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•The current status of thermal characterization in the MOF field could be described, charitably, as haphazard. This review lays out some recommendations on performing isothermal ...experiments to accurately identify a decomposition temperature.•The review encourages the use of variable temperature X-ray and differential scanning calorimetry, which would identify changes in phase/state that proceed without mass loss.•Where applicable, the review has highlighted the known pathways of decomposition of several prototypical MOFs.
In order to systematically engineer the properties of crystalline and amorphous metal-organic frameworks (MOFs) towards practical application, a thorough understanding of their high-temperature behaviour is required. However, key properties such as the thermal decomposition temperature of a given MOF are often overlooked in the literature, leading to a huge gap in our understanding of the thermal stability of this vast class of materials. Herein, we undertake a critical review of thermogravimetric analysis (TGA) data from the MOF literature, and question the consistency, accuracy and meaning of the data provided.
We use existing data to collate thermal decomposition temperature (Td) values for a series of archetypal coordination materials and their analogues. Several factors were identified which influence the thermal stability of MOFs. In particular: (i) the nature and position of functional groups, (ii) metal hardness, and, (iii) the presence of coordinated solvent molecules were all found to have a significant impact on decomposition temperature. Isoreticular expansion and interpenetration on the other hand were found to have a relatively modest impact.
Moreover, we propose that decomposition mechanisms in MOFs may be broadly separated into two categories, depending on whether the decomposition is ligand-centred, or inorganic node-centred. Whilst ligand stability appears to be the dominant factor in determining overall thermal decomposition temperature, the stability of the inorganic node is key in realising solid-liquid transitions and high-temperature recrystallisation within the family. Thus, both ligand and node stability must be considered when attempting to engineer the high-temperature properties of MOFs.
This study investigated the novel fabrication of polymer-metal composites using fused deposition modeling (FDM), and evaluated the mechanical and physical properties of the new materials. ...Specifically, an acrylonitrile butadiene styrene (ABS) – 420 stainless steel (SS) composite system was used, with 10, 15, and 23 wt% SS powder additions, and the resulting properties were compared to those of base ABS prepared using the same printing conditions. A new methodology to fabricate the composites was developed. The resulting materials were extruded into composite filaments, which were used to print test specimens. Tensile testing, modulated differential scanning calorimetry, and scanning electron microscopy were employed to characterize the composite materials and evaluate the effects of different print conditions. The results demonstrate, for the first time, the feasibility of using FDM to prepare ABS-SS composites that maintain or enhance mechanical properties as compared to the base polymer, while adding increased functionality.
Phase transition issues in the field of foods and drugs have significantly influenced these industries and consequently attracted the attention of scientists and engineers. The study of thermodynamic ...parameters such as the glass transition temperature (Tg), melting temperature (Tm), crystallization temperature (Tc), enthalpy (H), and heat capacity (Cp) may provide important information that can be used in the development of new products and improvement of those already in the market. The techniques most commonly employed for characterizing phase transitions are thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and differential scanning calorimetry (DSC). Among these techniques, DSC is preferred because it allows the detection of transitions in a wide range of temperatures (-90 to 550 °C) and ease in the quantitative and qualitative analysis of the transitions. However, the standard DSC still presents some limitations that may reduce the accuracy and precision of measurements. The modulated differential scanning calorimetry (MDSC) has overcome some of these issues by employing sinusoidally modulated heating rates, which are used to determine the heat capacity. Another variant of the MDSC is the supercooling MDSC (SMDSC). SMDSC allows the detection of more complex thermal events such as solid-solid (Ts-s) transitions, liquid-liquid (Tl-l) transitions, and vitrification and devitrification temperatures (Tv and Tdv, respectively), which are typically found at the supercooling temperatures (Tco). The main advantage of MDSC relies on the accurate detection of complex transitions and the possibility of distinguishing reversible events (dependent on the heat capacity) from non-reversible events (dependent on kinetics).
Solid crystalline wax content is an important parameter to control the quality and durability of asphalt binders produced for flexible pavement construction. Although a significant number of methods ...have been published for the quantification of wax in crude oil and lubricants, the direct use of these for asphalt binder characterization has faced a number of obstacles. In order to develop a precise, accurate and practical methodology for the measurement of in situ wax in binders at various temperatures, Variable-Temperature Fourier-Transform Infrared Spectroscopy (VT-FTIR) is used for the first time to determine wax contents in asphalt binders over the entire temperature domain that typical pavements experience. Cold Lake asphalt doped with varying amounts of n-alkanes, as well as a variety of commercial binders, were tested to validate the effectiveness of the VT-FTIR method. The results show that the crystalline wax contents determined by VT-FTIR are reasonably consistent with traditional cold precipitation and differential scanning calorimetry (DSC) methods. In contrast with DSC, the VT-FTIR method is able to observe the continuing increase in crystalline wax with decreasing temperature, and is able to do so over extended periods of time. On the whole, the VT-FTIR method provides a fundamental chemical analysis that probes the formation of crystalline unit cells in the asphalt binder. Hence, it is a promising approach for the accurate determination of solid crystalline wax content with the ultimate aim of controlling wax-associated pavement distresses such as raveling, cracking and moisture damage.
In Selective Laser Melting (SLM) layers of atomized powder are spread sequentially on a building platform and melted locally by a laser beam. The melt pool is quenched by the underlying material. SLM ...of AlSi10Mg alloys results in the development of microstructures consisting of supersaturated primary Al-rich phase surrounded by varied amounts of Al-Si eutectic. The origin of such microstructure is not fully understood. For insight into this issue, this work compares the results of processing AlS10Mgi alloys by SLM and by single-step rapid solidification techniques: Melt Spinning (MS) and Copper Mould Casting (CMC) achieving a range of cooling rates and microstructures which are analysed by means of microscopy, XRD and DSC.
The results obtained in these experiments together with the literature available on rapidly solidified Al-Si alloys suggest a correlation among microstructures of the products made with the three techniques. Data on lattice parameter and enthalpy of Si precipitation from primary Al concur in indicating that Si supersaturation scales in the order SLM > CMC > MS. The type and size of microstructural features, i.e. cells, columns, fibrous and lamellar eutectic, reveal the role of solidification conditions (undercooling, recalescence) and precipitation in the solid state for all techniques. Dendrite growth modelling validates the solidification results.
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•Diverse microstructures from different rapid solidification processes explained.•Si supersaturation in Al is 1 at% (MS), 2 at% (CMC) and 4 at% (SLM).•Eutectic fraction in Al-Si decreases is in the order: equilibrium > CMC > SLM ∼ MS (air side).•Growth rates of primary phase of 0.5 ÷ 1 m/s explain experimental eutectic fractions.
The mechanical and thermal properties of Kevlar fiber reinforced polymer composites are favorable. Researchers have extensively studied to improve these composites' mechanical characteristics, but ...research into improving their thermal properties is still limited. The aim of this study is to further improve the thermal properties of Kevlar fiber reinforced epoxy composite using surface modified Al2O3 nanoparticles. Vacuum-assisted resin infusion method (VARIM) was used to produce nanocomposite laminates. The surface of Al2O3 nanoparticles was modified with a silane coupling agent. Ultrasonication and magnetic mixing were used to incorporate Al2O3 at various wt.% (1, 2, 3, 4 and 5) into epoxy. Thermal stability was determined by performing differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA) and measuring thermal conductivity and thermal diffusivity. The results obtained revealed that the surface-modified Al2O3 nanoparticles exhibit good thermal barrier properties and improve thermal stability. In terms of degradation temperature and specific heat capacity, the thermal stability of Kevlar composites is best at 3 wt% Al2O3. SEM images and EDX processing were used to determine the morphological characteristics. The chemical composition was confirmed by EDX analysis, and SEM images revealed that the failure occurred in fiber and matrix-related mechanisms. Fourier transform infrared (FT-IR) spectra were recorded using a FT-IR spectrophotometer. It was seen that three new peaks appear with the addition of surface-modified Al2O3.
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•Surface modified alumina (Al2O3) was used to enhance the thermal properties of Kevlar composites.•Kevlar composite has the best thermal stability at 3 wt% Al2O3 regarding degradation temperature and specific heat capacity.•Thermal conductivity values showed an improvement of 23.68% at 3 wt% of Al2O3.•Fiber imprints, fiber breakages, matrix fractures, and matrix deformations were the most common damage mechanisms identified.
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•Asymmetric liposomes constituted from DPPC(out)/DPPS(in) lipids were prepared.•1H NMR spectra and DSC curves of successive heating runs confirmed the asymmetry.•The FTIR signatures ...of carbonyl, methyl and choline groups distinguish (a)symmetry.•MD simulations suggest hydration and lipid ordering differs due to (a)symmetry.
In contrast to symmetric unilamellar liposomes (sLUVs) prepared from a mixture of different lipids, asymmetric ones (aLUVs) with different lipid composition in the inner and outer membrane leaflets are more suitable model systems of eukaryotic plasma membranes. However, apart from the challenging preparation of asymmetric liposomes and small amounts of obtained asymmetric unilamellar liposomes (aLUVs), a major drawback is the qualitative characterization of asymmetry, as each of the techniques used so far has certain limitations. In this regard, we prepared aLUVs composed dominantly of DPPC(out)/DPPS(in) lipids and, along with 1H NMR and DSC characterization, we showed for the first time how FTIR spectroscopy can be used in the presence of (a)symmetry between DPPC/DPPS lipid bilayers. Using second derivative FTIR spectra we demonstrated not only that the hydration of lipids glycerol backbone and choline moiety of DPPC differs in s/aLUVs, but in addition that the lateral interactions between hydrocarbon chains during the phase change display different trend in s/aLUVs. Molecular dynamics simulations confirmed different chain ordering and packing between s/a bilayers, with a significant influence of temperature, i.e. membrane phase.
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