors Letters, Vol. 11, N° 12, 2013, pp. 2215-2217. Figure 1

Influence of melt superheating treatment on cast structure of Al–Sn alloys

The study covers the effect of Melt Superheating Treatment (MST) for Al–Sn alloys. To determine the optimal superheating temperature, the authors measured the temperature dependences of the kinematic viscosity, electrical resistivity, density and surface tension of Al–Sn melts with tin contents of 10, 20, 30, 40, and 50 wt.%. According to the measurement results, the temperature t* was determined for each Al–Sn alloy sample. Heating to this temperature breaks down the micro-inhomogeneous state and leads to the heterogeneous liquid – homogeneous liquid structural transition. Melt superheating (MST) results in a decrease in melt viscosity. It was found that the temperature t* rises with increasing tin concentration in the Al–Sn melt. An increase in the tin content in the Al–Sn melt also leads to a decrease in the absolute values of kinematic viscosity and surface tension, while the electrical resistivity and density increase accordingly. Thus, the Melt Superheating Treatment (MST) mode for Al–Sn alloys was determined. The effect of MST of Al–50wt.%Sn melt on the microstructure and mechanical properties of the ingot was studied in order to determine the structural sensitivity to the degree of melt overheating, and to find a new strategy to improve the shaping ability of the Al–Sn alloy two-phase structure. The results demonstrated that the method of resistivity and viscosity determination are more sensitive and effective for melt superheating temperature (MST mode) evaluation. In addition, the desired modified Al–Sn ingot structure can be formed under normal casting conditions; MST can contribute to the modified ingot structure formation by increasing the solidification time and decreasing the average solidification rate by reducing melt viscosity after superheating.