Effects Of Sub-zero Temperatures Research Articles

The Effect of Sub-Zero Temperature and High Strain Rate on Mechanical Strength of 9/7 ‘Oak’ Gun Propellant

The Effect of Sub-Zero Temperature and High Strain Rate on Mechanical Strength of 9/7 ‘Oak’ Gun Propellant

Unified hardening (UH) model for saturated frozen soils

The mechanical behavior of frozen soils is highly sensitive to temperature variations due to their complex micro-mechanisms. In regions with seasonal freeze-thaw cycles, the transition of soils between frozen and unfrozen states is significantly influencing their mechanical properties. This study synthesizes existing theories and data to categorize the effects of subzero temperatures on the properties of frozen soils. By introducing temperature and unfrozen water saturation, the phase change component of void ratio—decoupled from stress—is distinguished from actual voids, enabling the definition of the equivalent void ratio. Nonlinear relationships between temperature and other mechanical properties including elastoplastic deformation, cryogenic cohesion and ice segregation are established. Through the derivation of a loading-temperature yield equation, an elastoplastic constitutive model within a dual stress-variable framework of effective stress and temperature is established. This model captures key aspects of the behavior of frozen soils, including strength weakening due to ice segregation and temperature-induced strength changes. Applicable to conditions at or below the pore water melting point, the model's predictions align well with experimental observations.

Ballistic impact resistance of ultra-high-performance concrete (UHPC): A comprehensive experimental investigation and parametric analysis

This study explores the ballistic impact resistance of Ultra-High Performance Concrete (UHPC) through an extensive and detailed experimental investigation. This study will address various gaps related to the impact performance of UHPC. This investigation focused on understanding the ballistic resistance performance of UHPC based on (a) the influence of projectile/bullet parameters (hardness, velocity/energy, diameter, and nose shape), (b) the effect of hybrid fibers, (c) comparative analysis of the use of steel mesh and reinforcement bars and (d) effect of sub-zero temperature. In this parametric investigation, different types and shapes (flat, conical, and ogive) of hardened steel projectiles (diameter 12 and 18 mm) with impact velocities ranging from 103 to 182 m/s or impact energy ranging from 674 to 2103 J and UHPC slabs with thickness ranging from 36 to 100 mm was considered. Steel mesh and reinforcement bars at different spacing (with and without fiber UHPC) were also assessed, along with the effect of sub-zero temperature of −30 °C. The impact energy of the projectile and thickness of UHPC targets were statistically significant parameters affecting penetration depth in UHPC compared to other parameters with a p-value much less than 0.001. The Artificial Neural Network (ANN) model developed in the study could predict the penetration depth with reasonable accuracy (MAPE and RMSE were observed to be 7.69 and 12.12, respectively). The experimental results of this study will provide insights into the development of guidelines for using UHPC as a protective material, which can help ensure the safety and resilience of infrastructures in the face of potential threats.

Effect of sub-zero temperature on dynamic mode II fracture properties of saturated porous rocks

Investigating mode II (shear) fracture properties in alpine and arctic rock engineering is crucial. However, the effects of sub-zero temperatures on these fracture parameters of rocks remain unexplored. Thus, short core in compression (SCC) specimens are fabricated to investigate dynamic mode II fracture parameters of porous white sandstone (WS) across varying ambient temperatures (−55 °C, −40 °C, −25 °C, −10 °C, 0 °C, and 20 °C). Both dry and fully saturated WS SCC specimens were tested, with dry specimens as the control group. A high-speed camera was used with a dynamic-cryogenic testing setup to perform the dynamic tests on the rock specimens. Additionally, the fracture surface of recovered specimens was quantitatively characterized by laser scanning. The findings illustrate that the dynamic mode II fracture toughness and dynamic fracture energy (DFE) of WS specimens rise with the loading rate at several ambient temperatures. At a specific loading rate, the dynamic mode II fracture toughness of saturated WS demonstrates an initial growth as the temperature drops from 0 °C to −10 °C, subsequently decreases at −25 °C, and finally increases again as temperature continues to decrease below −25 °C. Nevertheless, the sub-zero temperatures slightly influence the mode II fracture toughness of dry WS. The fracture surface roughness of saturated and dry WS specimens in terms of the ambient temperature is consistent with the tendency of fracture toughness. However, the DFE of saturated and dry WS specimens remains nearly identical when the temperature is above 0 °C. In contrast, the DFE of dry WS specimens is larger than that of the saturated specimens when the temperature is below 0 °C. These phenomena are attributed to the water–ice phase transition in pores and the impact melting of ice at microcrack tips. Furthermore, it is found that the fractal dimension of shear fracture surfaces of SCC specimens decreases with rising loading rates, which is attributed to the enhanced occurrence of transgranular fracture during the dynamic failure process.

Effect of sub-zero temperatures on activation of basal slip, tension twinning and pyramidal slip in magnesium using micropillar compression

In this paper, the effect of sub-zero temperatures on the activation of basal <a> slip, {101‾2} tension twinning and <c+a> pyramidal slip in pure magnesium was investigated systematically at the micron length scale using micropillar compression. The micropillar compression was performed inside an SEM on selected grains of commercially pure polycrystalline magnesium between room temperature and ~ −94°C. A cryo-stub was designed and successfully employed to perform micropillar compression experiments below room temperature. Post-mortem SEM and TEM showed activation of basal <a> slip in [112‾1] oriented micropillars, {101‾2} tension twinning in [13‾20] oriented micropillars and <c+a> pyramidal slip in [0001] oriented micropillars. CRSSs were derived from the stress-strain curves as a function of test temperature for the three deformation modes. The CRSSbasal increased with decrease in test temperature for micron-sized pillars in comparison to an opposite trend for CRSSpyra: CRSStwin showed no change from room temperature to ~ −94°C.

The effect of subzero temperatures on the properties and structure of soy protein isolate emulsions

Different freezing temperatures (−5, −20, −40 and −80 ℃) could change soy protein isolate (SPI) structure and emulsion properties. After freezing at −5 ℃ and −20 ℃, the structure of the SPI loosened, the fluorescence intensity was red shifted, and the proportion of Phe, Tyr and Trp exposed increased. With decreasing temperature, the surface hydrophobicity (H0 × 100), the number of sulfhydryl groups and the number of disulfide bonds all rose, then fell (−40 ℃), and rose again (−80 ℃). The β-sheet content in the protein secondary structure increased from 32.71% (control) to 50.66% (−40 ℃) and then decreased to 37.05% (−80 ℃), while the β-turn and random coil contents showed the opposite pattern, which also confirmed aggregation. The emulsification performance of SPI after freezing treatment was decreased. The results of this study provide theoretical support for future production of frozen foods with added SPI.

Understanding the strength evolution of alkali-activated slag pastes cured at subzero temperature

As a kind of low-carbon cementitious materials, alkali-activated slag (AAS) can make contributions to the construction in cold areas because of its low freezing point of alkaline activator solution. This paper investigates the evolution mechanism of the strength and microstructure of AAS binders cured at −10 °C. Subzero-cured AAS pastes activated with different molarities of NaOH solution were made comparisons with the specimens cured at 20 °C in terms of various properties including setting time, compressive strength, pore structure, and microstructure. The results indicated that subzero temperature delayed the hydration process of AAS pastes, leading to prolonged setting times. The negative effects of subzero temperature were highlighted via the significantly decreased compressive strength, deteriorated pore structure, and porous microstructure. The phenomenon that subzero-cured AAS pastes exhibited lower compressive strength than ambient-cured specimens was attributed to the reduced reaction rate caused by subzero temperature, which influenced the formation position and compactness of hydration products. Interestingly, the compressive strength of subzero-cured AAS pastes still increased with the increase of NaOH molarity, which is believed to be associated with the fact that the increase in NaOH molarity caused the increase of hydration degree.

A temperature-dependent elastoplastic damage model for frozen sandstone

Subzero temperature has a significant effect on mechanical behaviors of rocks. When a sandstone is in subzero temperature environments, the sandstone exhibits temperature-dependent mechanical characteristics due to the freezing of pore water and the shrinkage of mineral particles. To disclose the influence of subzero temperature on the mechanical behaviors of the frozen sandstone, triaxial compressive tests were carried out at four confining pressures (0 MPa, 1 MPa, 3 MPa, 5 MPa) and four subzero temperatures (0 °C, −5 °C, −10 °C, −15 °C). The laboratory tests show that the frozen sandstone undergoes four typical deformation stages from initial loading to failure: compaction, elasticity, yield and postpeak failure stage. With the temperature decreasing, the compaction stage gradually shortens, and the nonlinear behavior of yield stage weakens. The peak strength and initial yield stress of the frozen sandstone increase, and the initial and ultimate yield surfaces expand gradually with decreasing temperature. Based on the deformation features of the frozen sandstone, an elastoplastic damage model is established by considering the effect of subzero temperature on the yield process and deformation. The elastoplastic damage model was calibrated by the experimental data, and the comparisons demonstrate that the proposed constitutive model well agrees with the stress–strain curves at the four subzero temperatures. An exponential evolution law of the macroscopic damage is found in the elastoplastic model, which is totally consistent with the microscopic damage variation monitored by the acoustic emission (AE) technology. Therefore, the temperature-dependent elastoplastic model can well predict the whole deformation behaviors of the sandstone under different subzero temperatures and gives a quantitative description on the damage evolution, and may serve as theoretical references for the construction and safety operation of frozen rock masses in cold regions.

Strength, Microstructure, and Deformation Behavior of Frozen Cemented Tailing Material

Cemented paste backfill (CPB) is a novel cementitious construction material that is used extensively for underground mine support and/or mine waste management worldwide. CPB consists of a mix of tailings (mine waste), water, and binder. In other words, tailings are transformed into construction materials. Key engineering properties required for the geotechnical design of CPB structures include strength and deformation behavior. However, there is a paucity of technical data and information about the effect of sub-zero temperatures on these properties. This paper presents an experimental study that focuses on the investigation of the influence of sub-zero curing temperatures on the strength, deformation behavior, and microstructure (e.g., pore structure, porosity, and binder hydration products) of CPB. Several CPB samples with various combinations of compositions (e.g., binder type, cement amount, and water content) cured at different temperatures (sub-zero and room temperatures) and times (7–90 days) are tested with regard to strength, stress–strain behavior, and microstructural characteristics. It is found that frozen CPB (FCPB) exhibits remarkable strength compared with unfrozen CPB and has a great resemblance to frozen soil. No obvious peak stress is observed from FCPB samples during compression, and strain hardening phenomena are commonly seen after the initial yield point. Factors that may affect the behavior of FCPB are thoroughly examined. Binder contents and types are found to be irrelevant; in contrast, water content plays a dominant role. This negligible effect of binder on the properties of FCPB is due to the inhibition of the binder hydration by sub-zero curing temperatures. FCPBs show coarser pore structure than unfrozen CPBs. The results obtained in this study have significant practical engineering applications for mines located in permafrost or cold regions.

Exposure duration and sub-zero temperature effects on concrete chloride diffusion decay index and binding

Accurate prediction of corrosion initiation period and service life of reinforced concrete structures requires accurate determination of their chloride diffusion coefficient and binding capacity. Temperature and exposure duration affect these properties, but there is paucity of knowledge regarding the effects of long-term exposure to sub-zero temperatures. Hence, the effects of sub-zero temperature, T, and exposure duration, t, on the chloride diffusion decay index, m, and binding in ordinary Portland cement concrete are investigated. Non-steady natural diffusion tests are conducted at + 5, 0, −5 and −15 °C for 3, 6, 12 and 18 months, and at the end of each period, the total and free chloride concentration profiles are determined. The index m is found to vary over a wide range, it being directly proportional to T and inversely to t. It drastically deviates from the commonly assumed value of 0.2 in the literature. Chloride binding is found to obey the Freundlich isotherm. The coefficient α of the isotherm equals 1.0 ± 0.16 and is only slightly dependent on T or t while its exponent β varies from 0.178 to 0.502, without exhibiting a systematic relationship with either parameter. Binding is noticed to increase with drop in temperature for the first three months of exposure, but over longer exposure durations, no systematic relationship is observed.

Long-Term Sequelae of Frostbite-A Scoping Review.

Frostbite is tissue damage caused by freezing temperatures and constitutes an important cause of morbidity in cold climate zones and high altitude. The direct effects of sub-zero temperatures lead to tissue freezing, electrolyte shifts and pH alterations, microvascular damage, and eventually to cell death. Upon rewarming, inflammatory reperfusion injury and thrombosis may lead to further tissue damage. Several studies and various case reports show that many patients suffer from long-term sequelae such as vasomotor disturbances (associated with susceptibility to refreezing), and neuropathic and nociceptive pain, as well as damage to skeletal structures. There are still many uncertainties regarding the pathophysiology of these sequelae. It has been shown that the transient receptor potential channel (TRP) family plays a role in cold allodynia. Botulinum Toxin type A (BTX-A) injections have been reported to be beneficial in vasomotor and neuropathic disturbances secondary to frostbite. Epidural sympathetic block has been used for short-term treatment of frostbite induced chronic pain. Furthermore, amitriptyline, gabapentinoids, and duloxetine may have some benefits. Frostbite arthritis clinically resembles regular osteoarthritis. In children there is a risk of epiphyseal cartilage damage leading to bone deformities. Despite some promising therapeutic concepts, the scarcity of data on frostbite long-term sequelae in the literature indicates the need of more in-depth studies of this pathology in all its aspects.

Характеристика суспензионных культур клеток Arabidopsis thaliana (L.) Heynh с изменённой экспрессией гена альтернативной оксидазы митохондрий АOX1a и анализ их морозоустойчивости

The enzyme alternative cyanide-resistant oxidase (AOX) localized in mitochondria is involved in the processes of plant adaptation to various unfavorable biotic and abiotic factors. Transfer of electrons from ubiquinone to oxygen by alternative oxidase has a nonprotonmotive character and, by bypassing two sites of H+ pumping in complexes III and IV, lowers the energy efficiency of respiration and energy of electron flow through AOX is released as heat. In this work, we characterized heterotrophic suspension cultures of Arabidopsis thaliana (L.) Heynh cells obtained from seeds of plants with altered (reduced (AS-12 line) and increased (XX-2 line)) expression of the alternative oxidase gene AOX1a and studied their viability under subzero temperature (-10 °С for 3, 6, 9 hours). Cell viability and reactive oxygen species (ROS) production were assessed using fluorescence microscopy with fluorescein diacetate (FDA) and propidium iodide (PI) for cell viability measurement and H2DCF-DA for ROS measurement. The proportion of living cells was calculated as the proportion of FDApositive and PI-negative cells. Differences between the studied lines were determined in the content of mitochondrial proteins of the respiratory chain (AOX, COXII, NDB) and uncoupling protein (UCP), as well as in the intensity of formation of ROS and frost resistance. The obtained results confirmed the higher content of the AOX protein and its high contribution to mitochondrial respiration in line XX-2. Suspension culture cells of the AS-12 line showed a decrease in the AOX protein content and its contribution to mitochondrial respiration, compared to the wild type (Col-0) and line XX-2. Simultaneously with a decrease in the AOX protein content in the AS-12 cell culture, an increase in the content of the uncoupling protein UCP and subunit II of cytochrome oxidase (COXII) was observed. ROS generation was reduced in cell cultures of both XX-2 and AS-12. The obtained results indicate that the cells of the wildtype (Col-0) suspension culture were subjected to the most significant effect of subzero temperature. Long-term exposure (for 9 h) under -10 °С revealed significant differences in the viability of wild-type culture cells and lines with altered AOX1a gene expression. Cells of line XX-2 with an increased content of AOX turned out to be more resistant to subzero temperature compared to wild-type and AS-12 cells. However, while the proportion of living cells in the culture of the AS-12 line 48 h after exposure remained at the same level as immediately after it, in the suspension culture of the wild type cell death developed over time. The obtained results indicate the importance of alternative oxidase in the development of frost resistance in plant cell.

The Aarhus Chamber Campaign on Highly Oxygenated Organic Molecules and Aerosols (ACCHA): particle formation, organic acids, and dimer esters from α-pinene ozonolysis at different temperatures

Abstract. Little is known about the effects of subzero temperatures on the formation of secondary organic aerosol (SOA) from α-pinene. In the current work, ozone-initiated oxidation of α-pinene at initial concentrations of 10 and 50 ppb, respectively, is performed at temperatures of 20, 0, and −15 ∘C in the Aarhus University Research on Aerosol (AURA) smog chamber during the Aarhus Chamber Campaign on Highly Oxygenated Organic Molecules and Aerosols (ACCHA). Herein, we show how temperature influences the formation and chemical composition of α-pinene-derived SOA with a specific focus on the formation of organic acids and dimer esters. With respect to particle formation, the results show significant increase in particle-formation rates, particle number concentrations, and particle mass concentrations at low temperatures. In particular, the number concentrations of sub-10 nm particles were significantly increased at the lower 0 and −15 ∘C temperatures. Temperature also affects the chemical composition of formed SOA. Here, detailed offline chemical analyses show that organic acids contribute from 15 % to 30 % by mass, with highest contributions observed at the lowest temperatures, indicative of enhanced condensation of these semivolatile species. In comparison, a total of 30 identified dimer esters were seen to contribute between 4 % and 11 % to the total SOA mass. No significant differences in the chemical composition (i.e. organic acids and dimer esters) of the α-pinene-derived SOA particles are observed between experiments performed at 10 and 50 ppb initial α-pinene concentrations, thus suggesting a higher influence of reaction temperature compared to that of α-pinene loading on the SOA chemical composition. Interestingly, the effect of temperature on the formation of dimer esters differs between the individual species. The formation of less oxidized dimer esters – with oxygen-to-carbon ratio (O:C)&lt;0.4 – is shown to increase at low temperatures, while the formation of the more oxidized species (O:C&gt;0.4) is suppressed, consequently resulting in temperature-modulated composition of the α-pinene-derived SOA. Temperature ramping experiments exposing α-pinene-derived SOA to changing temperatures (heating and cooling) reveal that the chemical composition of the SOA with respect to dimer esters is governed almost solely by the temperature at which oxidization started and is insusceptible to subsequent changes in temperature. Similarly, the resulting SOA mass concentrations were found to be more influenced by the initial α-pinene oxidation temperatures, thus suggesting that the formation conditions to a large extent govern the type of SOA formed, rather than the conditions to which the SOA is later exposed. For the first time, we discuss the relation between the identified dimer ester and the highly oxygenated organic molecules (HOMs) measured by chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometer (CI-APi-ToF) during the ACCHA experiments. We propose that, although very different in chemical structures and O:C ratios, many dimer esters and HOMs may be linked through similar RO2 reaction pathways and that dimer esters and HOMs merely represent two different fates of the RO2 radicals.

Effect of sub-zero temperatures on wear resistance of AISI P20 tool steel

In the present scenario, modernization of machine tool is the main objective to get the desired properties, so as to increase the tool steel life. In this research, we aim to improve the wear resistance of AISI P20 tool steel. Sub-cooling treatment is a simple technique that can extend the operational life of tool steel. In this study, P20 tool steel is treated at three different sub-cool temperatures. As the sub-cooled temperature decreases, the hardness and wear resistance of the metal increase significantly. The micro-hardness value of the − 150oC sub-cooled metal is found in the range of 320–330 HV which is having 35% more hardness as compared to the as-received untreated metal. There is also 28% increase in the wear resistance of − 150oC sub-cooled metal. The sub-zero temperature has shown its significant impact on microstructure, hardness and mechanical behavior of tool steel which directly improves the operational life of tool steel in the industrial environment.

Climate research in Arctic regions of Siberia

The paper shows the impact of recent changes in air temperature and wind speed on the living conditions of human population in the Arctic regions of Siberia (to the north of 66°N and within 66-162°E). The combined effect of subzero temperatures and wind speed on the thermal status of the exposed surface of the human body was evaluated as a proxy indicator for the severity of the climate. As average monthly temperatures increase, this indicator decreases in the Arctic territories of Siberia. The dynamics of daily fluctuations of threshold values of the weather severity index from October to April from 1966/67 to 2017/2018 has been analysed for the Verkhoyansk station. We found a decrease in the number of days prohibiting and an increase in the number of days restricting a person’s stay outdoors. In the more recent period from 1981/82 to 2017/18, a noticeable decrease in the number of days with prohibitive weather conditions was observed. At the same time, little variation was observed in the average number of days with restrictions in outdoor activities over longer time intervals: the difference between 1936-1964, 1966-2018, and 1981-2018 periods was less than five days.

Low-Temperature Behavior of Lithium Metal Anodes in Carbonate and Ether Electrolytes

The development of energy storage devices that can efficiently operate at temperatures below 0 °C is necessary for both terrestrial and space-based applications. NASA has previously carried out pioneering work that has enabled Li-ion batteries to operate at temperatures down to -30 °C through the development of new carbonate electrolyte formulations. [1-2] However, secondary batteries that utilize lithium metal anodes instead of conventional graphite anodes could significantly boost energy density and specific energy, but the behavior of lithium electrodes at low temperatures are largely unknown. In this work, we investigate the effect of sub-zero temperatures on electrochemical behavior, morphological evolution, and solid-electrolyte interphase (SEI) formation of Li metal anodes in carbonate and ether solvents. Electrolytes made with solvents such as ethylene carbonate and ethyl methyl carbonate with ester co-solvents (e.g., ethyl propionate) exhibit sufficient conductivity and show effective Li deposition/stripping down to -30 °C, but with relatively low average Coulombic efficiencies (CEs) of ~70 % (similar to the room-temperature CE of ~73 %). The introduction of SEI-stabilizing additives such as fluoroethylene carbonate (FEC) boosts the average Coulombic efficiency across a range of temperatures (CE = ~94 %). Ether-based electrolytes, on the other hand, enable Li deposition/stripping down to -70 °C with high CEs (~95 %). Interestingly, investigation of the Li morphology after cycling revealed that distinct Li morphologies form at different temperatures, which may be related to differences in SEI chemistry and structure. These results provide important new information for engineering Li metal anodes for efficient operation under a variety of conditions.

Sublethal effects of subzero temperatures on the light brown apple moth, Epiphyas postvittana: fitness costs in response to partial freezing.

Population responses to environmental extremes often dictate the bounds to species' distributions. However, population dynamics at, or near, those range limits may also be affected by sublethal effects. We exposed late instars and pupae of an invasive leafroller, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), to cold temperatures and measured the effects of exposure on subsequent survivorship, development, and reproduction. Cold temperature was applied as acute exposure to -10 °C (a low, but not immediately lethal temperature for this species) or the onset of freezing (the peak of the supercooling point exotherm). Survival was defined as the ability to successfully eclose as an adult. We measured immature development times, pupal mass, and adult longevity as proxies of fitness in survivors. Additionally, surviving insects were mated with individuals that had not been exposed to cold to measure fertility. There was no difference between the proportion of larvae or pupae that survived acute exposure to -10 °C and those exposed to the control temperature. Approximately 17% of larvae and 8% of pupae survived brief periods with internal ice formation and continued development to become reproductively viable adults. Importantly, surviving the onset of freezing came with significant fitness costs but not to exposure to -10 °C; most insects that survived partial freezing had lower fertility and shorter adult lifespans than either the -10 °C or control group. These results are discussed within the context of forecasting invasive insect distributions.

The effect of sub-zero temperature on the formation and composition of secondary organic aerosol from ozonolysis of alpha-pinene.

This study presents a newly constructed temperature controlled cold-room smog chamber at Aarhus University, Denmark. The chamber is herein utilized to study the effect of sub-zero temperature on the formation and chemical composition of secondary organic aerosol (SOA) from ozone initiated oxidation of α-pinene. The chemical composition of α-pinene SOA formed from dark ozonolysis of α-pinene at 293 K and 258 K was investigated using High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) and Ultra-High Performance Liquid Chromatography/Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry (UHPLC/ESI-qToF-MS). For comparison, an OH-initiated oxidation experiment was performed at 293 K. In ozonolysis experiments it was found that oxygen-to-carbon (O : C) ratios were higher in the particles formed at 293 K compared to 258 K. A total of 16 different organic acids and 30 dimers esters were quantified in the collected particles composing up to 34% of the total α-pinene SOA mass with increased mass fraction of carboxylic acids in particles from α-pinene ozonolysis at 258 K compared to 293 K. In contrast, dimer esters showed suppressed formation at the sub-zero reaction temperature, thus contributing 3% to SOA mass at 258 K while contributing 9% at 293 K. SOA formed in the OH-initiated oxidation of α-pinene at 293 K resulted in low concentrations of dimer esters supporting Criegee intermediates as a possible pathway to dimer ester formation. Vapour pressure estimates of the identified carboxylic acids and dimer esters are presented and show how otherwise semi-volatile carboxylic acids at sufficiently low temperatures may classify as low or even extremely low volatile organic compounds (ELVOC), thus may add to an enhanced particle formation observed at the sub-zero temperature through gas-to-particle conversion. The change in chemical composition of the SOA particles with temperature is ascribed to a combination of effects: the decreased vapour pressures and hence increased condensation of carboxylic acids from the gas phase to the particle phase along with suppressed formation of the high molecular weight dimer esters and different gas and particle phase chemistry results in particles of different chemical composition as a consequence of low reaction temperatures.

Thermal Destabilization of Collagen Matrix Hierarchical Structure by Freeze/Thaw.

This study aims to characterize and understand the effects of freezing on collagen structures and functionality. Specifically, thermodynamic destabilization of collagen at molecular- and fibril-levels by combination of low temperatures and freezing were experimentally characterized using modulated differential scanning calorimetry. In order to delineate the effects of sub-zero temperature and water-ice phase change, we hypothesized that the extent of destabilization can be determined based on post-thaw heat induced thermal denaturation of collagen. It is found that thermal denaturation temperature of collagen in hydrogel decreases by 1.4–1.6°C after freeze/thaw while no such decrease is observed in the case of molecular solution. The destabilization is predominantly due to ice formation. Exposure to low temperatures in the absence of ice has only minimal effect. Calorimetry measurements combined with morphological examination of collagen matrices by scanning electron microscopy suggest that freezing results in destabilization of collagen fibrils due to expansion of intrafibrillar space by ice formation. This fibril-level damage can be alleviated by use of cryoprotectant DMSO at concentrations as low as 0.5 M. A theoretical model explaining the change in collagen post-thaw thermal stability by freezing-induced fibril expansion is also proposed.

Optimization of Aging Cycle of Stainless Maraging Steel Using Dilatometric and Differential Scanning Calorimetric Analysis to Improve its Strength

04X13H5M5К9Л is a stainless maraging steel with high strength and excellent toughness. This alloy has been selected for making Impeller casting which is subjected to localized stresses reaching as high as 700 MPa at the tip of its vanes. The impeller rotates at 19000 RPM for 253 seconds.This alloy comprises of austenitic – martensitic dual phase at room temperature, in which martensite exhibits Body Centered Cubic (BCC) structure with moderate hardness (HRC 30) and high toughness. The required strength is achieved by precipitation of second phase particles in soft martensite matrix at suitable aging temperature. There is always a chance of reversion of martensite to austenite on heating either for extended period or at higher temperature. Thus optimization of aging temperature and time becomes highly critical keeping in mind the functional stress requirements of the component. The optimization has been done using Differential Scanning Calorimetric (DSC) analysis, dilatometric studies and experimental iterations of heat treatment temperatures and aging time.Different combination of strength and ductility could be achieved by varying heat treatment parameters. Martensite start and finish temperatures and austenitic reversion temperatures were established through DSC and dilatometric studies. Effect of subzero temperature on properties and microstructure of this steel is also presented here. Scanning Electron Microscopy and Energy Dispersive Spectroscopy were also carried out to analyze phases in different heat treatment conditions.