Enthalpy Release Research Articles

EFFECTS OF COMPOSITION OF GLUTEN-FREE BINARY GELS ON THE RETENTION AND RELEASE OF AROMA COMPOUNDS

Background. Gluten-free binary gels (BGs) composed of polysaccharides and animal proteins appear to be food products of great practical importance due to the constantly increasing risk of coeliac disease. Moreover, understanding the factors influencing the stability of aroma compounds (ACs) is very important because consumer acceptance of food is mainly dependent on aroma. The objective of this research was to determine the thermodynamic and kinetic behavior of ACs in BGs prepared with starches varying in amylose content depending on the concentration of myofibrillar proteins (MPs). BGs were prepared with 8 % of waxy rice starch (WRS) or tapioca starch (TS) and MPs ranged from 2 to 12 %. BGs were aromatized with: 1-heptanol, nonanal, 2-butanone, ethyl acetate, s-(+)-carvone. The studies included measurements of: rheological properties of BGs, non-covalent interactions between myosin and ACs, retention and release of ACs and enthalpy of ACs vaporization (ΔHvap). Results and conclusions. The main non-covalent interactions between myosin and ACs were hydrogen bonds and hydrophobic interactions. Retention and release parameters, determined during refrigerated storage (12 days) were statistically significantly (p ≤ 0.05) dependent on: starch type, MPs concentration and physicochemical properties of ACs. The ACs release rate constants were statistically significantly (p ≤ 0.05) correlated with those of rheological parameters derived from first order kinetic equation. ΔHvap values found in TS-based BGs were generally lower (p ≤ 0.05) than in counterparts prepared using WRS. BGs with a relatively high protein content can be intended for direct consumption and as subsystems for other food products to increase ACs stability.

Simple Growth-Metabolism Relations Are Revealed by Conserved Patterns of Heat Flow from Cultured Microorganisms.

Quantitative analyses of cell replication address the connection between metabolism and growth. Various growth models approximate time-dependent cell numbers in culture media, but physiological implications of the parametrizations are vague. In contrast, isothermal microcalorimetry (IMC) measures with unprecedented sensitivity the heat (enthalpy) release via chemical turnover in metabolizing cells. Hence, the metabolic activity can be studied independently of modeling the time-dependence of cell numbers. Unexpectedly, IMC traces of various origins exhibit conserved patterns when expressed in the enthalpy domain rather than the time domain, as exemplified by cultures of Lactococcus lactis (prokaryote), Trypanosoma congolese (protozoan) and non-growing Brassica napus (plant) cells. The data comply extraordinarily well with a dynamic Langmuir adsorption reaction model of nutrient uptake and catalytic turnover generalized here to the non-constancy of catalytic capacity. Formal relations to Michaelis–Menten kinetics and common analytical growth models are briefly discussed. The proposed formalism reproduces the “life span” of cultured microorganisms from exponential growth to metabolic decline by a succession of distinct metabolic phases following remarkably simple nutrient–metabolism relations. The analysis enables the development of advanced enzyme network models of unbalanced growth and has fundamental consequences for the derivation of toxicity measures and the transferability of metabolic activity data between laboratories.

Hydrogen Based Compounds as Energetic Catalysts for Liquid Rocket Engines: Implications and Applications

In the current scenario of space propulsion, liquid propellants have significantly proved useful in the upper stage rocket engines. Over the past couple decades, the world had inclined positively towards cryogenic fuel(s) viz., liquid oxygen and liquid hydrogen due to their high specific impulse. A higher specific impulse implies lower duration to achieve design cruise velocity for a given rocket initial and instantaneous mass. Liquid hydrogen and liquid oxygen as fuel and oxidizer can generate one of the highest enthalpy release in combustion, producing a specific impulse of up to 450 seconds at an effective exhaust velocity of 4.4 kilometres per second. Whereas, selected disadvantages are encountered in the form of storage and production. This indicates overdependence on cryogenic propellants and has necessitated the active research effort for better alternatives. As an interesting alternative, the combination of Dinitrogen Tetroxide (N2O4) and Monomethyl Hydrazine (MMH) have been used for many space applications owing to an extreme storage stability and hypergolic nature. Present study aims to express the effect of hydrogen-based compounds on the rocket performance. Four distinctive compounds from two groups of hydrogen-based compounds are tested with the varying oxidizer and fuel proportions to obtain a new, cost-effective and user-friendly composition that can be prepared at room temperature. The investigation attempt and explains the effect of hydrogen based energetic propellants using N2O4 and MMH as the base composition for upper stage performance. The work is motivated by the need of efficient space operations with attractive propulsive alternatives to minimize over-dependence on cryogenics, which will ultimately result in cost effectiveness. Various energetic materials were tested with the base composition by using standard NASA-CEA complex chemical equilibrium model. The performance was evaluated in terms of variation in specific impulse and characteristic velocity both of which are significant parameters. To, validate the practical utility, the role of chamber pressure, supersonic area ratio and optimal Oxidizer to fuel ratio (O/F) was determined. The work led to two interesting findings, a composition of beryllium hydride with base composition for high performance of rockets and the negative impact of hydrogen on liquid propellants.

Effects of Annealing on Enthalpy Recovery and Nanomechanical Behaviors of a La-Based Bulk Metallic Glass

Structural relaxation and nanomechanical behaviors of La65Al14Ni5Co5Cu9.2Ag1.8 bulk metallic glass (BMG) with a low glass transition temperature during annealing have been investigated by calorimetry and nanoindentation measurement. The enthalpy release of this metallic glass is deduced by annealing near glass transition. When annealed below glass transition temperature for 5 min, the recovered enthalpy increases with annealing temperature and reaches the maximum value at 403 K. After annealed in supercooled liquid region, the recovered enthalpy obviously decreases. For a given annealing at 393 K, the relaxation behaviors of La-based BMG can be well described by the Kohlrausch-Williams-Watts (KWW) function. The hardness, Young’s modulus, and serrated flow are sensitive to structural relaxation of this metallic glass, which can be well explained by the theory of solid-like region and liquid-like region. The decrease of ductility and the enhancement of homogeneity can be ascribed to the transformation from liquid-like region into solid-like region and the reduction of the shear transition zone (STZ).

Multi-Phase Impact on the Heat Load Characteristics of a Multi-Element Methane-Oxygen Rocket Thrust Chamber

Aiming at the capability to numerically model relevant operating conditions in rocket thrust chambers, this research work introduces a real gas multi-phase data extension to investigate the impact of water condensation within a widely studied sub-scale research test case. Following a dense gas approach and embedded into a state-of-the-art timescale based frozen non-adiabatic flamelet combustion model able to capture the characteristic hydrocarbon chemical reaction and recombination phenomena, the pre-computed manifold incorporates the underlying physical processes to describe the properties of the fluid mixture. The modeling framework is initially evaluated against simplified non-reacting experimental studies of water condensation along a cooled wall structure in the presence of non-condensable gases to confirm its ability to predict the phase change impact on the wall heat transfer. The good agreement with experimental heat load data is further confirmed in the frame of a conjugate heat transfer simulation of a multi-element methane-oxygen rocket thrust chamber, for which the design of the regenerative cooling system leads to localized condensation effects as the wall temperature remains below saturation conditions. The associated release of the formation enthalpy coupled with a change of the fluid properties captured by the newly developed model formulation leads to higher wall heat loads compared to the numerical predictions of an ideal gas single-phase model.

Kinetic incentive of hydrogen addition on nonpremixed laminar methane/air flames

In order to find out the respective influences of chemical reactivity and physical transport of hydrogen additive on nonpremixed flame, two fabricated hydrogen additions were introduced into nonpremixed methane/air flame modeling. Hydrogen addition was assumed as inert gas or partial reactivity fuel to respectively explore the kinetic reasons by the three aspects: the elementary reaction route, heat release, and physical diffusion of hydrogen addition. The analyses were implemented in terms of OH and H production. Results showed that, hydrogen addition can enhance OH and H production via elementary reactions, and causes flame reaction zone migration through the coupling interaction between the low-temperature heat enthalpy release and diffusion behavior of hydrogen addition. R84 (OH + H2=H + H2O) and R38 (H + O2=O + OH) are the most important elementary reactions related to OH and H production. The physical incentive of hydrogen addition can hardly work without the chemical effects of hydrogen addition.

Optically Triggered Synchronous Heat Release of Phase‐Change Enthalpy and Photo‐Thermal Energy in Phase‐Change Materials at Low Temperatures

AbstractPhase‐change materials (PCMs) are used in several energy recycling utilization systems due to their latent‐heat‐storage and ‐release ability. However, the inability of PCMs to release heat at temperatures below their freezing point limits their application in distributed energy utilization systems. This paper reports optically‐triggered low‐temperature heat release in PCMs based on a solid–liquid phase change (PC) controlled by the trans–cis (E–Z) photo‐isomerization of azobenzene. To achieve this, a photo‐responsive alkyl‐grafted Azo is incorporated into tetradecane (Ted) to create a photo‐sensitive energy barrier for the PC. The Azo/Ted composite exhibits controllable supercooling (4.04–8.80 °C) for heat storage and achieves synchronous heat release of PC enthalpy and photo‐thermal energy. In addition, the Azo reduces the crystallization of Ted by intercalating into its molecular alignment. Furthermore, under light illumination, the Azo/Ted composite releases considerable heat (207.5 J g−1) at relatively low temperatures (−1.96 to −6.71 °C). The temperature of the annular device fabricated for energy utilization increases by 4 °C in a low‐temperature environment (−5 °C). This study will pave the way for the design of advanced distributed energy systems that operate by controlling the energy storage/release of PCMs over a wide range of temperatures.

Chemical effects of hydrogen addition on low-temperature oxidation of premixed laminar methane/air flames

Hydrogen addition exhibits its chemical incentive on hydrocarbon flames via its reactivity in elementary reactions and the heat enthalpy release in self-oxidation. In the study, a one-step reaction kinetics was provided to fictitious hydrogen addition for separating the hydrogen reactivity effect on radical formation from the heat release effect on flame temperature change. The numerical investigation was conducted by adding the fictitious hydrogen into premixed flame modeling. The structures of the flame preheat zone were compared in terms of the profiles of flame temperature, the build-up of active radical pool and the consumption rates of methane and hydrogen addition. Comparative analyses showed that the reactivity and the heat release of hydrogen addition could both lead to the early boosting of flame oxidation, but showed different dependences on hydrogen enrichment rate. Reaction path analyses were conducted to explore the kinetic reasons for the above observations. The results suggested that OH + H2 = H + H2O was an apparent reaction route to show the reactivity of hydrogen addition, whereas the enhanced triangular reaction loops between OH, H and HO2 were the key factors for strengthening the low-temperature oxidation of hydrogen-enriched methane/air flames. Besides, the effects of the heat release and the reactivity of hydrogen addition on the early build-up of flame kernel were quantified.

Engineering medium-range order and polyamorphism in a nanostructured amorphous alloy

Like crystalline materials, the properties of amorphous materials can be tailored by tuning the local atomic-to-nanoscale structural configurations. Polyamorphism is evident by the coexistence of kinetically stabilized amorphous structures with tailorable short-to-medium-range orders, providing a viable means to engineer the degree of local order and heterogeneity. Here, we report experimental evidence of the coexistence of liquid-like and solid-like amorphous phases in a Ni82P18 amorphous alloy with enhanced thermal stability and plasticity prepared by pulsed electrodeposition. The two amorphous phases, of comparable volume fraction of ~50% each, have similar short-range order but are distinguished by packing at the medium-range length scale (>6 Å). Upon heating, a structure crossover at ~450 K was observed, where the liquid-like structure transforms to the solid-like structure, as evidenced by the enthalpy release and an anomalous contraction of atomic structure over the medium-range length scale, due to the metastable nature of the liquid-like structure.

Structural evolution in a melt-quenched zeolitic imidazolate framework glass during heat-treatment.

A pronounced enthalpy release occurs around 1.38Tg in the prototypical metal-organic framework glass formed from ZIF-4 [Zn(C3H3N2)2], but there is no sign for any crystallization (i.e., long-range ordering) taking place. The enthalpy release peak is attributed to pore collapse and structural densification.

Enthalpy relaxation of low molecular weight amorphous styrene oligomers measured with an adiabatic calorimeter

Herein, we measured the enthalpy relaxation of three styrene oligomers with different molecular weights (styrene oligomer with Mw = 4.53 × 102: PSA-300, styrene oligomer with Mw = 5.89 × 102: PSA-500, and styrene oligomer with Mw = 1.01 × 103: PSA-1000) near their respective glass transition temperatures (Tg) using an adiabatic calorimeter. We determined the relaxation rates and the amounts of configurational enthalpy released from the temperature dependence of the temperature drift rates around Tg. Based on our experimental findings, we found the amounts of configurational enthalpy release per monomer unit to be 0.8, 3.5, and 1.6 kJ mol−1 for PSA-300, PSA-500, and PSA-1000, respectively. These values were 3.9–18 times larger than that of glycerol, which is a typical glass-forming liquid.

Thermal analysis of electrodeposited nano-grained Ni-Mo alloys

Enthalpy release of electrodeposited nano-grained Ni and Ni-Mo alloys with grain size ranging from 29.3 to 5.9 nm and their corresponding annealed samples were investigated by using differential scanning calorimetry. Grain boundary energy of the annealed samples was estimated from enthalpy release and corresponding grain size. Results indicated that the enhanced grain boundary stability in the as-annealed ng Ni-Mo alloys is ascribed to kinetically grain boundary pinning by Mo instead of grain boundary energy reduction.

The pitfalls of empirical fitting of glass relaxation data with stretched exponents

A stretched exponent is commonly used to fit experimental relaxation data for glasses, which typically exhibit a range of time constants. While it has been supported by theory for various processes, mostly near and above the glass transition temperature, Tg, it is also commonly used in phenomenological fits below Tg without a mechanistic model. The properties of the stretched exponent, and sample data for the enthalpy release and dynamic-mechanical response of metallic glasses, are used to show that in the absence of a mechanistic model, stretched-exponent fits easily lead to artifacts and flawed conclusions.

A sensitive calorimetric technique to study energy (heat) exchange at the nano-scale.

Every time a chemical reaction occurs, an energy exchange between reactants and the environment takes place, which is defined as the enthalpy of the reaction. During the last few decades, research has resulted in an increasing number of devices at the micro- or nano-scale. Sensors, catalyzers, and energy storage systems are more and more developed as nano-devices which represent the building blocks for commercial "macroscopic" objects. A general method for the direct evaluation of the energy balance of such systems is not available at present. Calorimetry is a powerful tool to investigate energy exchange, but it usually requires macroscopic sample quantities. Here, we report on the development of an original experimental setup able to detect temperature variations as low as 10 mK in a sample of ∼10 ng using a thermometer device having physical dimensions of 5 × 5 mm2. This technique has been utilized to measure the enthalpy release during the adsorption process of H2 on titanium-decorated monolayer graphene. The sensitivity of these thermometers is high enough to detect a hydrogen uptake of ∼10-10 moles, corresponding to ∼0.2 ng, with an enthalpy release of about 23 μJ. The experimental setup allows, in perspective, scalability to even smaller sizes.

Preparation of mono-dispersed, high energy release, core/shell structure Al nanopowders and their application in HTPB propellant as combustion enhancers

Mono-dispersed, spherical and core/shell structure aluminum nanopowders (ANPs) were produced massively by high energy ion beam evaporation (HEIBE). And the number weighted average particle size of the ANPs is 98.9 nm, with an alumina shell (3–5 nm). Benefiting from the passivation treatment, the friction, impact and electrostatic spark sensitivity of the ANPs are almost equivalent to those of aluminum micro powders. The result of TG-DSC indicates the active aluminum content of ANPs is 87.14%, the enthalpy release value is 20.37 kJ/g, the specific heat release S1/Δm1* (392–611 °C) which determined the ability of energy release is 19.95 kJ/g. And the value of S1/Δm1* is the highest compared with ANPs produced by other physical methods. Besides, the ANPs perfectly compatible with hydroxyl-terminated polybutadiene (HTPB), 3 wt. % of ANPs were used in HTPB propellant replaced micron aluminum powders, and improved the burning rate in the 3–12 MPa pressure range and reduced the pressure exponential by more than 31% in the 3–16 MPa pressure range. The production technology of ANPs with excellent properties will greatly promote the application of ANPs in the field of energetic materials such as propellant, explosive and pyrotechnics.

Phase transitions and glass transition in a hyperquenched silica‐alumina glass

AbstractWe investigate phase transitions, glass transition, and dynamic behavior in the hyperquenched 69SiO2–31Al2O3 (mol%) glass (SA glass). Upon reheating, the SA glass exhibits a series of thermal responses. Subsequent to the sub‐Tg enthalpy release, the glass undergoes a large jump in isobaric heat capacity (ΔCp) during glass transition, implying the fragile nature of the SA glass. The mullite starts to form before the end of glass transition, indicating that the SA glass is extremely unstable against crystallization. After the mullite formation, the remaining glass phase exhibits an increased Tg and a suppressed ΔCp. The formation of cristobalite at 1553 K indicates the dominance of silica in the remaining glass matrix. The cristobalite gradually re‐melts as the isothermal heat‐treatment temperature is raised from 1823 to 1853 K, which is well below the melting point of cristobalite, while the amount of the mullite remains unchanged.

Intrinsic correlation between β-relaxation and spatial heterogeneity in a metallic glass.

β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.

Dynamic CO2 adsorption performance of internally cooled silica‐supported poly(ethylenimine) hollow fiber sorbents

The dynamic adsorption behavior of CO2 under both nonisothermal and nearly isothermal conditions in silica supported poly(ethylenimine) (PEI) hollow fiber sorbents (Torlon®‐S‐PEI) is investigated in a rapid temperature swing adsorption (RTSA) process. A maximum CO2 breakthrough capacity of 1.33 mmol/g‐fiber (2.66 mmol/g‐silica) is observed when the fibers are actively cooled by flowing cooling water in the fiber bores. Under dry CO2 adsorption conditions, heat released from the CO2‐amine interaction increases the CO2 breakthrough capacity by reducing the severity of the diffusion resistance in the supported PEI. This internal resistance can also be alleviated by prehydrating the fiber sorbent with a humid N2 feed. The CO2 breakthrough capacity of prehydrated fibers is adversely affected by the release of the adsorption enthalpy (unlike the dry fibers); however, active cooling of the fiber results in a constant CO2 breakthrough capacity even at high CO2 delivery rates (i.e., high adsorption enthalpy delivery rates). In full RTSA cycles, a purity of 50% CO2 is achieved and the adsorption enthalpy recovery rate can reach ∼72%. Studies on the cyclic stability of uncooled fiber sorbents in the presence of SO2 and NO contaminants indicate that exposure to NO at 200 ppm over 120 cycles does not lead to a significant degradation of the sorbents, but SO2 exposure at a similar high concentration of 200 ppm causes 60% loss in CO2 breakthrough capacity after 120 cycles. A simple amine reinfusion technique is successfully demonstrated to recover the adsorption capacity in poisoned fiber sorbents after deactivation by exposure to impurities such SO2. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3878–3887, 2014

High-Resolution Polarimetric Radar Observations of Snow-Generating Cells

AbstractHigh-resolution X-band polarimetric radar data were collected in 19 snowstorms over northern Colorado in early 2013 as part of the Front Range Orographic Storms (FROST) project. In each case, small, vertically erect convective turrets were observed near the echo top. These “generating cells” are similar to those reported in the literature and are characterized by ~1-km horizontal and vertical dimensions, vertical velocities of 1–2 m s−1, and lifetimes of at least 10 min. In some cases, these generating cells are enshrouded by enhanced differential reflectivity ZDR, indicating a “shroud” of pristine crystals enveloping the larger, more isotropic particles. The anticorrelation of radar reflectivity factor at horizontal polarization ZH and ZDR suggests ongoing aggregation or riming of particles in the core of generating cells. For cases in which radiosonde data were collected, potential instability was found within the layer in which generating cells were observed. The persistence of these layers suggests that radiative effects are important, perhaps by some combination of cloud-top cooling and release of latent enthalpy through depositional and riming growth of particles within the cloud. The implications for the ubiquity of generating cells and their role as a mechanism for ice crystal initiation and growth are discussed.

Structural and calorimetric studies of two crystallization stages of Ag10As30S60 glassy alloys

The structure of the as-prepared and thermal annealed Ag10As30S60 chalcogenide glass is characterized using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) curves recorded at four different heating rates are analyzed to determine the glass and crystallization transition temperatures, thermal stability and enthalpy release. Two separated crystallization peaks are observed in the DSC curves. XRD results indicate the precipitation of AgAsS4 crystal phase is responsible for the first peak. Numerous phases with S8 dominant phase are accountable for the second peak. The crystallization kinetics such as the activation energy for the crystallization (Ec), the frequency factor (Ko) and the crystallization rate constant K are determined for each crystallization stage. The results show that the crystallization rate constant for the first crystallization stage is about six times larger than that of the second crystallization step.