Mid-high Temperature Research Articles

Effect of Ti microalloying and laser remelting on the microstructure and mechanical properties of laser powder bed fusion Al–12Si alloy

This study explores the impact of micron-Ti microalloying and laser remelting on the microstructure and mechanical properties of laser powder bed fusion (LPBF) Al–12Si alloy, analyzing the underlying reasons. The findings show that the tensile strength and elongation at 300 °C (184 ± 13 MPa and 7.4 ± 1.4 %) are on par with other LPBF heat-resistant Al alloys, such as near-eutectic Al–Ce and Al–Ni. Introducing micron Ti (1 wt%) to AlSi12 effectively eliminates the α-Al <100> texture and refines the grain structure, thanks to the strong nucleation effect from the formed D022-(Al, Si)3Ti, leading to enhanced tensile strength in both as-built and heat-treated (300 °C for 2 h) samples, while maintaining ductility within the typical range for LPBF near-eutectic Al–Si alloys. Laser remelting further decreases the presence of unmelted Ti powder in AlSi12Ti and encourages the formation of D022-(Al, Si)3Ti, offering insights into LPBF Al alloy composition design through promoting the melt of the modified powder and improving material homogeneity, especially for incorporating micron powders with high melting points. Additionally, the superior mid-high temperature properties (300 °C) of AlSi12+1 wt% micron-Ti contribute to expanding the database of LPBF Al alloys.

Influence of electronic transport mechanism optimization on the thermoelectric properties of ZnO based functional ceramics

ZnO is one of the most promising mid-high temperature thermoelectric materials because of low cost, easy preparation and favorable chemical stability. Herein, the influence mechanism of Alkaline earth metal Ba doping on the thermoelectric properties of ZnO was studied. It is found that Ba doping can effectively improve the carrier mobility and electrical conductivity for that Ba doping can shorten the Zn–O bond and enhance the overlap and hybridization of electron orbitals. The electronic density of states of Zn and O is significantly enhanced. The highest power factor of ∼7.95 μWcm−1K−2 was obtained at 873 K, which is ∼1.4 times higher than that of the undoped sample. Due to the decrease of Young’s modulus caused by Ba doping and the enhancement of phonon scattering of heavy element, the thermal conductivity decreases obviously in the whole temperature range. Ba doping enhanced the effect of phonon glass-electron crystal, and the ZT value of ∼0.345 was obtained at 873 K, which is ∼3.52 times as that of the pristine ZnO. The results indicated that alkaline earth metal doping is an effective way to improve the thermoelectric properties of ZnO functional materials.

The Effects of Indium Additions on Tribological Behavior of Spark Plasma Sintering-Produced Graphene-Doped Alumina Matrix Composites for Self-Lubricating Applications

Alumina (Al2O3) ceramics are interesting for low-weight and mid-high temperature applications. The addition of indium (In) and graphene nanoplatelets (GNPs) can be used to reduce the density and modify the functional properties and mechanical performance of the ceramic matrix. GNP and In-reinforced Al2O3 matrix composites were prepared by the spark plasma sintering (SPS) technique. Monolithic Al2O3 and Al2O3 matrix composites with either 5 or 10 wt.% of In and 2 wt.% of GNPs (Al2O3-5In-2GNPs and Al2O3-10In-2GNPs) were compacted into disc-shaped samples. The microstructure was studied and characterized with light-optical microscopy (LOM) and scanning electron microscopy (SEM). Hardness was determined using the Vickers technique and tribological properties were studied by the ball-on-disk method. The coefficient of friction (COF) and specific wear rates were evaluated from tribological tests. Worn surfaces were studied by SEM and confocal microscopy. Interdiffusion transition regions were formed among individual microstructural constituents (Al2O3, In, GNPs) under high sintering temperatures, which were responsible for the balanced hardness and low porosity of the produced composites. The addition of In and graphene nanoplatelets resulted in smaller COF and wear rates indicating good improvement in the tribological behavior. The prepared Al2O3-5In-2GNP and Al2O3-10In-2GNP composites represent promising nanocomposites for self-lubricating applications.

Power factor enhancement of β-Zn4Sb3-InSb composites via arrays of p-n junction: Mixing p-type and n-type thermoelectric materials

The lead-free and abundantly available nature of β-Zn4Sb3 makes it a promising thermoelectric material for mid-high temperature applications. We produce composites by embedding 1 wt %, 3 wt %, and 5 wt % of n-type InSb into the p-type β-Zn4Sb3 host, enhancing both electrical conductivity and thermopower. At 600 K, the electrical resistivity decreases to 55 μΩ-m from 71 μΩ-m for the β-Zn4Sb3 host. Surprisingly, the thermopower of the β-Zn4Sb3/3 wt % InSb composite increases to 209 μV/K from 180 μV/K for the β-Zn4Sb3 host, contrary to expectations. This enhancement can be attributed to p-n junctions between the p-type β-Zn4Sb3 and n-type InSb. The power factor of the β-Zn4Sb3/3 wt % InSb composite increased to 787 μW/mK2 from 452 μW/mK2 for the β-Zn4Sb3 host. Despite higher thermal conductivity, the β-Zn4Sb3/3 wt % InSb composite achieved the highest zT value of ∼0.67 at around 600 K, 67.5 % higher than the β-Zn4Sb3 host's zT value of 0.40.

GIS model for geothermal advantageous target selection

As the particularly popular green energy, geothermal resources are gradually favored by countries around the world, and the development model centered on geothermal dew point cannot meet the increasing geothermal demand. In this paper, a GIS model combining PCA and AHP is proposed, aiming to select the advantages of geothermal resources at the regional scale and analyze the main influencing indicators. Through the combination of the two methods, both data and empirical can be considered, then the geothermal advantage distribution on the area can be displayed through GIS software images. A multi-index evaluation system is established to qualitatively and quantitatively evaluate the mid-high temperature geothermal resources in Jiangxi Province, and carry out the evaluation of the dominant target areas and the analysis of geothermal impact indicators. The results show that it is divided into 7 geothermal resource potential areas and 38 geothermal advantage targets, and the determination of deep fault is the most critical index of geothermal distribution. This method is suitable for large-scale geothermal research, multi-index and multi-data model analysis and precise positioning of high-quality geothermal resource targets, which can meet the needs of geothermal research at the regional scale.

A discrete spectral contributions based statistical narrow-band model for high-temperature combustion systems

In this work, we present a discrete spectral contribution based statistical narrow-band model (DSC-SNB) to solve the radiative transfer of high-temperature combustion systems, which employs a discrete spectral strategy to considerably improve the accuracy of the traditional SNB model. Based on the contribution of different gases to radiation, the DSC-SNB model divides the spectral lines into different groups within the spectral interval based on the low-level transition energies (E“). Treating the Line-by-Line (LBL) approach as the benchmark solution, several inhomogeneous high-temperature H2O-CO2-CO mixture columns are adopted to evaluate the DSC-SNB model. Results show that the DSC-SNB model performance is comparable with the traditional SNB model for isothermal gas mixture columns. The DSC-SNB model exhibits high accuracy under mid-high temperature or pure high temperature without low-temperature wrapping, outperforming the traditional SNB model. These findings could be used to reconstruct the multi-dimensional flame temperature field by building a highly accurate and efficient mathematical model for flame radiation prediction.

Significantly improved thermoelectric properties of Nb-doped ZrNiSn half-Heusler compounds

ZrNiSn-based n-type half-Heusler (HH) compounds are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric properties because they have a high power factor (PF). However, the thermal conductivity of HH compounds remains high, especially for the lattice thermal conductivity and bipolar thermal conductivity, which limits their application. In this work, we have enhanced the thermoelectric and mechanical properties of ZrNiSn-based compounds by Nb doping, which is realized by replacing Zr sites with Nb. As a consequence, the PF is enhanced from 3.25 mW mK−2 for pristine ZrNiSn to 4.78 mW mK−2 for Zr0.96Nb0.04NiSn, while the lattice thermal conductivity respectively declines from 4.5 W m-1K−1 to 2.58 W m-1K-1at 1123 K. Combined experimental results and ab-initio calculations revealed the increase in PF is because of that Nb-doping can significantly improve the electrical conductivity through a lot of carrier injection. The reduction of lattice thermal conductivity is primarily due to that Nb doping can generate quantum dots in the matrix causing the phonon scattering for the Nb-rich quantum dots and cause the mass and stress field fluctuations, and Nb-doping can efficiently weaken the bipolar thermal effect further reducing the thermal conductivity. A maximum ZT value of 0.89 within single-doped ZrNiSn compounds is achieved at 1123 K for Zr0.96Nb0.04NiSn, which is 53% higher than the pristine ZrNiSn. Furthermore, Nb doping effectively improves the microhardness, shear modulus, and Young's modulus of the ZrNiSn HH samples. The synthesized Hf-free HH compounds establish Nb as an effective dopant for attaining a high ZT and implying the excellent potential of these materials for industrial applications.

Enhancement in thermoelectric properties of ZrNiSn-based alloys by Ta doping and Hf substitution

Though half-Heusler alloys are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric and mechanical properties, their thermal conductivities remain high and limit their practical applications. In this work, the thermal conductivities and electrical conductivities of ZrNiSn-based alloys were synergistically optimized by Ta doping and Hf substitution. The samples were fabricated using levitation melting combined with spark plasma sintering. When compared with the pristine ZrNiSn, the electrical conductivity in Zr0.68Hf0.3Ta0.02NiSn increases from 7.3 × 104 to 12.9 × 104 S·m−1, and the lattice thermal conductivity declines from 4.1 to 2.7 W·m−1·K−1 at 923 K. As a result, an enhanced ZT of 0.94 for Zr0.68Hf0.3Ta0.02NiSn is achieved, which is a nearly 56% enhancement over pristine ZrNiSn. Furthermore, the microhardness, Young's modulus, and shear modulus in Zr1-x-yHfxTayNiSn (x = 0, 0.2, 0.3, 0.4; y = 0, 0.02) alloys are substantially improved upon increasing the Ta and/or Hf content, far exceeding those in other conventional thermoelectric materials.

Preparation and infrared properties of Ni3Al–Cr3C2 composite films deposited by electrohydrodynamic atomization technology

The purpose of this work is to provide a clean and efficient preparation process for Ni3Al–Cr3C2 composite films. In this work, two kinds of slurries were prepared to deposit Ni3Al–Cr3C2 composite films on a 316L stainless steel substrate by electrohydrodynamic atomization (EHDA) technology. The slurry used for deposition was prepared with magnetic stirring and ball milling, respectively. The influence of the atomization parameters (atomization distance, operating voltage, and flow rates) on the characteristics of deposited films was investigated. Homogenous and compact films, with a structure of ∼10 layers, were fabricated under the optimized deposition parameters. The composition and micro-morphology of the composite films were examined. The spectral reflectivity of the Ni3Al–Cr3C2 composite films was characterized by the Fourier transforms infrared spectroscopy (FTIR) to study the infrared properties of the composite films. The results showed that because the slurries used to deposit the composite films were prepared by various methods, the infrared reflectivity and emissivity of the two composite films were different in the middle and far-infrared region. However, in the whole middle and far-infrared spectrum, the emissivity of the two films was significantly low. Therefore, the Ni3Al–Cr3C2 composite films deposited by EHDA are expected to be used in mid-high temperature solar films.

Reevaluating a non-conventional procedure to microencapsulate beneficial lactobacilli: assessments on yield and bacterial viability under simulated technological and physiological conditions.

Maintaining viability of beneficial microorganisms applied to foods still constitutes an industrial challenge. Many microencapsulation methodologies have been studied to protect probiotic microorganisms and ensure their resistance from manufacturing through to consumption. However, in many Latin-American countries such as Argentina there are still no marketed food products containing microencapsulated beneficial bacteria. The objectives of this work were: (i) to obtain microcapsules containing Lactobacillus fermentum L23 and L. rhamnosus L60 in a milk protein matrix; and (ii) to evaluate the viability of microencapsulated lactobacilli exposed to long-term refrigerated storage, mid-high temperatures and simulated gastrointestinal conditions. The method of emulsification/rennet-catalyzed gelation of milk proteins used in this study led to high encapsulation yields for both strains (98.2-99%). Microencapsulated lactobacilli remained viable for 120 days at 4°C, while free lactobacilli gradually lost their viability under the same conditions. Microencapsulation increased the resistance of lactobacilli to mid-high temperatures, since they showed survival rates of 95-99.3% at 50 °C, and of 72.5-74.4% at 65 °C. Under simulated gastric conditions, the microencapsulated lactobacilli counts were higher than 8.5 log CFU mL-1 and showed survival rates between 96.61% and 97.74%. Furthermore, in the presence of bile (0.5-2% w/v) the survival of microencapsulated strains was higher than 96%. The microencapsulation process together with the matrix of milk proteins used in this study protected beneficial Lactobacillus strains against these first simulated technological and physiological conditions. These findings suggest that this microencapsulation method could contribute to secure optimal amounts of living lactobacilli cells able to reach the intestine. © 2021 Society of Chemical Industry.

Tribological behaviors and microstructure evolution of Inconel 718 superalloy at mid-high temperature

Revealing the microstructure evolution of materials during the sliding process is crucial to their long-term application. In this study, the friction and wear behaviors of Inconel 718 under different conditions were systematically investigated. The wear surface and detailed microstructure evolution of Inconel 718 after sliding were also characterized by three-dimensional (3D) profilometry, scanning electron microscopy (SEM-EDS) and electron backscattered diffraction (EBSD). The results indicated that the applied load and test duration significantly influenced the tribological performance and microstructure evolution of Inconel 718. Under an applied load of 3 N, the main wear mechanisms of Inconel 718 gradually changed from single adhesive wear to mixed adhesive and abrasive wear as the test duration increased. However, the wear mechanisms of Inconel 718 were continuously dominated by adhesive wear and oxidative wear under a load of 5 N. Furthermore, clear lattice distortion occurred on the rubbing area, as observed by the EBSD characterization, resulting in the nucleation and propagation of microcracks during the rubbing process. In this case, the material was peeled off from Inconel 718 under the action of friction when the microcracks expanded and interconnected. These results have significance in guiding the application of Ni-based superalloys in aeroengines.

Constrains of eruption environment and hydrothermal fluid on the Permian pyroclastic reservoirs in the Sichuan Basin, SW China

A breakthrough has been recently made in the hydrocarbon exploration of the Permian pyroclastic rocks in the Jianyang Area, western Sichuan Basin, China. With an aim to decipher the impacts of the eruption environment, the temperature of hydrothermal fluids, and the paleo-salinity on the formation of authigenic mineral assemblage and pores, this study implements comprehensive petrographic and geochemical studies through the integrated core and thin section analyses. The data presented demonstrate that the Permian volcanic intervals are intensively affected by an event of Emei taphrogeny. During basaltic magma upward migration, fractional crystallization of anorthose results in slightly-alkaline magma. The specific pyroclastic rocks are formed by the eruption of slightly-alkaline magma in the sea or a salt lake and subsequent hydrothermal alteration. During deposition and diagenesis, the authigenic mineral association is constrained jointly by the sodium-rich and high salinity water environment, and mid-high temperature, high-salinity hydrothermal fluid. Specifically, the sodium-rich hydrothermal fluid, which may sustain till the late diagenesis stage, caused pervasive albitization of pyroclastic rocks, then leading to mineral transformation and formation of a series of mineral associations. Therefore, zeolitization of volcanic glass and vesicle-infillings of zeolite is an essential condition for later mineral transformation and dissolution. Albitization of analcite, recrystallization induced by deep hydrothermal fluids, and both meteoric and deep burial dissolution expanding the micro-pore space ultimately formed porous pyroclastic reservoirs.

Current Research Status of Skutterudite-based Thermoelectric Materials and Modularization Techniques for Mid-high Temperature Applications

Current Research Status of Skutterudite-based Thermoelectric Materials and Modularization Techniques for Mid-high Temperature Applications

Current Status of Nanostructured Thermoelectric Materials for Mid-High Temperature Applications

Current Status of Nanostructured Thermoelectric Materials for Mid-High Temperature Applications

Study on the denitrification performance of FexLayOz/activated coke for NH3-SCR and the effect of CO escaped from activated coke at mid-high temperature on catalytic activity.

Currently, activated coke is widely used in the removal of multiple pollutants from industrial flue gas. In this paper, a series of novel FexLayOz/AC catalysts was prepared by the incipient wetness impregnation for NH3-SCR denitrification reaction. The introduction of Fe-La bimetal oxides significantly improved the denitrification performance of activated coke at mid-high temperature, and 4% Fe0.3La0.7O1.5/AC exhibited a superior NOx conversion efficiency of 90.1% at 400°C. The catalysts were further characterized by BET, SEM, XRD, Raman, EPR, XPS, FTIR, NH3-TPD, H2-TPR, et al., whose results showed that the perovskite-type oxide of LaFeO3 and oxygen vacancies were produced on the catalysts' surfaces during roasting. Fe-La doping enhanced the amount of acid sites (mainly Lewis and other stronger acid sites) and the content of multifarious oxygen species, which were beneficial for NOx removal at mid-high temperature. Moreover, it was investigated that the effect of released CO from activated coke at mid-high temperature on the NOx removal through the lifetime test, in which it was found that a large amount of CO produced by pyrolysis of activated coke could promote the NOx removal, and long-term escaping of CO on the activated coke carrier did not have a significant negative impact on catalytic performance. The results of the TG-IR test showed that volatile matter is released from the activated coke while TG results showed that the weight loss rate of 4% Fe0.3La0.7O1.5/AC only was 0.0015~0.007%/min at 300-400°C. Hence, 4% Fe0.3La0.7O1.5/AC had excellent thermal stability and denitrification performance to be continuously used at mid-high temperature. Finally, the mechanisms were proposed on the basis of experiments and characterization results.

The steady thermodynamics of a flash-binary geothermal power system based on correction models validated by static tests

The operation parameters and performance of a flash-binary power system are analyzed based on four scenarios of the experiment, optimum design, error correction Model 1 and error correction Model 2. The operation parameters of a flash-binary power system are tested to explain steady in the experiment with heat source temperature ranges from 100-150?C. The results show that the simulation data of error correction Model 2 have a good agreement with the experiment and most parameter values of relative error, ?, are less than 5%. The expander with low isotropic efficiency results in the largest exergy losses in the flash-binary power system. The flash-binary power system has better performance to adapt to the mid-high temperature geothermal resource in China. Some valuable data obtained from the research can be applied in the future power plant construction in China?s southern Tibet, western Yunnan and Sichuan.

Three dimensional ordered macroporous zinc ferrite composited silica sorbents with promotional desulfurization and regeneration activity at mid-high temperature

A series of 3DOM zinc ferrites and silica composites were prepared and applied on the hydrogen sulfide (H2S) removal at 500 °C. With ∼230 nm-sized macropores and 15 nm-sized walls, the morphology analysis showed the as-prepared ZnFe2O4-SiO2 materials were conducted as a typical 3DOM structure. As a solifidation agent, silica significantly influenced not only the composites structure, which induced the homogeneity and stability of 3DOM structure, but also the desulfurization behavior, in which the active phase utilization decreased if the compounded silica amount was too high or too low. Anyway, the 3DOM sorbents exhibited remarkable desulfurization performance due to the unique structure features with high surface area, nano zinc ferrite particles, well arranged macropores with interconnected mesopores, which significantly enhances gaseous reactant diffusion during sulfidation compared with those of bulk sorbent. Sulfates could be formed during both sulfidation and regeneration process, while Fe2(SO4)3 in sulfided sample and ZnSO4 in regenerated sample. 3DOM structure and silica participation enhanced the breakthrough capacity of 50 wt% ZnFe2O4 compounded sorbent and maintained higher desulfurization capacities upon multiple sulfidation/regeneration cycles more ascribed to its special structural feature.

Design and performance investigation of a biogas fueled combined cooling and power generation system

Nowadays, different renewable sources and energy storages are attracting world attention due to the major concerns about the excessive emissions and the global energy crisis. Hence, this paper studies design and performance investigation of a biogas fueled building cooling and power system, which comprises of an air compressor, an air preheater, a combustion chamber, a gas turbine, some compression chillers and aggregated plug-in electric vehicles. In the proposed coproduction facility, biogas is used as an alternative fuel to reduce the total greenhouse gas emissions and increase the economic saving in the consumption of fossil fuels. The feasibility and robust performance of this microgrid is evaluated under extremely-hot summer days. Three compression chillers are installed to satisfy total cooling demand of a large hotel building located in Ahwaz, Iran. Moreover, the total energy requirement of chillers and building electricity demand is supplied through the biogas driven gas turbine cycle. A mid-high temperature waste heat is recovered from the exhaust gases of gas turbine to be used in the air preheating stage. A comprehensive thermodynamic analysis is developed to solve a mixed integer non-linear programming problem with the aim of minimizing total electricity procurement cost of the proposed zero-emission cogeneration micro-plant taking into account the operational constraints of gas turbine cycle, compression chillers, and electric drive vehicles.

Design and performance investigation of a novel absorption ice-making system using waste heat recovery from flue gases of air to air heat pump

Nowadays, different renewable sources and energy storages are attracting world attention due to the major concerns about the excessive emissions and global energy crisis. Hence, this paper studies design and optimal short-term evaluating of a green absorption ice-making system, which comprises of an air to air heat pump (AAHP) and a Stirling engine. Energy-exergy analysis is used for model investigation. In the proposed system, the AAHP is equipped with an inside evaporator fan, an outside condenser fan, an expansion valve, compressor, condenser and an evaporator to supply total cooling demand of large residential buildings during the extremely hot summer days or transient heat waves. In the presented model, the delicate ice crystals can be formed through the AAHP’s refrigeration cycle by blowing the chilled inside air at the lowest temperature of −8.6 °C to a water tank. Expected energy requirement of compressor, evaporator and condenser fans can be supplied either by Stirling engine or by local electricity grid. Meanwhile, a mid-high temperature waste heat is recovered from the exhaust air of the condenser fan in order to drive the Stirling engine for the next hour. A comprehensive analysis is developed as a non-linear programming (NLP) problem to minimize total energy procurement cost over a 24-h time interval taking into account the operational constraints of AAHP and Stirling engine. Simulation results reveal the validity and effectiveness of the proposed ice-harvesting system with zero-CO2 emissions and near-zero energy consumption.

Experimental Performance of a Micro-ORC Energy System for Low Grade Heat Recovery

The state-of-the art of ORC energy systems is mainly dominated by large scale units in the MW range of power output, in the field of heat recovery at mid-high temperature levels (around 200-500°C), where multiple commercial realizations are available. Nevertheless, the cutting-edge niche of micro-ORC energy systems offers good solutions for low-temperature heat recovery. Many prototypes are currently under investigations, but a leading technology is not yet established. This work reports an experimental activity carried out for performance characterization of a prototypal micro-ORC energy system. In particular, the paper presents the test bench developed in the laboratories of the University of Bologna and the first obtained results in terms of thermodynamic performance and main components characteristics. The ORC system comprises a small reciprocating three-piston expander, run on R134a as operating fluid. Heat is provided to the ORC from an external source, via hot water at temperature below 100 °C, in order to simulate a low-enthalpy heat recovery process. The system rejects unused heat via a water cooled condenser. Thus, the investigated ORC is a plug and play system, requiring only to be connected to the hot and cold heat sources. The ORC system has been tested for prolonged operation at various thermal input conditions. In particular, the behavior of the key cycle parameters and performance indexes (e.g. max. and min. pressures, superheating temperature, expander isentropic efficiency, electric power output, etc.) are investigated as function of pump rotational speed (i.e. organic fluid mass flow rate), for three different set point values of the hot source (65 °C, 75 °C, 85 °C). The operating thermodynamic cycle has been completely characterized by means of a real-time measurement and acquisition tool, developed in LabVIEW environment. Performance variations of the system have been monitored: the electric power output ranges between 0.30 to 1.2 kW, with gross efficiency in the range 2.9-4.4 %, while the expander “electro-isentropic” efficiency results in the range of 35-42 %.