Direct Heating Research Articles

Operational strategies and well design for reservoir thermal energy storage (RTES)

The intermittency of renewable energy sources necessitates effective energy storage solutions. This study narrows in on reservoir thermal energy storage (RTES) as a system to bridge the supply–demand gap through the storage and recovery of heated water for periods ranging from daily to monthly timescales. By injecting hot water into subsurface formations and later retrieving it for use in electricity generation or direct heating, the viability of RTES is explored for load-levelling applications, typically on daily to monthly timescales rather than extended seasonal or multi-year storage. Many factors including formation parameters (e.g. permeability, porosity, thermal conductivity of rock, thickness and insulating layers), completion parameters such as injection well design, operational parameters such as injection and production rates, and schedule affect the facility's performance. The two key performance indicators (KPI) considered herein are the produced water temperature and the heat recovery factor. In this study, three operational strategies and four different well completions were investigated using a coupled fluid–thermal simulation model. Heat loss from the upper and lower boundaries of the reservoir was also studied. While the immediate impact of initial hydro-thermal charging on produced water temperature may not persist in the long-term operation, it does influence the overall system efficiency by reducing the cumulative heat recovery. Through detailed analysis, it is demonstrated that vertical injection across the entire well length offers a balanced approach between minimizing pumping costs and maximizing heat extraction efficiency. This study establishes basic calculations for developing innovative operational and completion strategies to maximize the long-term economic benefit of RTES.

Impact of forest fire severity on soil physical and chemical properties in pine and scrub forests in high Andean zones of Peru

Forest fires are the main threat to ecosystems and human life. The frequency, seasonality, extent and severity of fires affect ecosystems and the physical and chemical properties of the soil by direct (heating) and indirect (ash) effects. This could affect biodiversity and forest residency in the face of climate change. In this study, we evaluated the impact of fire severity on soil physical and chemical properties caused by forest fires in a high Andean area of Peru. For this purpose, the severity levels, the degree of hydrophobicity, and the physical and chemical properties of the soil were analyzed by sampling in affected areas (Pinus radiata D. Don plantation and shrubland) and unaffected areas. The results showed a very low severity in soil components, with a strong hydrophobicity, more persistent in the forest plantation area than in the shrubland. The physical properties of the soil did not show variations; however, in the Pinus plantations they showed variations in their chemical properties such as pH, electrical conductivity, organic matter, nitrogen and cation exchange capacity compared to areas not affected by the forest fires. Likewise, in the study area an adequate regeneration process was evidenced; in fact, it is important to apply mechanisms to accelerate the restoration of the vegetation cover and the physical and chemical quality of the vegetation.

Removal performance of Nitrogen, sulfur and chlorine pollutants during chemical looping combustion of textile dyeing sludge using red mud as an oxygen carrier

Textile dyeing sludge (TDS) is a solid waste produced by the textile industry, which contains a large number of N, S and Cl components. Direct heat treatment of TDS will cause a large number of N, S, Cl pollutants to be released. This study verified the possibility of chemical looping combustion (CLC) of TDS using alkaline WS900 red mud (RM) oxygen carrier, which is rich in Na/Ca, to remove N, S and Cl pollutants at the same time. Raising the reaction temperature in the range of 750–900 °C, prolonging the residence time and increasing the oxygen excess coefficient can promote the oxidation performance of WS900 to NOx precursor, while CO2 atmosphere inhibits the oxidation ability. After 10 cycles of CLC, the Cl fixation rate of WS900 is still close to 100% and its S fixation rate is more than 97%. It can maintain more than 90% of carbon conversion and 86% of CO2 selectivity. Compared with the pyrolysis condition, the removal rate of N pollutants is about 67%. Alkaline RM is also neutralized after absorbing acid gases during CLC, which is convenient for its further treatment and utilization.

A new reactor with porous baffle for thermochemical heat storage: Design and performance analysis

The Ca(OH)2/CaO reaction system presents broad development prospects for thermochemical heat storage. However, the poor thermal conductivity of heat storage material and the complex internal flow limit the performance of the reactor. To improve the performance of the reactor, this work designs a new improved fixed bed reactor with baffle. The comparison with and without baffle, as well as the effects of operation conditions and structural parameters of the improved reactor on heat storage and release performance, are numerically investigated. The results show that the improved reactor can enhance heat transfer and flow inside the reactor, resulting in performance improvement. After baffle installation, there is a 64% decrease in the pressure drop, a 29.14% reduction in the heat storage time, and a 35.45% reduction in the heat release time. Besides that, the improved reactor can maintain the high temperature for longer at the outlet during heat release. The orthogonal test design analysis reveals that the inlet velocity of direct heat transfer fluid has the greatest impact on heat storage, while the inlet velocity of indirect heat transfer fluid has the smallest impact, and the same applies to heat release. The thermal conductivity and permeability of the porous baffle have minor impacts, but the air distribution chamber has a noticeable influence. This work can guide the optimization design and operation adjustment of the reactor.

Rapid 24 GHz microwave sintering of alumina – yttria-stabilized zirconia ceramic composites

Samples of alumina – 3 % yttria-stabilized zirconia (YSZ) composites were sintered in rapid processing regimes using 24 GHz microwave heating at rates of up to 200 °C/min and zero hold time. The final relative density was 96–99 % for the samples containing 1.5 and 7.5 wt % YSZ and 98–99 % for the samples containing 13 wt % YSZ. The microwave sintering kinetics were compared for the processes carried out by direct and susceptor-assisted microwave heating. Under direct microwave heating, the effect of an intense microwave electromagnetic field with an estimated absorbed power density of up to 130 W/cm3 resulted in a shift of the shrinkage curves by about 100 °C towards lower temperatures compared to the case of susceptor-assisted heating. The grain size of the samples sintered by direct microwave heating decreased with an increasing heating rate. The mechanical properties were slightly higher for the materials sintered under susceptor-assisted microwave heating. The samples containing 13 wt % YSZ exhibited a microhardness of about 20 GPa and a fracture toughness of about 7 MPa m1/2.

Sustainable conversion of agricultural waste into solid fuel (Charcoal) via gasification and pyrolysis treatment

Managing agricultural waste by burning it in the fields is a straightforward method, but leads to significant pollution. One promising alternative is to convert agricultural waste into solid fuel, such as charcoal, to support renewable energy from biomass. The quality of barbecue charcoal depends upon selecting suitable materials and employing heating methods to ensure efficient transformation. This research aims to study the charcoal conversion process from agricultural waste using two types of kilns: 1) direct heating (gasification kiln: GK) and 2) indirect heating (pyrolysis kiln: PK) designed to recirculate syngas from wood as fuel for the pyrolysis process. The study tested three types of agricultural waste materials, including coconut shells (CS), cassava rhizome (CR), and acacia wood (AW), to examine the differences in charcoal produced by the two heating methods. The tests revealed that the maximum temperatures inside the kilns were 792.45 ± 127.18 °C, 907.67 ± 37.3 °C, and 980.07 ± 110.56 °C for the GK, and 921.88 ± 57.84 °C, 801.93 ± 10.16 °C, and 937.82 ± 95.85 °C for the PK. The charcoal from the PK exhibited higher calorific values than the GK, with 7474.68 ± 36.62, 6429.04 ± 72.22, and 7268.33 ± 52.86 calories per gram. The charcoal yield was also higher in the PK, at 31.29 ± 4.39, 34.33 ± 3.39, and 17.58 ± 2.09 percent for coconut shells charcoal (CSC), cassava rhizome charcoal (CRC), and acacia wood charcoal (AWC), respectively. However, the PK required more fuel and longer ignition times. The resulting charcoal from the slow pyrolysis process in the PK is suitable as barbecue fuel due to its size, which is similar to the original material. In contrast, the charcoal from the GK, which tends to shrink or break into smaller pieces, is more suitable for grinding into briquettes. This study provides a guideline for producing high-quality barbecue charcoal, offering commercial benefits including the gasification and pyrolysis processes that improve combustion efficiency and reduce pollution by producing high-quality gas for fuel, unlike traditional kilns that emit a large amount of CO during the conversion of wood to charcoal and enabling the selection of appropriate raw materials for different heating methods to maximise the utility of the products. This approach adds value to agricultural raw materials and helps effectively manage agricultural waste (zero waste) for further utilisation and development.

Analytical solution to the two-dimensional inverse heat conduction problem in an axisymmetric cylindrical coordinate system

The inverse heat conduction problem involves estimating an unknown cooling or heating action based on internal temperature histories in a body. Such a problem is ill-posed because the estimated results are highly sensitive to input data noise. An analytical solution is developed for the two-dimensional inverse heat conduction problem in an axisymmetric cylindrical coordinate system using the Laplace transform technique. On the basis of the known temperatures at multiple measuring points distributed on two cylindrical measuring surfaces along the axial direction inside the solid, the expressions for the surface temperature and heat flux are explicitly obtained in the form of a power series of time and eigenfunctions of the space variable. A comprehensive estimation error assessment is conducted using the internal temperature distribution that is obtained using a direct heat conduction analysis under the prescribed stationary and moving boundary conditions. The constant thermal condition is well reproduced using the present inverse solution, except in the vicinity of the discontinuity point. In addition, the effective heat flux and effective heat transfer coefficient are introduced to obtain the best estimation of the transient heat transfer in the major cooling area of the rotating cylinder.

Effect of precursor concentration on the growth of magnesium oxide nanomaterials by direct heating method

Various methods have been developed for the preparation of MgO nanomaterials. However, they often suffer from limitations such as high cost, lengthy synthesis times, and excessive power consumption. In response, a direct heating (DH) method has been developed to address these challenges, enabling rapid (< 10 min) and cost-effective production of MgO nanomaterials. In this study, the DH process was optimized by investigating the influence of precursor solution concentrations (0.01 M to 0.10 M) on the growth of MgO nanomaterials on wire surfaces. Analyses utilizing Fourier Transform Infrared Spectroscopy (FTIR), Energy Dispersive X-ray Spectroscopy (EDX), and X-ray Photoelectron Spectroscopy (XPS) confirmed the formation of MgO. Field Emission Scanning Electron Microscopy (FESEM) revealed that the nanomaterials predominantly nano-walls and particles. The optimized synthesis conditions for MgO nanomaterials requires the heating of 0.10 M Mg(NO3)2·6H2O precursor at 50 W.h. for 10 min. Under this specific condition, a favourable surface coverage of 81.54% was achieved where the MgO nanowalls produced has demonstrated promising capability to remove Methylene Blue (MB) dye under UV irradiation. In conclusion, DH technique is a favourable alternative route to synthesize MgO nanomaterial in a simple, cost-and -time efficient and environmentally friendly way.

100% renewable heat supply in Berlin by 2050 – A model-based approach

The energy crisis posed major challenges, especially for the heating sector. The European gas price increased from 16€/MWh in March 2021 to 227€/MWh in March 2022, which led to a debate in Germany around reshaping the heat supply to decrease energy import dependence. However, many actors on the heating market are reluctant to make the necessary investments to reduce dependence on natural gas. Although, independence of fossil energy imports by 2050 would be possible with a switch to a 100% renewable heat supply for Berlin.Based on the linear open-source energy system model GENeSYS-MOD, a 100% renewable-based energy system is modeled. GENeSYS-MOD is extended to model district heating, which is crucial for Berlin's heat supply. Furthermore, the modeling of heat pumps and a data set for Berlin is revised.The modeling considers the tightened climate protection targets, which means a 95% CO2 reduction by 2045 compared to 1990. By 2050, 100% of the transport, industry, electricity, and building sectors` emissions are mitigated.The results suggest the immediate exploitation of the existing renewable energy potential and seasonal heat storages, as well as a necessity for a major increase in renovation rates. The upcoming coal phase-out in Berlin in 2030 should be met by a mix of heat pumps and biomass. If heat pump resources are exhausted, direct electric heating is deployed as last resort option.

Mechanism and structure of micro-nanofluid directional heat conduction in asphalt pavement in plateau permafrost regions

Solar radiation in plateau permafrost regions is strong. The asphalt pavement strongly absorbs and slowly dissipates heat, leading to significant heat accumulation on the pavement. This accumulation disturbs the underlying permafrost and eventually causes serious pavement damage. To improve the heat resistance and dissipation capabilities of asphalt pavement, a nanofluid directional heat conduction structure (N-DHCS) was suggested and analyzed in this paper. The designed structure can resist heat in the daytime due to the low thermal conductivity of liquid and dissipates heat at night through natural convection. The finite element method and laboratory irradiation experiment were employed to performed thermal analyses of N-DHCS. The results demonstrated that establishing the N-DHCS in asphalt pavements can enhance active heat dissipation capacity, which is beneficial for protecting the frozen soil in plateau permafrost regions.

(Invited) Battery Safety and Self-Quenching Battery Pack Concept: D3 (Dissipate, Direct, and Douse Heat and Fire)

Battery pack safety following an individual cell runaway is an important aspect to address as we pack more energy into a small volume and mass. Thermal runaway incidents are extremely rare but can be catastrophic. The runaway can be triggered by local overcharge, thermal or mechanical abuse.In this talk, we will first go into the details of thermal runaway initiation and propagation. Second, solution to quench the thermal runaway from propagating within the cell through a novel and passive detection of incipient thermal runaway temperature via thermoplastics will be introduced; and that is followed by discussion on the coolant release trigger to quench the runaway. In addition, this solution prevents propagation from cell-to-cell and reduce the chance for secondary combustion. This solution also leverages the fast thermal conductivity that is offered by the Cu and Al current collectors within the cell. The role of current collectors in relaying the information from the source of thermal runaway to the surface of the new format cell will be discussed.The solution proposed here is chemistry agnostic and can be adapted to applications such as in transportation of cells and packs, grid energy storage, and mobility. The talk concludes by describing the next steps and value chain partnerships to make this safer solution a reality.Below is an image that shows the self-quenching battery concept. Figure 1

Highly Anisotropic Thermally Conductive Dielectric Polymer/Boron Nitride Nanotube Composites for Directional Heat Dissipation.

An ideal dielectric material for microelectronic devices requires a combination of high anisotropic thermal conductivity and low dielectric constant (ɛ') and loss (tan δ). Polymer composites of boron nitride nanotubes (BNNTs), which offer excellent thermal and dielectric properties, show promise for developing these dielectric polymer composites. Herein, a simple method for fabricating polymer/BNNT composites with high directional thermal conductivity and excellent dielectric properties is presented. The nanocomposites with directionally aligned BNNTs are fabricated through melt-compounding and in situ fibrillation, followed by sintering the fibrous nanocomposites. The fabricated nanocomposites show a significant enhancement in thermal properties, with an in-plane thermal conductivity (K‖) of 1.8 Wm-1K-1-a 450% increase-yielding a high anisotropy ratio (K‖/K⊥) of 36, a 1700% improvement over isotropic samples containing only 7.2 vol% BNNT. These samples exhibit a 120% faster in-plane heat dissipation compared to the through-plane within 2 s. Additionally, they display low ɛ'of ≈3.2 and extremely low tan δ of ≈0.014 at 1kHz. These results indicate that this method provides a new avenue for designing and creating polymer composites with enhanced directional heat dissipation properties along with high K‖, suitable for thermal management applications in electronic packaging, thermal interface materials, and passive cooling systems.

Platinum-Ruthenium Bimetallic Nanoparticle Catalysts Synthesized Via Direct Joule Heating for Methanol Fuel Cells.

Platinum-Ruthenium (PtRu) bimetallic nanoparticles are promising catalysts for methanol oxidation reaction (MOR) required by direct methanol fuel cells. However, existing catalyst synthesis methods have difficulty controlling their composition and structures. Here, a direct Joule heating method to yield highly active and stable PtRu catalysts for MOR is shown. The optimized Joule heating condition at 1000°C over 50 microseconds produces uniform PtRu nanoparticles (6.32wt.% Pt and 2.97 wt% Ru) with an average size of 2.0 ± 0.5 nanometers supported on carbon black substrates. They have a large electrochemically active surface area (ECSA) of 239 m2 g-1 and a high ECSA normalized specific activity of 0.295mA cm-2. They demonstrate a peak mass activity of 705.9mA mgPt -1 for MOR, 2.8 times that of commercial 20 wt.% platinum/carbon catalysts, and much superior to PtRu catalysts obtained by standard hydrothermal synthesis. Theoretical calculation results indicate that the superior catalytic activity can be attributed to modified Pt sites in PtRu nanoparticles, enabling strong methanol adsorption and weak carbon monoxide binding. Further, the PtRu catalyst demonstrates excellent stability in two-electrode methanol fuel cell tests with 85.3% current density retention and minimum Pt surface oxidation after 24 h.

6E analysis and particle swarm optimization of a novel ultra-high temperature solar cogeneration system fusing thermochemical energy storage and multistage direct heat transfer

The reshaping of the energy development model depends on the penetration of renewable energy on the input side and the substitution of electrical energy on the supply side. Therefore, this research proposes a novel solar cogeneration system in which the Brayton cycle possesses triple pressure and can be flexibly coupled directly to each subsystem under supercriticality without intermediate exchangers. Firstly, a series of research tasks are executed sequentially, including model establishment, cycle potential comparison, operating parameter analysis, and multi-objective particle swarm optimization. Secondly, the exergy flow rate, consumption rate, and 6E (energy, exergy, exergoeconomic, exergoenvironmental, emergoeconomic, and emergoenvironmental) performances of each subsystem and subcomponent are thoroughly discussed. Lastly, the operating performance, economic benefits, and regional characteristics of each subsystem and the cogeneration system are explored. After optimization, the exergy efficiency, average advanced exergy-emergy factor, and advanced exergy-emergy impact difference of the system are 22.72 %, 56.17 %, and 23.97 %. Compared with similar research, the energy efficiency of the system is improved by 8.2 %, which can repay the capital cost in 8.83 years, and the economic benefit reaches $5,503,086 for the entire service year. The system embodies excellent 6E performance, aligns with the goals of reshaping the energy development model, and provides guidelines for the development of ultra-high temperature solar cogeneration.

Optimization of iron nanoparticle sizethrough green synthesis using leaf extract from selective members of citrus family

Nanoparticles (NPs) are known for their unique attributes due to their small size that lies in atomic domain. The unique properties of NPs have prompted researchers to explore ways to make these materials with controlled size and morphology. The green synthesis of NPs is the most promising way of preparing NPs due to its convenience, environmentally friendly nature and cost effectiveness. This research work was mainly concerned with the green synthesis of iron nanoparticles (INPs) using three members of the citrus family (i.e., grapefruit, lemon, orange) under direct heating and ultrasound mediated conditions. The prepared NPs were investigated by different characterization techniques including Particle Size Analyzer (PSA), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The results revealed and demonstrated that the NPs thus formed differed not only in their sizes but also in the number of phases in PSA. The NPs prepared from extract of grapefruit leaves showed uniformity when lower concentration of extract was used; however, when higher volume were used formation of two phases was observed. In the case of ultrasound mediated synthesis, the formation of two phases was observed even at lower volume of grapefruit leave extract. In the case of nanoparticles (NPs) prepared using lemon leaf extract, both heating and ultrasound-mediated conditions resulted in the formation of multiple phases. Conversely, for NPs prepared with orange leaf extract, ultrasound-mediated conditions consistently led to the formation of a single phase, regardless of the volume of plant extract used. However, the size of these phases varied, showing an irregular trend with the volume of the plant extract. The FTIR spectra, DLS results, and SEM images reflected quite drastic differences in NPs before and after calcination. All synthesized NPs exhibited the same surface plasmon resonance of 700 nm.

Recent advancements in solar collector-evaporator for direct expansion solar heat pump

The direct expansion solar heat pump (DX-SHP) is a competent and sustainable option for domestic heating and drying applications. The solar collector-evaporator is a vital component of DX-SHP, which operates on solar energy and low-temperature ambient energy. The heat collection capacity, coefficient of performance, and evaporation temperature are used to quantify its performance. In this context, a systematic literature review on recent advancements in the collector-evaporator of DX-SHP is presented. This paper highlights and discusses the various configurations of solar collector-evaporators, theoretical and experimental investigations, and the selection method of eco-friendly refrigerants. The impact of different performance factors, such as ambient conditions, structural parameters, and types of refrigerants, on solar collector-evaporators is also reviewed. The optimal range for solar radiation, wind speed, and temperature is found to be 350 W/m2-700 W/m2, 0.5 m/s -2.5m/s, and 5°C to 35°C. The DX-SHP system produces hot water from 15°C to 60°C, with an average COP of 1.5 to 4.5. The finned solar collector-evaporator with R290 refrigerant performs optimally in various climatic conditions. Integrating solar collector-evaporators with innovative technologies such as microchannel, heat pipe, thermoelectric generators, photovoltaic thermal collectors with compound parabolic and Fresnel lenses can improve the system's performance. In this review, researchers can find scope for performance enhancement of the collector-evaporator used in DX-SHP.

Effect of induced electric field parameters on physiochemical characteristics and microorganism of orange juice

AbstractThe induced electric field (IEF) was produced via an oscillating magnetic field, for direct heating of orange juice within a winding coil. During the process, the applied magnetic field was at the range from 0.07 to 1.43 T with 50 kHz. Then, the numerical relationships between excitation voltage, magnetic field, IEF, induced current density, magnetomotive force and energy efficiency were investigated. Since a rectangular wave of excitation voltage was applied, the sawtooth waveform of induced voltage or IEF loaded on the juice was observed. As the magnetic field increased, outlet temperature and induced current density in the juice was improved, the maximum values were 99.5°C and 0.50 A/cm2, respectively, at a conductivity 4.53 mS/cm for the residence time of 4.5 min. However, the efficiency of the process exhibited a negative correlation with the escalation of excitation voltage. At initial colony count about 4.50 log colony‐forming units/mL, the IEF pasteurization reduced all the aerobic microorganisms, molds, and yeasts in the juice, which were evaluated at detectable limits below 1 log colony‐forming units/mL. Moreover, all treatments had no significant effects on pH, total soluble solids and titratable acidity of the juice (p &gt; 0.05). In particular, there was also no significant change in color parameters after IEF pasteurization (p &gt; 0.05). It indicated a significant decrease in vitamin C and total phenolic content of the control groups (p &gt; 0.05), but no significant change in carotenoid content was observed in all treated juice (p &gt; 0.05). heating technology using IEF has the potential to the pasteurization of fruit juice.Practical applicationsInduction electric field technology has progressed rapidly in the past 2 years, it can use oscillating alternating magnetic field to realize the direct heating of liquid food. The process has thermal and non‐thermal effects, at 65–70°C, within 15–30 s to achieve the commercial asepsis of acidic liquid food. After the pasteurization, it can better maintain the color and flavor of the juice. We have assembled a laboratory prototype for testing in the early stage, the processing capacity is at the range of 0–10 L/h. Currently, the system has been able to reach the pilot production scale with the processing capacity of 100–500 L/h. Through the use of highly permeable magnetic materials, the energy consumption per ton of juice processing is controlled at 120 k·Wh level for the pasteurization and is expected to achieve a larger processing capacity in the future.

Optimising design and operation of sewage source heat pump: techno-economic and environmental assessment

ABSTRACT Heat pump can be used to recover abundant thermal energy contained in the discharge of municipal wastewater treatment plants. While there are some design standards for common heat pump systems, the design of a sewage source heat pump (SSHP) system is still often based on a fixed heat load and neglects the interdependencies between the equipment sizing and operating parameters. To address the issue that previous design methods have not balanced investment and operational costs well from a global optimisation perspective, this work formulates the simultaneous optimisation of SSHP design and operation as a non-linear programming problem. The proposed model features the consideration of multiple working conditions caused by the impact of ambient temperature variation on the heat load of the SSHP system. The feasibility and potential benefits of the optimised SSHP system are also evaluated by incorporating techno-economic performances and environmental impact analyses into the mathematical framework. A case study is carried out to demonstrate the effectiveness of the proposed methodology. The results show that the total annual cost of the optimally designed and operated SSHP in Harbin could be 9% lower than in Beijing and 39% lower than in Shanghai, suggesting that constructing and running the SSHP system in severe cold regions with great heating demands might be more economical than in less cold regions. The CO2, SO2, and NOx emissions of the SSHP could be approximately 50% less than that of coal-fired boiler heating, and 80% less than that of direct electric heating with coal-fired electricity.

Insight into phase stability and thermoelectric properties of semiconducting iron silicides with manganese substitution: β-Fe1−xMnxSi2 (0≤x≤0.05)

Metal-doped iron silicides (β-FeSi2) are promising thermoelectric materials for high-temperature applications. However, the addition of metal dopants usually causes the formation of secondary metallic phases, resulting in the degradation of thermopower. Here we report the stability of semiconducting β-phase (higher than 95 %) of Mn-doped β-FeSi2 obtained at Mn concentration from 1 % to 5 %, where the samples were prepared through direct arc melting and heat treatment process. The carrier density remarkably increases with Mn content, but the mobility decreases. The electrical resistivity significantly decreases with Mn content. The Seebeck coefficient of Mn-doped samples is improved and stable at high-temperature regions because of the reduction of the bipolar effect and β-phase stability. The thermal conductivity slightly increases with Mn. As a result, the maximum thermoelectric power factor PF = 970 μWm−1K−2 and dimensionless figure of merit ZT = 0.12 at 800 K are obtained in a 3 % Mn-doped sample. The stability of β-phase and the improved carrier density are the origins of enhanced thermoelectric properties, where the Seebeck coefficient is improved, and the electrical resistivity is reduced. Our study provides insight into the importance of phase stability for enhancing thermoelectric transport in Mn-doped β-FeSi2.

Effects of Direct Aging Heat Treatments on the Superelasticity of Nitinol Produced via Laser Powder Bed Fusion

This work investigates the effects of short-time direct aging heat treatments on the mechanical properties and microstructure of additively manufactured Nitinol (NiTi) alloy. Cylindrical samples were produced through laser powder bed fusion (L-PBF), directly aged at different temperatures and compared to the solution annealed and aged conditions. Compression tests were carried out at room temperature both in cyclic mode at constant strain and incremental cyclic mode, to provide a comprehensive analysis on the superelastic features of NiTi after direct aging heat treatments. Furthermore, cyclic compression tests were performed at 37 °C to evaluate the superelastic effect at the body temperature and, therefore, the possibility to use these treatments for biomedical components. The effects of direct aging on the microstructure were characterized using transmission electron microscopy (TEM). High cyclic stability and superelastic recovery up to 10 pct of deformation emerged for the direct aged alloys. The comparable results obtained with and without the solution treatment points out that this step was not necessary in reaching superelasticity, proving the effectiveness of direct aging.