Volume Transition Temperature Research Articles

Operational and structural parametric analysis of a photovoltaic/thermal system integrated with phase change material

Applying PCM to the photovoltaic/thermal (PV/T) system can adjust the operating temperature of the photovoltaic cell and further enhance system efficiency. Hence, in the present study, a novel PV/T-PCM collector by connecting the PCM with the heat-absorbing plate in parallel is first proposed. Then, the dynamic model of the PV/T-PCM system is established and verified. Finally, the sensitivity analysis is performed on key parameters, including the water mass flow rate, water tank volume, PCM thickness, and phase transition temperature. The results show that raising the water mass flow rate improves electrical gain, total heat gain, and total system efficiency; however, their change gradient is all inconspicuous as the water mass flow rate is greater than 0.02 L/s. As the water tank volume expands from 75 L to 105 L, electrical gain, total heat gain, and total system efficiency are improved. Thickening the PCM can promote the electrical gain, while the total system efficiency and total heat gain will increase slightly at first and then decrease. The best thickness is 0.015 m. When the phase transition temperature increases from 40 ℃ to 50 ℃, the total system efficiency and total heat gain decline, but the electrical gain decreases insignificantly. The changes of the above four parameters all affect the complete melting ratio of PCM as well as the phase transition start time on the entrance side and exit side of the collector.

Gas Permeability through Polyimides: Unraveling the Influence of Free Volume, Intersegmental Distance and Glass Transition Temperature.

The relationships between gas permeability and free volume fraction, intersegmental distance, and glass transition temperature, are investigated. They are analyzed for He, CO2, O2, CH4, and N2 gases and for five similar polyimides with a wide range of permeabilities, from very low to extremely high ones. It has been established here that there is an exponential relationship between permeability and the free volume fraction, and between permeability and the most probable intersegmental distance as measured by WAXS; in both cases, with an exponential coefficient that depends on the kinetic gas diameter as a quadratic polynomial and with a preexponential positive constant. Moreover, it has been proven that the intersegmental distance increases linearly with the free volume fraction. Finally, it has been established that the free volume fraction increases with the glass transition temperature for the polymers tested, and that they depend on each other in an approximate linear way.

Study on the effect of bismuth oxide on the structural, optical and dielectric properties of strontium phosphate glasses

ABSTRACT The melt-quenching technique was used to prepare strontium phosphate glasses with bismuth oxide. The impact of replacing Bi3+ with Sr2+ on density (d), molar volume (Vm), and glass transition temperature (Tg) was investigated. The FTIR spectra revealed that the structural units of the samples consisted of [BiO3], [BiO6], [PO3], and [PO4]. UV-visible spectroscopy was used to study the optical transmittance, energy band gap (Eopt), and Urbach energy (Etail). The Wemple-Didomenico model successfully described the dispersive behavior of the refractive index. Impedance spectroscopy is utilized to determine the electrical and dielectric properties. The AC conductivity followed Jonscher's universal power law, and it was observed that as the concentration of Bi2O3 increased, the frequency exponent (s) decreased. The dielectric constant (ϵ’) and dielectric loss (tan δ) were found to be frequency and composition dependent. The parameters describing the dispersion of dielectric constant are obtained through the Cole-Davidson relation.

Negative magnetization, shielding current effect and divalent manganese in CaMn1-xTaxO3 manganites

Crystal and electronic structures, and magnetic properties of polycrystalline tantalum substituted manganites CaMn1-xTaxO3 (x = 0.00, 0.03, 0.06, 0.09 and 0.12) were studied by x-ray diffraction, x-ray absorption near edge spectroscopy (XANES) at Mn K-edge, and by magnetization measurements. Large changes of cell volume and magnetic transition temperature, in addition to shift of Mn K-edge spectra and change of shape of XANES spectra, indicate the existence of Mn3+ as well as Mn2+ species induced by the substitution of Mn4+ by Ta5+. Cell parameters and cell volume increase with increasing x because the Mn3+, Mn2+ and Ta5+ ions are larger that substituted Mn4+ ion. According to XANES results, at least the x = 0.00 sample is oxygen deficient. The negative magnetization was observed in the CaMnO3-δ sample, and a possible explanation of the phenomenon is given considering two canted (almost antiferro)magnetic sublattices and a shielding current, associated with the itinerant eg electrons, induced by the oxygen vacancies.

Galactose-functionalized injectable thermosensitive microgels for target specific uptake in hepatoma cells.

Microgels acts as a potential candidate for responsive composite materials, which has been favored by scientists because of its excellent colloid stability, easy integration, and most of their surface area can be used as support after modification. Specififically, microgels are fascinating capable of maintaining good biocompatibility and controlled-release in vivo and making the possible for applications in biomaterials and biomedicines. Besides, in the process of microgel synthesis, some targeting factors can be combined to achieve the purpose of cell targeting and uptake. Therefore, how to fundamentally design microgels is an urgent problem to be solved. In this study, we design and synthesize an injectable microgel P(DEGMA-co-OVNGal) that is made of 2-methyl-2-acrylate-2-(2-methoxy ethoxy) ethyl ester (DEGMA) and glycopolymer (OVNGal) that is thermoresponsive and contains galactose. When the content of crosslinking agent is regulated, the microgel will realize the transformation from sol to gel at the temperature around human body, causing the controlled release of the loaded drugs. With the increase of crosslinker content from 1% to 7%, the appearance of microgel changed from loose and ordered to compact and hard morphology, the swelling ratio of microgel decreased from 187% to 142%, and the phase volume transition temperature decreased from 29.2°C to 28°C. The results indicated that when the monomer ratio (DEGMA: OVNGal) increased from 2:1 to 40:1 with the amount of crosslinking agent at 1%, the particle size of microgel increased from 460nm to 660nm. In vitro released studies confirmed that the cumulative released DOX (doxorubicin, as a selected model drug) from the microgel can reach 50% after 7days. Furthermore, in vitro experiments demonstrated that the injectable microgel P(DEGMA-co-OVNGal) can target HepG2 cells effectively and meantime dispalys excellent biocompatibility. Therefore, the injected microgels based on P(DEGMA-co-OVNGal) have the potential to serve as a robust and promising drug delivery carrier for targeted cancer therapy.

Effect of tungsten metal-oxide addition on physical, structural, and electrical properties of borophosphate glasses

Demand for electrical energy storage (EES) is growing, necessitating the development of improved batteries. Inorganic glassy materials are a class of essential electrodes due of their excellent conductivity and stable structures, they are appropriate for both cathode and anode applications. The most important characteristics that determine the dynamic behavior and rate performance of batteries are the ionic and electronic conductivities of active materials. In this sense, many crystalline, vitreous, and glass ceramics are investigated for their electrical properties. To create uniformly mixed glasses within the xLi2WO4–(50–x)Li2O–40P2O5–10B2O3 system, the melt-quenching technique is employed. Glasses containing up to 50 mol% Li2WO4 are synthesized with a substantial region of glass formation. They are homogeneous, transparent, and bubble-free. The structural approach and selected physical parameters of these glasses, such as density, molar volume, electrical conductivity, and glass transition temperature, are evaluated. The study emphasizes that an increase in the Li2WO4 amount leads to an elevation in both the density (ρ) and molar volume (Vm). Infrared spectroscopy identifies the presence of numerous structural units within the glass framework. Electrical conductivity measurements of the glasses are investigated using impedance spectroscopy. The measurements are taken over a wide frequency range of 10 Hz to 1 MHz and across temperatures ranging from 303K to 403K. It is highlighted that the electric conductivity rises as the Li2WO4 content is added. The glass with x = 30 mol% exhibits a dc conductivity of 8.63 × 10−7 (Ω−1 cm−1) at room temperature and 8.90 × 10−5 (Ω−1 cm−1) at 150°C. To gain insight into the conductivity relaxation mechanism, the frequency dependence of the glasses' conductivity is investigated. The results indicate that the ion transport in the glasses is governed by the correlated barrier hopping mechanism, which follows Jonscher's power behavior. The scaling conductivity spectra reveal that the conduction mechanism in the glasses is influenced by both temperature and composition.

Influence of Radioactive Sludge Content on Vitrification of High-Level Liquid Waste

The radioactive sludges formed at the bottom of high-level liquid waste (HLW) storage tanks pose challenges when the HLWs are vitrified. This study aims to determine the influence of the sludge content (enriched in Na2O, Al2O3, NiO, Fe2O3, and BaSO4) on the structure and properties of waste glasses in order to find the optimal ratio of sludges to HLW during vitrification. In the experiments, the simulated sludge and simulated HLW were mixed at different ratios from 0:8 to 4:4, with an overall waste content of 16 wt %, in a borosilicate glass wasteform. It is found that the glass density, molar volume, sulfur retention, and glass transition temperature changed little when increasing the sludge content of the glasses, while the viscosity, chemical durability, and crystallization features of the glasses varied notably. The crystals formed in the glasses during the thermal treatment were exclusively Fe-substituted diopside (Ca, Mg, Fe)2Si2O6. An increase in the Al2O3 and NiO content of the glasses may have been responsible for the increased crystallinity at high temperatures. The leaching rate of Si, B, Na, and Cs from the glasses declined with the increasing addition of sludge to the glasses. Although all the glasses fulfilled the requirements for vitrification processing and glass-product performance, it is recommended that the sludge content of the whole waste should not exceed 25 wt %. This study guides further research on the immobilization of high-level sludges.

Synthesis and characterization of positive volume phase transition hydrogel membrane prepared using a cellulose substrate

ABSTRACT Thermo-responsive hydrogels display swell-collapse behavior in response to changing temperature. While the hydrogels themselves have many applications such as environmental and chemical separations, the hydrogel can be incorporated within a membrane substrate to improve its mechanical properties and expand the opportunities for development and implementation. The present work describes synthesis of positive volume transition temperature responsive poly(acrylamide-co-acrylic acid) hydrogels on cellulose paper substrates. Effects of factors including co-monomer ratios, degree of crosslinking and mass loading are assessed for Series A random copolymer hydrogels in terms of permeability response. Examination of the membranes is then extended to investigate inter-penetrating networks (IPN) between acrylamide and acrylic acid homo-polymer hydrogels. Series B membranes explore the impact of nonpolar ligands on temperature response through addition of butyl methacrylate co-monomer. For both random copolymer and IPN hydrogel membranes, mass loading dominates permeability performance, though certain criteria must be met. Performance of the Series A membranes was found to be ideal when monomer ratio was 1:1 and there was a lower degree of crosslinking. Addition of butyl methacrylate did not appear to have a consistent impact on the membrane response, appearing to be tied to the degree of crosslinking for random copolymer hydrogel coatings.

A controlled recognizing and releasing glycoprotein based on temperature-responsive phenylboronic microgels for colorimetric analysis of complex samples.

It is challenging to capture glycoprotein from complicated samples with both high specificity and high recovery. Herein, we synthesized temperature-responsive poly (N-isopropylacrylamide-4-vinylbenzeneboronic acid) (PNIPAM-VPBA) microgels having enhanced affinity capability towards glycoprotein, resulting signal amplification in colorimetric analysis subsequently. Through temperature control, the target glycoprotein can be captured and released by the reported microgels unbiasedly. The microgels fabricated were characterized in terms of particle size, phase transition temperature, and toxicity. It was revealed that obvious changes in the particle diameter ranged from 600 to 323nm between 25°C and 55°C, the volume transition temperature was close to human physiological temperature, and low toxicity towards HeLa cells after 24h incubation. Using glucose oxidase as a model target, the microgels were applied as adsorbent materials for preconcentration of glucose oxidase, resulting 22-fold signal enhancement and a limit of detection (LOD) as low as 16.7 U/L. In beer sample analysis, satisfied recoveries of 78.7%-101.6% with the relative standard deviations (RSDs) less than 7.2% was observed. Subsequently, the microgels were combined with enzyme-linked immunosorbent assay (ELISA) for alpha fetoprotein (AFP) determination, by immobilizing horseradish peroxidase (HRP) and anti-AFP antibodies on the surface of microgels. A meaningful low concentration range of 2.5×10-2 to 1.0μg/L was obtained for AFP detection using the microgels based colorimetric method with a LOD of 8.4ng/L. In human serum sample analysis, good accuracy was achieved from method comparing with commercial ELISA kit, satisfied recoveries of 86.1%-96.4% with the RSDs less than 6.0% was observed. The proposed PNIPAM-VPBA microgels have great potential applications in the fields of food and clinical analysis.

Indentation and temperature response of liquid metal/hydrogel composites

The volume transition of thermo-responsive hydrogels has been investigated for various applications such as soft actuators, sensors, drug delivery systems, and bioelectronics. Nonetheless, issues such as poor mechanical properties, slow transition rates, and limited volume change windows have prompted the development of hydrogel composites. Gallium (Ga), a liquid metal, is combined with poly(N-isopropyl acrylamide) (PNIPAm) with varying composite structures in this work. Ga's melting point is close to PNIPAm's volume transition temperature. The introduction of Ga particles into PNIPAm improves both the indentation resistance and the transition rate of the hydrogel. Microchannel structure can further improve the transition rate by increasing the water diffusion rate. The relatively fast thermal conduction of the Ga phase in anisotropic composites causes fast deswelling of Ga-rich regions, resulting in unique double bending behavior. The double bending behavior was utilized to hold irregularly shaped objects firmly. These results offer a useful design strategy of composites for the development of novel responsive smart systems.

Water-pumping and purifying hydrogels driven by diurnal temperature variation

Diurnal temperature variation is an untapped energy source, although it has seldom been exploited as such. In this study, a novel system of thermo-responsive hydrogels uses diurnal temperature variation as the only power source to pump and purify water. Water is pumped at night and released during the day using an anisotropic xylem-mimetic structure whose composite phase functions as a water channel valve. The water-pumping efficiency ranges from 78% to 92%, and the weights of the pumped water per cycle are 2–3 times larger than those of the pumping materials. The anisotropic structure and the hydrophobic silicone phase are critical in determining pumping efficiency, which allow irreversible water flow upon temperature variation. Seasonal and regional temperature changes can be matched using thermo-responsive hydrogels with different volume transition temperatures. Purification of dye-contaminated water and separation of oil–water mixtures are also successfully achieved based on the hydrophilic and the pollutant-adsorbing nature of hydrogels. Since temperature changes even faster than the diurnal temperature cycle can also power this water pumping and purification system, this novel mechanism could provide a new operational solution for future water management.

Li‐Salt Doped Single‐Ion Conducting Polymer Electrolytes for Lithium Battery Application

AbstractTraditionally solid polymer electrolytes (SPEs) for lithium battery application are made by dissolving a Li‐salt in a polymer matrix, which renders both the Li+ cations, the charge carriers of interest, and the anions, only by‐standers, mobile. In contrast, single‐ion conductors (SICs), with solely the Li+ cation mobile, can be created by grafting the anions onto the polymer backbone. SICs provide the safety, mechanical stability, and flexibility of SPEs, but often suffer in ionic conductivity. Herein an intrinsically synergetic design is suggested and explored; one dopes a promising SIC, LiPSTFSI (poly[(4‐styrenesulfonyl) (trifluoromethanesulfonyl)imide]), with a common battery Li‐salt, LiTFSI. This way one both increases the Li+ concentration and transport. Indeed, systematically exploring doping, it is found that 50–70 wt% of LiTFSI renders materials with considerable improvements in both the (Li+) dynamics and the ionic conductivity. A deeper analysis allows to address connections between the ion transport mechanism(s) (Arrhenius/VTF), the charge carrier speciation and concentration, and the free volume and glass transition temperature. While no silver bullet is even remotely found, the general findings open paths to be further explored for SPEs in general and Li‐salt doped SICs in particular.

Nanoscale Mechanical Properties of Core–Shell-like Poly-NIPAm Microgel Particles: Effect of Temperature and Cross-Linking Density

Poly-NIPAm microgel particles with two different cross-linking densities were prepared with the classical batch polymerization process. These particles were adsorbed onto modified silica surfaces, and their nanomechanical properties were measured by means of atomic force microscopy. It was found that these particles have a hard core-soft shell structure both below and above the volume transition temperature. The core-shell-like structure appears due to a higher reaction rate of the cross-linker compared to that of the monomer, leading to depletion of cross-linker in the shell region. The microgel beads with lower average cross-linking density were found to be less stiff below the volume transition temperature than the microgel with higher cross-linking density. Increasing the temperature further to just above the volume transition temperature led to lower stiffness of the more highly cross-linked microgel compared to its less cross-linked counterpart. This effect is explained with the more gradual deswelling with temperature for the more cross-linked microgel particles. This phenomenon was confirmed by dynamic light scattering measurements in the bulk phase, which showed that the larger cross-linking density microgel showed a more gradual collapse in aqueous solution as the temperature was increased.

Effect of TiO2 on the chemical durability and optical properties of Mo-based phosphate glasses

The phosphate glasses belonging to the 20Li2O−(40−x)Li2MoO4−xTiO2−40P2O5 (0≤x≤10 mol%) system were prepared via melt quenching route at 900 ±15°C. The prepared samples are characterized by X−Ray diffraction and differential scanning calorimetry to check possible crystalline residues and the glassy state. The structural properties of these glasses were investigated by some physical parameters such as density, molar volume, and glass transition temperature (Tg). Density values, molar volume values, and Tg values were found to depend strongly on the chemical composition of the glasses. The density decreased from 3.68 ±0.01 g/cm3 to 3.27 ±0.01 g/cm3, and Tg increased from 372 ±2°C to 407 ±2°C with the TiO2 content increased from 0 mol% to 10 mol%. The structural approach is verified by Infrared (IR) and Raman spectroscopies. The chemical durability tests on the glasses have shown that the compositions containing pyrophosphate (Q1) and orthophosphate (Q0) units are more water−resistant. The optical properties have been evaluated by determining some parameters, like optical band gap energy (Eg) and Urbach energy (ΔΕ) and their values are determinate. These parameters vary in opposite; Eg decreased from 2.12 eV to 1.35 eV and ΔΕ increased from 0.38 eV to 0.48 eV with increasing in TiO2 mol %.

Temperature-Responsive On-Off Control over Water Evaporation Achieved via Sweat-Gland-Mimetic Composites.

Responsive cooling materials that mimic sweat glands have gained popularity because they are efficient and do not require artificial energy sources. Temperature-responsive hydrogels sweat above their volume transition temperature through the release of water and exhibit excellent cooling ability. However, thus far, practical applications have not been possible because the water in these materials cannot be preserved in cool environments. To address this issue, this paper presents a simple composite of poly(N-isopropylacrylamide) and polydimethylsiloxane that offers excellent on-off control over water evaporation and can be used repeatedly; the proposed composite features an evaporation rate of 2.97 g/h above the lower critical solution temperature (LCST) and 0.08 g/h below the LCST. This 35.7-fold change in the water evaporation rate is comparable to that in mammalian sweat glands. The responsive on-off control relies on the structures of the composite and the dry layers formed on the surface of the composite in cool environments. The proposed material effectively regulates water evaporation and offers a novel, low-cost cooling strategy suitable for numerous applications.

Thermal, optical and electrical properties of MnO2-doped mixed sodium potassium phosphate glasses

Glasses in the system (1 − x)(0.5NaPO3–0.5KPO3)–xMnO2, with 0 ≤ x ≤ 50 mol%, have been prepared using a melt-quench route. The glasses exhibit a yellow to dark color with the increase in manganese content owing to the presence of Mn2+ and Mn3+ ions in the network. The amorphous state of the glasses is evidenced by the X-ray diffraction. In order to get an insight into the physical and structural aspects of these vitreous materials, we have determined some of their parameters such as density, molar volume and glass transition temperature. From differential thermal analysis scan on heating, we evaluated the glass transition temperature (Tg) of each glass, which corresponds to the phase transition temperature from solid to viscous liquid. The density (ρ) as a structural index is found to increase while the corresponding molar volume decreases with MnO2 content. The structural approach of the studied glasses is evaluated by infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies. IR technique allowed us to identify the coexisting bond vibration modes in the glass network, and it has shown that many structural phosphates units coexist, mainly pyrophosphate and metaphosphate structural groups. EPR experiments have shown the presence of Mn2+ centers in the glasses. The UV–Visible absorption is utilized to estimate the values of the optical band gap (Eg) and Urbach energy (ΔE). The optical band gap energy is determined from both the absorption spectrum fitting (ASF) and Tauc’s methods. These optical parameters are composition dependence. The dc conductivity of the glasses is determined in the temperature range from 303 to 473 K. It decreases with increasing manganese content. It is thermally activated and followed an Arrhenius behavior. The crystallization of glasses is realized by submitting them to heat treatments, and the crystallized phases are identified by XRD analysis. The crystallization kinetic was studied under non-isothermal conditions. The activation energy (Ec) and the Avrami parameter (n) were determined.

Effect of V2O5 on crystallization tendency and chemical durability of Mo-bearing aluminoborosilicate glass

AbstactThe effect of V2O5 addition on molybdates crystallization tendency, glass structure and chemical durability of aluminoborosilicate glass belonging to SiO2-B2O3-CaO-Na2O-Al2O3-MoO3 system has been studied. The results confirm that V2O5 addition can effectively suppress the crystallization tendency of powellite and enhance the molybdenum solubility in the glass. The MoO3 solubility limit is found to be 2.8 mol% in the V2O5-containing aluminoborosilicate glass. Raman results reveal that V2O5 addition seems to modify the local structural environment of isolated MoO4 units and increase their chemical disorder in the glass, which is favored for molybdenum incorporation in the glass. The molar volume and glass transition temperature of samples are found to depend on V2O5 content. Product consistency test (PCT) results show that the normalized leaching rates of the V2O5-containing aluminoborosilicate glass maintain at a fairly low level compared with standard borosilicate glassy waste form.

Effect of V2O5 on the Properties of Lithium Bismuth Borate Glasses

The series of the glasses with general formula (40-x)Li2O:30B2O3:30Bi2O3:xV2O5, with x = 0, 0.5, 1.0, 1.5 and 2.0 mol% was prepared using conventional quenching technique. The glass transition temperature Tg for these samples were determined from Differential Thermal Analysis. The glass transition temperature increases beyond 0.5 mol%. It was found that the glasses under study consist of randomly connected BO3 & BO4 structural units. The density and molar volume of glasses were found to depend on V2O5 content. Initially the density increases, molar volume and glass transition temperature decrease with the addition of V2O5. This suggests that when V2O5 is added to the glass initially up to 0.5 mol% it may be entering as a modifier. Beyond 0.5 mol% density decreases and molar volume increases as V2O5 goes as former. This reveals the role of V2O5 as a glass network former beyond 0.5 mol%. Optical band gap energy decreases and cut off wavelength increases with increase in V2O5 content.

Criteria for colloidal gelation of thermo-sensitive poly(N-isopropylacrylamide) based microgels

HypothesisSuspensions of the poly(N-isopropylacrylamide) (PNIPAM) based temperature(T)-sensitive microgels can undergo colloidal gelation forming a three-dimensional sparse network-like structure in the hydrophobic and shrunken state of T > T* (T*: volume transition temperature), despite their considerably low particle volume fractions (<0.2). The effective surface charge density is expected to be a key factor governing the colloidal gelation and gel modulus. ExperimentsThe combined analysis of the viscoelasticity and electrophoretic mobility (EPM) was performed varying systematically pH and ionic strength (I). The microgels containing the extremely small content of electrolyte (0.1 mol%) with the T* and swelling degree being insensitive to pH and I were employed to facilitate the exclusive analysis of their effects on colloidal gelation. FindingsThe results unambiguously reveal (1) that the gelation requires the adequate suppressions of the interparticle charge repulsion, and (2) that a reduction in the interparticle charge repulsion results in an increase in gel modulus by several orders of magnitude. The long-term linear creep behavior show that the colloidal gels are identified as a viscoelastic fluid with a long relaxation time and a high viscosity whereas they behave elastically at relatively short timescale in conventional oscillatory tests.

Effect of Dy2O3 on Electrical Conductivity, Dielectric Properties and Physical Properties in Lithium Borosilicate Glasses

Glass series with general formula 30Li2O-(70-x) [1/7SiO2:6/7B2O3]-xDy2O3 was prepared using conventional melt quench technique. The Impedance spectroscopy (IS) measurement were carried out by Impedance Analyzer in the frequency range 2 mHz to 20 MHz in the temperature range 423 K to 673 K. Addition of dysprosium oxide in borosilicate glasses affects density, molar volume and glass transition temperature. Electric modulus data has been analyzed to study relaxation behavior. The dielectric constant (ε’) and dielectric loss (tan δ) of prepared glasses gradually decreases with increase in frequency and decrease in temperature due to electron hopping.