Thermally Assisted Research Articles

Projection Stereolithography 3D-Printed Bio-Polymer with Thermal Assistance.

A stereolithography process with thermal assistance is proposed in this work to address the tradeoff between the flowability and the high concentration of solute loadings at room temperature, through which the improved performance of polymers prepared using stereolithography 3D printing can be achieved. For the experiment, polyethylene glycol diacrylate (PEGDA) with a high molecular weight of 4000 is adopted to improve the mechanical properties of 2-Hydroxyethyl methacrylate (HEMA). For the polymer of HEMA, the highest soluble concentration of PEGDA is about 20 wt% at 25 °C (room temperature) while the concentration could be raised up to 40 wt% as the temperature increases to 60 °C. The 3D printing tests showed that the objects could be easily fabricated with the HEMA polymer loaded with 40 wt% of PEGDA through the thermally assisted projection stereolithography technology. By adding the 40 wt% of PEGDA, the Young's modulus has been enhanced by nearly 390% compared to the HEMA resin without solute, of which the Young's modulus is 63.31 ± 2.72 MPa. The results of the cell proliferation test proved that the HEMA resin loaded with PEGDA led to a better biocompatibility compared to the HEMA resin without the loading of the PEGDA solute. All of the results demonstrate that the polymer loaded with high solute is feasible to be precisely 3D-printed using the projection stereolithography process with thermal assistance, and the improved mechanical properties are beneficial for biomedical applications.

Macro-micro behaviors of Ti–22Al–26Nb alloy under near isothermal electrically-assisted tension

To reveal the non-thermal effect of electric current in macro-micro behaviors Ti–22Al–26Nb alloy sheet with bimodal initial structure, electrically-assisted (EA) near isothermal tensile tests and thermally-assisted (TA) tensile tests have been conducted comparatively at equivalent temperatures by adjusting the applied current-time curve. The stress responses, morphological evolution of constituent phases and the phase transformation behaviors have been characterized by means of EBSD and TEM. Result shows that EA tension is more effective in reducing deformation resistance, while TA tension plays a more important role in plasticity enhancement. The overall electrically-induced stress drop in EA tension increases monotonically with current density, but its non-thermal component first decreases with current density and then increases significantly from the critical value of 12.40A/mm2 in EA tension. The variation of the non-thermal stress drop during EA deformation is attributed to the competition between softening effect and strengthening effect of the electric current, viz. electric current causes softening effect by activating dislocation slip and inducing dislocation annihilation on one hand, and the electrically-driven dynamic precipitation of nanoscale O laths and dynamic globularization of O lamellae leads to additional strengthening effect on the other hand. Moreover, electric current promotes the process of O→B2 phase transformation from both thermodynamic and kinetic at 16.01–20.87 A/mm2.

Thermally assisted 3D printing of bio-polymer with high solute loading with improved mechanical properties

In this study, a stereolithography process with thermal assistance is presented to address the tradeoff between the flowability and the high concentration of solute loadings at room temperature, by which the improved performance of polymers prepared by stereolithography 3D printing can be achieved. As a demonstration, an easily available polymer of polyethylene glycol diacrylate (PEGDA) with the solute loading of Polyethylene glycol (PEG) was designed for the potential biomedical applications. The maximum soluble concentration of PEG in PEGDA is less than 10 wt% at room temperature while the concentration reaches to 30 % with the temperature of 70 ℃. The comparative experiments proved that the concentration of high molecular weight PEG had a positive effect on the mechanical property of the PEGDA resin. The hardness of the sample printed by the PEGDA resin with PEG by the concentration of 30 wt% is three times higher than that of the PEGDA resin without solute. As the concentration of PEG increases from 15 wt% to 30 wt%, the Young’s modulus has been enhanced from 67.9 MPa to 131.4 MPa while the Young’s modulus for the PEGDA resin without solute is 39.6 MPa. The compression tests with 3D printed lattice structure illustrated that the compressive modulus of the PGEDA resin has been improved by nearly 600 % with the 30 wt% of PEG. The cell proliferation experiments showed that the PEGDA resin with the loading of PEG led to the higher cell activity comparing to the PEGDA resin without solute. The results indicate that the thermally assisted 3D printed PEGDA resin with the high loading of PEG has excellent mechanical properties for the potential biomedical application. This study is expected to demonstrate the thermally assisted stereolithography 3D printing process for the polymer with high solute loading, which contributes to explore the printable materials with improved performance.

Influence of thermal assistance on the biodegradation of organics during food waste bio-drying: Microbial stimulation and energy assessment

Recently, bio-drying was highlighted in the drying pretreatment of high-moisture organic wastes for their energy recovery. In this study, to investigate the influences of thermal assistance on microbial stimulation and energy utilization in organic degradation, thermally assisted bio-drying (TB) was conducted on food waste (FW) and was compared with conventional bio-drying (CB) and thermal drying (TD). As expected, more water was removed in TB, which exhibited no lag phenomenon and intensified microbial activity. Corresponding with the stimulated enzyme activity, more readily degradable carbohydrates, lipids and lignocellulose were decomposed in TB than those in CB, and lipid degradation generated a significant proportion of the total bio-heat generated (43.13%–45.83%). Furthermore, according to the microbial analysis (qPCR and 16S rRNA gene sequencing), Bacillus was found to be the dominant genus involved in the degradation of organics during TB and CB. In the initial phase, rather than Weissella, as in CB, Ureibacillus was notable in TB for the degradation of readily degradable substrates. In the late phase, Pseudoxanthomonas and Saccharomonospora were enriched for degrading lignocellulose. In addition, heat balance and life-cycle energy assessment demonstrated that a small amount of thermal heat (11.96%) upgraded bio-drying with high energy efficiency. Compared with the CB and TD processes, the TB trial consumed less thermal energy (0.58 MJ/kg H2O) and achieved a higher energy output/input ratio (3.64). This research suggests that thermal assistance is a promising approach to enhancing FW bio-drying, which exhibits efficient drying performance and great potential for energy recovery.

Conduction-Based Thermally Assisted Micromilling Process for Cutting Difficult-to-Machine Materials

The increasing demand for complex and wear-resistant forming tools made of difficult-to-machine materials requires efficient manufacturing processes. In terms of high-strength materials; highly suitable processes such as micromilling are limited in their potential due to the increased tool loads and the resulting tool wear. This promotes hybrid manufacturing processes that offer approaches to increase the performance. In this paper; conduction-based thermally assisted micromilling using a prototype device to homogeneously heat the entire workpiece is investigated. By varying the workpiece temperature by 20 °C < TW < 500 °C; a highly durable high-speed steel (HSS) AISI M3:2 (63 HRC) and a hot-work steel (HWS) AISI H11 (53 HRC) were machined using PVD-TiAlN coated micro-end milling tools (d = 1 mm). The influence of the workpiece temperature on central process conditions; such as tool wear and achievable surface quality; are determined. As expected; the temporary thermal softening of the materials leads to a reduction in the cutting forces and; thus; in the resulting tool wear for specific configurations of the thermal assistance. While only minor effects are detected regarding the surface topography; a significant reduction in the burr height is achieved.

Energetic enhancement of thermal assistance in the cooling stage of biodrying by stimulating microbial degradation

In this study, thermal assistance was employed in the cooling stage of conventional biodrying. The results indicated that thermal assistance greatly enhanced water removal with improved vapor-carrying capacity of air-flow, and rapidly decreased moisture contents (MCs) from 45.15% to 49.42% to 15.20–25.85% in 6 days, which were much lower than those of conventional biodrying (CB, 34.90–40.85%). More importantly, a synergistic enhancement of physical and biological effects was observed in thermally assisted biodrying (TB) in terms of stimulated enzymes activity and microbial metabolism (higher oxygen uptake rate and degradation coefficient k). Among the degraded organics, lignocellulose was noted to be important for bio-heat generation in cooling stages, especially for straw as bulking agent. Heat balance results suggested that small fractions of thermal heat (19.76–24.73%) were required to upgrade CB processes for water evaporation with higher energy efficiency. Based on economic viability analysis and with consideration of the further drying for CB products, thermally assisted biodrying presented more economic benefits with less investment and shorter payback period. This research provided an efficient engineering approach to upgrade the cooling stage of conventional biodrying with low external heat cost.

Demonstration of Thermally-Assisted Programming With High Speed and Improved Reliability for Junctionless Nanowire NOR Flash Memory

This study demonstrated the thermally-assisted (TA) programming of a NOR flash memory device, which was composed of a junctionless silicon nanowire with gate-all-around structure. Fast programming was achieved with the aid of Joule heat, which was generated in a homogeneously heavily doped SiNW by a voltage difference between the source and drain. Its programming speed is the fastest among various NOR flash memories ever reported. The mechanism for programming NOR flash memory inevitably degrades the quality of the tunneling oxide. But the proposed thermally-assisted programming provides simultaneous curing of the damaged tunneling oxide.

Investigation on thermally assisted optically stimulated luminescence (TA – OSL) signal in various sodium chloride samples

Sodium chloride (NaCl) is a phosphor with potential significance in retrospective dosimetry and geological dating. NaCl has been extensively studied for practical use in OSL dosimetry, however, the exact mechanism of the OSL emission is not well explained. This work attempts to extend the information on NaCl luminescent properties by establishing the occurrence of very deep traps in NaCl using the thermally assisted OSL (TA – OSL) method. The studied material was sodium chloride in different forms: halite minerals from Kłodawa salt mine in Poland and NaCl in chemically pure form. The isothermal TA – OSL signal was measured at various temperatures between 25 °C and 280 °C after a prior irradiation and TL erasing of shallow and main traps. The appearance of a strong TA – OSL signal indicates the occurrence of very deep traps in all forms of investigated salt. The temperature dependence of TA – OSL was determined and the activation energy for thermal assistance corresponding to deep traps in NaCl was estimated. For selected temperatures of TA – OSL readout (200 °C and 280 °C) the dose response was examined in wide dose range (1–1000 Gy). Sublinearity was found in different dose range depending on the type of samples.

Transition from thermally assisted to mechanically driven boundary migration and related apparent activation energies

Grain boundary migration during deformation of nanostructured materials has been reported for a variety of materials and loading conditions. As boundary migration occurs even under cryogenic conditions, it is believed to be mainly mechanically driven, although results suggest thermal assistance. Yet, these contributions could not be separated and the underlying apparent activation energies remain uncertain. Here we present an approach that is capable to determine the average activation energies for mechanically driven boundary migration. While the apparent activation energy is increasing with temperature, below critical temperatures it becomes zero, indicating the existence of an entirely mechanically driven regime.

Properties of quartz TA – OSL signal with a peculiar decay shape for SAR TA – OSL applications

A rapidly decaying curve shape, similar to the conventional optically stimulated luminescence (OSL) curve, is observed when thermally assisted (TA –) OSL signal from quartz is measured at a steady elevated temperature. In some cases, a peculiar shape consisting of one flat signal is yielded. The effects of thermal assistance and dose applied on the features of this peculiar type of curve were studied using quartz samples collected from Anatolia, Turkey. The results indicated the absence of the initial rapidly decaying part of the conventional OSL for this peculiar curve of TA-OSL for all stimulation temperature and dose ranges attributed within this work. The intensity of the flat decay curve signal was observed to be dependent on both stimulation temperatures and doses. It was demonstrated that for this specific signal, thermally assisting activation energy of almost 1 eV stands as a prevalent feature, indicating that this TA – OSL signal with flat shape originates from very deep traps with activation energy under the conduction band of the order of 3 eV. This suggests that such a trap could provide signal with adequate stability for dating back to a million years, or even more. The dose response linearity for high attributed doses strongly supports the application of Single Aliquot Regenerative TA – OSL for equivalent dose estimations. However, the high recuperation values, of the order of 30%, indicating possible underestimation of the equivalent doses, along with recycling ratio values of around 1.25, stand as drawbacks that limit the use of such TA – OSL signal in single aliquot regenerative protocols for equivalent dose estimations.

Analysis of self-heating of thermally assisted spin-transfer torque magnetic random access memory.

Thermal assistance has been shown to significantly reduce the required operation power for spin torque transfer magnetic random access memory (STT-MRAM). Proposed heating methods include modified material stack compositions that result in increased self-heating or external heat sources. In this work we analyze the self-heating process of a standard perpendicular magnetic anisotropy STT-MRAM device through numerical simulations in order to understand the relative contributions of Joule, thermoelectric Peltier and Thomson, and tunneling junction heating. A 2D rotationally symmetric numerical model is used to solve the coupled electro-thermal equations including thermoelectric effects and heat absorbed or released at the tunneling junction. We compare self-heating for different common passivation materials, positive and negative electrical current polarity, and different device thermal anchoring and boundaries resistance configurations. The variations considered are found to result in significant differences in maximum temperatures reached. Average increases of 3 K, 10 K, and 100 K for different passivation materials, positive and negative polarity, and different thermal anchoring configurations, respectively, are observed. The highest temperatures, up to 424 K, are obtained for silicon dioxide as the passivation material, positive polarity, and low thermal anchoring with thermal boundary resistance configurations. Interestingly it is also found that due to the tunneling heat, Peltier effect, device geometry, and numerous interfacial layers around the magnetic tunnel junction (MTJ), most of the heat is dissipated on the lower potential side of the magnetic junction. This asymmetry in heating, which has also been observed experimentally, is important as thermally assisted switching requires heating of the free layer specifically and this will be significantly different for the two polarity operations, set and reset.

Thermally assisted OSL (TA-OSL) from various luminescence phosphors; an overview

The current status of the results on the thermally assisted optically stimulated luminescence (TA-OSL) signal performed on various naturally occurring as well as artificial luminescent phosphors is described. TA-OSL of anion deficient alumina doped with carbon and quartz has been reported to be effectively used towards dosimetry purposes, especially for large accumulated doses. Basic TA-OSL features, such as the stability, dose response, thermal assistance and TA-OSL curve shapes, along with the prevalence and the ubiquity of these features are discussed.

In Situ Nanolithography with Sub-10 nm Resolution Realized by Thermally Assisted Spin-Casting of a Self-Assembling Polymer.

In situ nanolithography is realized based on warm spin-casting of block copolymer solutions. This advancement is based on Si-containing block copolymers with an appropriate thermodynamic driving force for spontaneous phase-separation combined with the thermal assistance provided by slight temperature elevations during the spin-casting. Sub-10 nm half-pitch nanoscale patterns are produced within 30 s without a separate annealing process.

Numerical analysis of thermally assisted spin-transfer torque magnetization reversal in synthetic ferrimagnetic free layers

The spin transfer torque magnetization reversal of synthetic ferrimagnetic free layers under pulsed temperature rise was numerically studied by solving the Landau–Lifshitz–Gilbert equation, taking into account the stochastic random fields, the temperature dependence of magnetic parameters, and the spin torque terms. The anti-parallel magnetization configuration was retained at the elevated temperature, due to interlayer dipole coupling. A significant thermal assistance effect, resulting in a 40% reduction in the switching current, was demonstrated during a nanosecond pulsed temperature rise up to 77% of the Curie temperature.

Thermal assistance in TA – OSL signals of feldspar and polymineral samples; comparison with the case of pure quartz

Thermally assisted optically stimulated luminescence (TA – OSL) is studied for the cases of polymineral, rich in K-feldspar sample as well as one pure sample of K-feldspar. For both cases, the shape of the TA – OSL signal indicates an initial, fast decaying part which is followed by a flat, very slowly decaying part with intensity much larger than the ordinary background noise signal. Thermal assistance characteristics indicate that for the case of pure K-feldspar, the signal originates from a unique very deep trap. The experimental features of the TA – OSL signal in the case of polymineral sample resemble much the corresponding TA – OSL features of pure quartz, in terms of both glow curve shape, especially at high stimulation temperatures, as well as signal intensity and its dependence on the stimulation temperature. Nevertheless, TA – OSL stimulation at low, ambient temperatures provide strong hints towards the contribution of K – feldspars to this signal. Proper selection of the stimulation temperature could possibly discriminate the TA – OSL signal originating from quartz and feldspars.

Laser-assisted machining process simulation for precision mould/die manufacturing

Laser-assisted machining (LAM) is a kind of thermally assisted manufacturing process which preheats workpiece material with a highly-localized laser beam and then is followed by a conventional machining operation. It could greatly improve the machinability and gives better machining quality, especially suitable for hard-to-cut materials such as superalloys, tool steels. For manufacturing the high-precision moulds/dies that could fabricate micro and precision components, the thermal assistance at the local cutting zone, will help to lower the cutting force and consequently improve the internal surface quality of the machined moulds/dies. In this paper, a numerical study of the laser-assisted cutting of plastic mould steel NAK80 is reported. The simulation of LAM of NAK80 is realized by combining two procedures: simulation of moving laser heat source applied on the local surface of workpiece which causes the corresponding temperature field arise and material soften, and simulation of the subsequent cutting processes. This research will offer the die/mould manufacturers a more efficient planning tool for the fabrication of the micro, precision dies/moulds of high surface quality, and hence, for forming the high-quality micro components as well as for longer die/mould life.

Asymmetric Composite Free Layers With Compensated Magnetization for Ultrahigh Density Integration of STT-MRAM

This paper analyzes the magnetization switching behavior of an asymmetric synthetic antiferromagnetic (AF) free layer for spin-transfer torque random access memory and numerically demonstrates thermally assisted magnetization switching. Optimization of the free-layer thickness and the magnetic properties enables successful magnetization switching while retaining the AF structure during the switching process. The thermal stability was improved by increasing the lateral aspect ratio of the free layers while also maintaining writability with a reduced current density under the thermal assistance.

Optically stimulated luminescence (OSL) and thermally assisted OSL in Eu2+ – Doped BaSO4 phosphor

BaSO4:Eu2+ phosphor has been investigated for its photoluminescence (PL), thermoluminescence (TL), TL kinetics, optically stimulated luminescence (OSL) and thermally assisted OSL (TA-OSL) response. PL spectra showed the characteristic emission of Eu2+ ion at 375 nm when excited by 320 nm. The luminescence lifetime has been measured as 40 and 628 μs of fast and slow components respectively. The TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined. The phosphor is found to be 6 and 4 times more sensitive than CaSO4:Dy and α-Al2O3:C, respectively, in TL mode. However, its OSL sensitivity is 75% of α-Al2O3:C. It is found to possess three OSL components having photoionization cross-sections of 1.4 × 10−17, 1.2 × 10−18 and 5.2 × 10−19 cm2 respectively. The temperature dependence of OSL studies showed that integrated TA-OSL signal increases with stimulation temperature between 50 and 250 °C, while between 260 and 450 °C the signal intensity decreases. This behavior is interpreted to arise from competing effects of thermal assistance (activation energy EA = 0.063 ± 0.0012 eV) and depletion of trapped charges. This increase of OSL at elevated temperature can be employed for enhancing the sensitivity of phosphor for radiation dosimetry.

Thermally assisted high efficiency ductile machining of nanocrystalline hydroxyapatite: A numerical study

Abstract This study investigates the role of thermal assistance and tool geometry on ductile regime machining of a nanocrystalline (nano-HAP) bioceramic using numerical simulation. AdvantEdge FEM Version 5.9 is used to conduct the simulation of orthogonal machining of the nano-HAP material. Thermal boundary conditions are specified to approximate laser preheating of the work material. The effects of operating conditions (preheat temperature, rake angle, and edge radius) on critical depth of cut, cutting force, and thrust force are investigated. Based on the pressure-based criterion for ductile regime machining (DRM), the dependence of critical depth of cut on preheating temperature is examined. It is found that for different combinations of rake angle and edge radius, the critical depth of cut increases as thermal boundary temperature increases. In addition, it is concluded that using higher thermal boundary temperature for smaller negative or 0° rake angle, we can achieve the comparable critical depth of cut obtained by using a higher negative rake angle, which usually generates higher thrust force and consequently deteriorates the dimensional accuracy of finished parts.

Thermally assisted MRAMs: ultimate scalability and logic functionalities

This paper is focused on thermally assisted magnetic random access memories (TA-MRAMs). It explains how the heating produced by Joule dissipation around the tunnel barrier of magnetic tunnel junctions (MTJs) can be used advantageously to assist writing in MRAMs. The main idea is to apply a heating pulse to the junction simultaneously with a magnetic field (field-induced thermally assisted (TA) switching). Since the heating current also provides a spin-transfer torque (current-induced TA switching), the magnetic field lines can be removed to increase the storage density of TA-MRAMs. Ultimately, thermally induced anisotropy reorientation (TIAR)-assisted spin-transfer torque switching can be used in MTJs with perpendicular magnetic anisotropy to obtain ultimate downsize scalability with reduced power consumption. TA writing allows extending the downsize scalability of MRAMs as it does in hard disk drive technology, but it also allows introducing new functionalities particularly useful for security applications (Match-in-Place™ technology).