High‐Mobility Helical Tellurium Field‐Effect Transistors Enabled by Transfer‐Free,Low‐Temperature Direct Growth

The transfer‐free direct growth of high‐performance materials and devices can enable transformative new technologies. Here, room‐temperature field‐effect hole mobilities as high as 707 cm² V^?1 s^?1 are reported, achieved using transfer‐free, low‐temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO₂/Si. The Te nanostructures exhibit significantly higher device performance than other low‐temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high‐performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO₂/Si and HfO₂. The morphology of the Te films can be tailored by the growth temperature, and different carrier scattering mechanisms are identified for films with different morphologies. The transfer‐free direct growth of high‐mobility Te devices can enable major technological breakthroughs, as the low‐temperature growth and fabrication is compatible with the severe thermal budget constraints of emerging applications. For example, vertical integration of novel devices atop a silicon complementary metal oxide semiconductor platform (thermal budget <450 °C) has been theoretically shown to provide a 10× systems level performance improvement, while flexible and wearable electronics (thermal budget <200 °C) can revolutionize defense and medical applications.     

A new route to graphene starting from heavily ozonized fullerenes: Part 1—thermal reduction under inert atmosphere

It is shown that heavily ozonized C₆₀ or C₇₀ fullerenes (known also as “fullerene ozopolymers”) are suitable substrates for the preparation of graphene or nanographene in place of graphite oxide (GO) by thermal reduction in inert atmosphere. TGA-FTIR study shows that the release profile of CO₂ and CO from fullerene ozopolymers in the temperature range between 25°C and 900°C is comparable to that shown by GO. Furthermore, the FT-IR spectral evolution of fullerene ozopolymers from room temperature to 630°C under inert atmosphere is once again strikingly comparable to that observed on GO under the same conditions.     

A STUDY ON THE THERMAL STABILITY TO 1000°C OF VARIOUS CARBON ALLOTROPES AND CARBONACEOUS MATTER BOTH UNDER NITROGEN AND IN AIR

ABSTRACT

The thermal behavior of graphite, C₆₀ fullerene, fullerene black (carbon soot containing fullerenes), extracted fullerene black and diamond has been analyzed to 1000°C by TGA–DTA (thermogravimetric analysis and differential thermal analysis) under a nitrogen flow at a heating rate of 20°C/min. Very small weight losses have been recorded in the case of graphite and diamond. Furthermore no diamond graphitization has been observed. The sublimation of pure C₆₀ and the fullerene fraction of fullerene black (both pristine and extracted) has been observed and discussed.

The combustion reaction in air flow of graphite, C₆₀ and C₇₀ fullerenes, fullerene black (both unextracted and extracted), carbon nanotubes and diamond has been studied by TGA–DTA at a heating rate of 20°C/min. C₇₀ fullerene and fullerene black have been found to be the most reactive carbon materials with O₂. The role played by C₇₀ in the degradation of fullerites has been discussed. Among the carbon materials examined, the best resistance to O₂ attack has been shown by diamond and carbon nanotubes. The behavior of graphite is intermediate between diamond and fullerene blacks. The behavior of C₆₀ fullerene appears closer to that of graphite although it appears to be more reactive with O₂. Samples of graphite and carbon blacks N375 and N234 have been studied by TGA–DTA in air flow before and after a radiation treatment with neutrons or γ radiation. The effect of the radiation damage in the combustion reaction of these carbon materials has been discussed.     

Investigation of cyclic thermal shock behaviour of fibre reinforced glass matrix composites using non-destructive forced resonance technique

Abstract

A non-destructive forced resonance technique was used to assess the damage development in SiC fibre reinforced glass matrix composite materials subjected to cyclic thermal shock. Both elastic modulus and internal friction measurements were conducted. The thermal shock tests involved quenching the specimens from high temperatures (590–710°C) to room temperature in a water bath. Damage in theform of matrix microcracks was induced by quenchingfrom 620 and 660°C, and the extent of damage increased with the number of thermal shock cycles. After a certain number of shocks, this damage was detected by a decrease in the Youngs modulus and a simultaneous increase in the internal friction. The non-destructive dynamic forced mechanical resonance technique employed was shown to be more sensitive than a destructive three point flexural technique for detecting crack development in the early stages of thermal shock damage. The technique was also used to confirm the occurrence of a crack healing process in the thermally shocked specimens: after an annealing heat treatment for 12 h at 550°C, the initial values of Young's modulus and internal friction were recovered. This was attributed to crack closure due to viscous flow of the glass matrix.     

Solution‐Processed All‐Ceramic Plasmonic Metamaterials for Efficient Solar–Thermal Conversion over 100–727 °C

Low‐cost and large‐area solar–thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large‐scale solar–thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concentrating solar power. Few state‐of‐the‐art selective absorbers are qualified for both low‐ ( < 200  ° C) and high‐temperature ( > 600  ° C) applications due to insufficient spectral selectivity or thermal stability over a wide temperature range. Here, a high‐performance plasmonic metamaterial selective absorber is developed by facile solution‐based processes via assembling an ultrathin ( ≈ 120 nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in‐plane plasmon and out‐of‐plane Fabry–Pérot resonances, the all‐ceramic plasmonic metamaterial simultaneously achieves high, full‐spectrum solar absorption (95%), low mid‐IR emission (3% at 100  ° C), and excellent stability over a temperature range of 100–727  ° C, even outperforming most vacuum‐deposited absorbers at their specific operating temperatures. The competitive performance of the solution‐processed absorber is accompanied by a significant cost reduction compared with vacuum‐deposited absorbers. All these merits render it a cost‐effective, universal solution to offering high efficiency (89–93%) for both low‐ and high‐temperature solar–thermal applications.     

Toughness and fatigue behaviour of eutectic and hypereutectic Al–Si–Cu–Mg alloys produced through lost foam and squeeze casting

Abstract

The strength and toughness of four high silicon content Al–Si–Mg–Cu alloys have been studied at room temperature (RT), 200°C and 300°C. The fatigue behaviour has also been investigated. The alloys were produced using two very different processing routes: lost foam and squeeze casting. In the tensile tests, the ductility was low for alloys produced via both routes irrespective of the testing temperature. The strength was similar at RT and 200°C, but at 300°C it fell abruptly. The toughness followed the same trend with testing temperature. Direct observation of fatigue cracks revealed that the brittle silicon and intermetallic particles broke ahead of the crack tip; the fatigue crack advanced by linking the main crack with cracks formed ahead of it. The T6 thermal treatment improved fatigue resistance in the squeeze cast material, especially at high D K values.     

Ultra low and negative expansion glass-ceramic materials produced from pyrophyllite and blast furnace slag

Ultra low and negative expansion glass-ceramic materials have been obtained from pyrophyllite and blast furnace slag. The batch composition was modified with the addition of lithium carbonate, hydrated alumina, boric acid and nucleating agent (titania). The batch was melted at 1400°C followed by casting in the form of bars and annealed at 510°C for 4 h. The annealed specimens were subjected to heat treatment at predetermined temperatures selected from DTA study of the parent glass. Thermal expansion measurement and X-ray diffraction analysis revealed that the specimen nucleated at 545°C for 4 h and crystallized at 720°C for 2 h which resulted in negative coefficient of thermal expansion [(-) 9 to (-) 2 x 10^-7/°C] over the temperature range (30-600°C) due to the formation ofβ bd-eucryptite while other heating schedule showed the formation of spodumene and lithium aluminium silicates. The samples showed excellent flexural strength value and varied in the range 120–200 MPa depending upon the phases present.     

Ceramic Tool Concepts for the Semi‐Solid Processing of Steel Alloys

Two different ceramic tool concepts for the semi‐solid processing (Thixoforming) of steel alloys are presented. Materials selection is adapted to forming technology (Thixoforging, Thixoextrusion), preset die temperature, and resulting process conditions. Gas‐pressure sintered silicon nitride (Si₃N₄) is chosen as die material in low tool temperature (300...400 °C) thixoforging experiments due to its high strength and outstanding thermal shock resistance. High purity dense alumina (Al₂O₃) is applied as die material for high temperature (1200 °C) thixoextrusion tests. Thixoforging results using Si₃N₄ dies pre‐heated to 300 °C show sufficient thermal shock and corrosion resistance of Si₃N₄ and confirm the applicability of this tool concept. The high temperature tool concept developed at the Institute of Mineral Engineering (GHI) effectively reduced thermal shock impacts on extrusion dies. As expected, corrosion resistance of Al₂O₃ proved to be excellent. Further research will be carried out concerning long‐term behaviour of Si₃N₄ thixoforging dies as well as on the influence of extrusion speed and tool temperature on the quality of products extruded through Al₂O₃ dies at high temperature.     

Thermal stability of retained austenite in Cr–Ni weld metals at low temperatures

Abstract

As environmental temperature decreases, the amount of retained austenite is more likely to greatly reduce due to the thermal austenite–martensite transformation caused by the decreased thermal stability of retained austenite, probably making its amount lower than the required content. In the present study, the thermal stability of retained austenite in Cr–Ni weld metals was investigated to see whether sufficient retained austenite can be maintained at low temperatures. The specific experimental procedure is as follows: briefly, the samples were cooled in turn from room temperature to 0, ?20, ?40, ?60, ?80, ?100 and ?196°C; the amount of retained austenite at the above temperatures was measured using X-ray diffraction. Through investigating the dependence of the content of retained austenite on temperature, it was revealed that when the content of retained austenite is <20%, retained austenite can be maintained until ?196°C.     

Temperature compensated fiber Bragg grating using fiber reinforced polymeric composites

Abstract

The thermal drift coefficient of the Bragg wavelength of fiber Bragg grating (FBG) is unacceptably high when the FBG is used as a wavelength reference or a wavelength‐selective passive component, especially in a dense wavelength division multiplexing (DWDM) system. A light, small and robust carbon fiber reinforced polymeric composite structure is proposed to compensate for the temperature induced wavelength drift in FBG. It has been shown to be able to reduce the temperature induced wavelength shift of a fiber Bragg grating from 10 pm/°C to below 1 pm/°C. Stability in long term performance over a two month period has also been demonstrated. A fabrication route that leads to optimal compensation performance has also been proposed.     

Highly Safe Electrolyte Enabled via Controllable Polysulfide Release and Efficient Conversion for Advanced Lithium–Sulfur Batteries

Conventional lithium–sulfur batteries often suffer from fatal problems such as high flammability, polysulfide shuttling, and lithium dendrites growth. Here, highly‐safe lithium–sulfur batteries based on flame‐retardant electrolyte (dimethoxyether/1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropyl ether) coupled with functional separator (nanoconductive carbon‐coated cellulose nonwoven) to resolve aforementioned bottle‐neck issues are demonstrated. It is found that this flame‐retardant electrolyte exhibits excellent flame retardancy and low solubility of polysulfide. In addition, Li/Li symmetrical cells using such flame‐retardant electrolyte deliver extraordinary long‐term cycling stability (less than 10 mV overpotential) for over 2500 h at 1.0 mA cm^?2 and 1.0 mAh cm^?2. Moreover, bare sulfur cathode–based lithium–sulfur batteries using this flame retardant electrolyte coupled with nanoconductive carbon‐coated cellulose separator can retain 83.6% discharge capacity after 200 cycles at 0.5 C. Under high charge/discharge rate (4 C), lithium–sulfur cells still show high charge/discharge capacity of ≈350 mAh g^?1. Even at an elevated temperature of 60 °C, discharge capacity of 870 mAh g^?1 can be retained. More importantly, high‐loading bare sulfur cathode (4 mg cm^?2)–based lithium–sulfur batteries can also deliver high charge/discharge capacity over 806 mAh g^?1 after 56 cycles. Undoubtedly, the strategy of flame retardant electrolyte coupled with carbon‐coated separator enlightens highly safe lithium–sulfur batteries at a wide range of temperature.     

Miniature thermal cycling tests on aluminium alloy metal matrix composites

Abstract

A new miniaturised electrothermomechanical test system has been used to study the thermal cycling response of a number of aluminium alloy metal matrix composites reinforced with either Al₂0₃ or SiC particles. Tests were also performed on a monolithic 2618 aluminium alloy for comparison. The system showed good test discrimination between the different materials for both constant load-constant temperature (creep) tests and constant load-temperature cycling (50–200°C) tests. The system was also used to compare the yield behaviour at 200°C, and the thermal expansion and thermal diffusivity of several of the materials.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

The International Foundry and Heat Treatment Conference ‘82

Abstract

Sheet materials for hot gas path components in modern land based gas turbines demand high strength over the temperature range 650–950°C and freedom from serious in-service embrittlement. This is particularly critical where a gas turbine is subject to cycling since thermal stresses can lead to the cracking of such components. It is also highly desirable that if cracking does occur, components can be repaired safely and easily. Haynes 230, a modern alloy, is relatively immune to in-service embrittlement, particularly in comparison to some older materials, but may require a rejuvenation heat treatment to facilitate repairs after in-service exposure. A rejuvenation heat treatment at 1177°C for 0.5 hour was shown to restore stress rupture and weld ductility to those required by AMS 5878A and Section IX of the ASME Vessel and Boiler Code. Stress rupture results for the aged samples indicated that lives were in excess of those for materials in the as-received condition. This was ascribed to grain boundary precipitation.     

Hollow-grained “Voronoi foam” ceramics with high strength and thermal superinsulation up to 1400 °C

Nanostructures tend to be unstable at high temperatures due to large capillary energies, and therefore nanotechnology has not yet found many high-temperature applications at 1000 °C and above. By taking advantage of the high-temperature stability of refractory ceramics, here we develop a new approach of making hollow nano-grained materials to achieve thermal superinsulation across a wide temperature range, where the gaseous voids are mostly isolated within individual grain, with size comparable to the mean free path of air molecules to lower the thermal conduction by Knudsen effect. We have proved this general concept with hollow-grained La₂Zr₂O₇ ceramic, and demonstrated exceptionally low thermal conductivity (0.016 W/(m⋅K)), the lowest ever reported for hard materials at or above room temperature. The centimeter-scale samples also have ultrahigh compressive strength (251 MPa), tensile strength in bending up to 100 MPa, and excellent thermal stability up to 1400 °C in air, due to monodispersity of pores that delays coarsening.     

Silicon Oxycarbide Coatings Consisting of Defined Bottom–Up-Grown Nanostructures (Small 41/2023)

Silicon oxycarbide (SiOC) materials have arisen in the past few decades as a promising new class of glasses and glass-ceramics thanks to their advantageous chemical and thermal properties. Many applications, such as ion storage, sensing, filtering, or catalysis, require materials or coatings with high surface area and might benefit from the high thermal stability of SiOC. This work reports the first facile bottom–up approach to textured high surface area SiOC coatings obtained via direct pyrolysis of polysiloxane structures of well-defined shapes, such as nanofilaments or microrods. This work further investigates the thermal behavior of these structures by means of FT-IR, SEM, and EDX up to 1400 °C. The rods shrink in volume by ≈30% while their aspect ratio remains unaffected by pyrolysis until at least 1100 °C. The nano-sized filaments show signs of viscous flow already at a comparably low temperature of 900 °C which is very probably due to the nano-size effect. This might open a way to experimentally study the size-effect on the glass transition temperature of oxide glasses, an experimentally unexplored but very relevant topic. These structures have great potential, for example, as ion storage materials and supports in high temperature catalysis and CO₂ conversion.     

Single‐Crystalline,Metallic TiC Nanowires for Highly Robust and Wide‐Temperature Electrochemical Energy Storage

Customized electrode materials with good temperature adaptability and high‐rate capability are critical to the development of wide‐temperature power sources. Herein, high‐quality TiC nanowires are uniformly grown on flexible carbon cloth as free‐standing electric‐double‐layer supercapacitor electrode. The TiC nanowires, 20–40 nm wide and 3–6 µm long, are single‐crystalline and highly conductive that is close to typical metal. Symmetric supercapacitors are constructed with ionic liquid electrolyte and TiC nanowires electrodes as wide‐temperature and long‐cycle stable power source. Ultrastable high‐rate cycling life of TiC nanowire arrays electrodes is demonstrated with capacitance retention of 96.8% at 60 °C (≈440 F g^?1), 99% at 25 °C (≈400 F g^?1), and 98% at ?25 °C (≈240 F g^?1) after 50 000 cycles at 10 A g^?1. Moreover, due to high electrical conductivity, the TiC nanowire arrays show ultrafast energy release with a fast response time constant of ≈0.7 ms. The results demonstrate the viability of metal carbide nanostructures as wide‐temperature, robust electrode materials for high‐rate and ultrastable supercapacitors.     

Study of microstructural and mechanical properties of weld heat affected zones of 2024-T3 aluminium using Gleeble simulation

Abstract

The effect of the microstructural properties on the mechanical properties of welding thermal cycles and post-weld heat treatment of the heat affected zone (HAZ) in 2024-T3 aluminium alloy has been investigated. Gleeble HAZ simulation, differential scanning calorimetry, TEM and tensile test have been utilised to investigate the regions representative of HAZ microstructures. The decay of strength in the weld HAZ is primarily due to the precipitation and coarsening of stable S phases. The welded HAZ in the region at peak temperature of 414°C has the lowest strength after natural aged temper. Post-weld T81 artificial aging (PWAA-T81) heat treatment at 190°C for 12 h has no effect on improving the HAZ strength; the HAZ strength of 2024-T3 alloy obtained by PWAA-T81 treatment is less than that obtained by natural aging, and its lowest strength is shifted to the region of the peak temperature, which is 452°C. Scanning electron microscopy observation reveals that the fracture mode changes from transgranular to intergranular failure when the 2024 specimen is exposed to a thermal cycle up to a peak temperature of 550°C. This is caused by the liquation of grain boundary segregates or formation of a eutectic structure while the specimen is subjected to high temperature thermal cycles during welding, which results in a decrease in the strength and ductility of the grain boundary. It is also shown that the decrease in ductility in this high temperature HAZ cannot be improved using the PWAA-T81 heat treatment.     

Effect of alumina particle size and thermal condition of casting on microstructure and mechanical properties of stir cast Al–Al2O3 composites

Abstract

Metal matrix composites are considered as a distinct category of the advanced materials, which have low weight, high strength, high modulus of elasticity, low thermal expansion coefficient and high wear resistance. Among them, Al–Al₂O₃ composites have achieved significant attention due to their desired properties. In the present research, Al–Al₂O₃ composites with 5 vol.-% alumina were produced by stir casting at a temperature of 800°C. Two different particle sizes of alumina were used as 53–63 and 90–105 μm. The microstructure of the samples was evaluated by SEM. In addition, the mechanical properties of the samples were measured, and hence, the optimum temperature and particle size of alumina to be added to the Al matrix were determined. The results demonstrated the positive effect of alumina on improving the properties of Al–Al₂O₃ composites.     

Synthesis of Lanthanofullerenes by Laser Ablation Using Pulsed and CW-Nd:YAG Lasers

Abstract

Endohedral lanthanum fullerenes, La@C₆₀, La@C₈₂ and La₂C₈₀, were synthesized by ablation of graphite and La₂O₃ in flowing Ar buffer gas with a 1064 nm beam from a pulsed-Nd: YAG laser in which the graphite was heated by the CW-Nd:YAG laser up to 2500 °C. The La-fullerenes were also prepared by laser ablation of a lanthanum-graphite composite rod. The extractant fullerenes from the carbon soot were analyzed by laser-desorption time of flight mass spectrometry (LD-TOF MS). The relative yield of La@C₈₂, increased with increasing temperature of the rod in the range of 700-2300 °C.