Showing papers on "Thermal stability published in 2014"

Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells.

Abstract: Organic–inorganic perovskite solar cells have recently emerged at the forefront of photovoltaics research. Power conversion efficiencies have experienced an unprecedented increase to reported values exceeding 19% within just four years. With the focus mainly on efficiency, the aspect of stability has so far not been thoroughly addressed. In this paper, we identify thermal stability as a fundamental weak point of perovskite solar cells, and demonstrate an elegant approach to mitigating thermal degradation by replacing the organic hole transport material with polymer-functionalized single-walled carbon nanotubes (SWNTs) embedded in an insulating polymer matrix. With this composite structure, we achieve JV scanned power-conversion efficiencies of up to 15.3% with an average efficiency of 10 ± 2%. Moreover, we observe strong retardation in thermal degradation as compared to cells employing state-of-the-art organic hole-transporting materials. In addition, the resistance to water ingress is remarkably enhanced...

Native Cellulose: Structure, Characterization and Thermal Properties.

Abstract: In this work, the relationship between cellulose crystallinity, the influence of extractive content on lignocellulosic fiber degradation, the correlation between chemical composition and the physical properties of ten types of natural fibers were investigated by FTIR spectroscopy, X-ray diffraction and thermogravimetry techniques. The results showed that higher extractive contents associated with lower crystallinity and lower cellulose crystallite size can accelerate the degradation process and reduce the thermal stability of the lignocellulosic fibers studied. On the other hand, the thermal decomposition of natural fibers is shifted to higher temperatures with increasing the cellulose crystallinity and crystallite size. These results indicated that the cellulose crystallite size affects the thermal degradation temperature of natural fibers. This study showed that through the methods used, previous information about the structure and properties of lignocellulosic fibers can be obtained before use in composite formulations.

Structural changes and thermal stability of charged LiNixMnyCozO2 cathode materials studied by combined in situ time-resolved XRD and mass spectroscopy

Abstract: Thermal stability of charged LiNixMnyCozO2 (NMC, with x + y + z = 1, x:y:z = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes ...

Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis

Abstract: The thermal stabilities of 66 ionic liquids (ILs) were investigated using the thermogravimetric analysis (TGA) method. Isothermal TGA studies on the ILs showed that ILs exhibit decomposition at temperatures lower than the onset decomposition temperature (Tonset), which is determined from ramped temperature TGA experiments. Thermal decomposition kinetics of ILs was analyzed using pseudo-zero-order rate expression and their activation energy was obtained. Parameter T0.01/10h, the temperature at which 1% mass loss occurs in 10 h, was used to evaluate the long-term thermal stability of ILs. The thermal stability of the ILs was classified to five levels according to Tonset. The ILs thermal stability is dependent on the structure of ILs, i.e., cation modification, cation and anion type. The correlations between the stability and the hydrophilicity of ILs were discussed. Finally, the thermal stabilities of acetate-based ILs, amino acid ILs, and dicyanamide ILs were analyzed.

Nanotwinned diamond with unprecedented hardness and stability

Abstract: Nanotwinned diamond synthesized with onion carbon nanoparticles as precursors has much higher hardness and thermal stability than natural diamond; its enhanced hardness is due to the reduced size of its twin structures. Even diamond has its limitations when used in tools to cut and shape the hardest of materials. Materials scientists have therefore sought to synthesize materials that are harder than natural diamond, preferably with increased thermal stability. In air, natural diamond starts to oxidize at about 800 °C, leading to severe wear at high temperatures. Attempts to increase the hardness of diamond by decreasing its grain size have succeeded, but at the cost of even poorer thermal stability. Yongjun Tian and colleagues report the synthesis of synthetic diamond that is both ultrahard and has a dramatically enhanced thermal stability with an oxidization temperature of more than 1,000 °C. The material is synthesized using onion carbon nanoparticles as precursors and owes its enhanced hardness to a nanoscale structure consisting not of tiny grains, but of crystal 'twins' — domains of the crystal lattice related by symmetry. This result, which follows similar success with nanotwinned cubic boron nitride, suggests a general approach to making new, advanced carbon-based materials with exceptional properties. Although diamond is the hardest material for cutting tools, poor thermal stability has limited its applications, especially at high temperatures. Simultaneous improvement of the hardness and thermal stability of diamond has long been desirable. According to the Hall−Petch effect1,2, the hardness of diamond can be enhanced by nanostructuring (by means of nanograined and nanotwinned microstructures), as shown in previous studies3,4,5,6,7. However, for well-sintered nanograined diamonds, the grain sizes are technically limited to 10−30 nm (ref. 3), with degraded thermal stability4 compared with that of natural diamond. Recent success in synthesizing nanotwinned cubic boron nitride (nt-cBN) with a twin thickness down to ∼3.8 nm makes it feasible to simultaneously achieve smaller nanosize, ultrahardness and superior thermal stability5. At present, nanotwinned diamond (nt-diamond) has not been fabricated successfully through direct conversions of various carbon precursors3,6,7 (such as graphite, amorphous carbon, glassy carbon and C60). Here we report the direct synthesis of nt-diamond with an average twin thickness of ∼5 nm, using a precursor of onion carbon nanoparticles at high pressure and high temperature, and the observation of a new monoclinic crystalline form of diamond coexisting with nt-diamond. The pure synthetic bulk nt-diamond material shows unprecedented hardness and thermal stability, with Vickers hardness up to ∼200 GPa and an in-air oxidization temperature more than 200 °C higher than that of natural diamond. The creation of nanotwinned microstructures offers a general pathway for manufacturing new advanced carbon-based materials with exceptional thermal stability and mechanical properties.

Preparation and characterization of surface modified boron nitride epoxy composites with enhanced thermal conductivity

Abstract: Hexagonal boron nitride (h-BN) microparticles, modified by surface coupling agent 3-aminopropyl triethoxy silane (APTES), were used to fabricate thermally conductive epoxy/BN composites, and the effects of modified-BN content on the thermal and insulating properties were investigated. It was found that incorporation of h-BN particles in the epoxy matrix significantly enhanced the thermal conductivity of the composites. With 30 wt% modified-BN loading, the thermal conductivity of the composites was 1.178 W m−1 K−1, 6.14 times higher than that of the neat epoxy. Fabricated epoxy/BN composites exhibited improved thermal stability, storage modulus, and glass transition temperature with increased BN content. The composites also possessed excellent electrical insulation properties. These results revealed that epoxy/BN composites are promising as efficient heat-releasing materials for thermal management and microelectronic encapsulation.

Polylactic acid (PLA) biocomposites reinforced with coir fibres: Evaluation of mechanical performance and multifunctional properties

Abstract: The effects of fibre content (5–30 wt%) and fibre treatment on surface morphology, tensile, flexural, thermal and biodegradable properties of polylactic acid (PLA)/coir fibre biocomposites were evaluated via scanning electron microscopy (SEM), mechanical testing, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and soil burial method. Similar decreasing trends were found for tensile and flexural strengths with higher strength values obtained for PLA/treated coir fibre biocomposites. 20 wt% treated coir fibres were determined to achieve optimum tensile and flexural strengths of biocomposites. Regardless of fibre treatment, the thermal stability of biocomposites is worsened with increasing the fibre content. The decreased cold crystallisation temperatures of biocomposites further confirms the effective nucleating agent role of coir fibres. The biocomposites undergo much faster degradation than PLA, with the maximum weight loss of 34.9% in treated fibre biocomposites relative to 18% in PLA after 18-day burial, arising from the hydrophilic nature of coir fibres.

Manipulating the Electronic Structure of Li-Rich Manganese-Based Oxide Using Polyanions: Towards Better Electrochemical Performance

Abstract: Lithium-rich manganese-based layered oxides show great potential as high-capacity cathode materials for lithium ion batteries, but usually exhibit a poor cycle life, gradual voltage drop during cycling, and low thermal stability in the highly delithiated state. Herein, a strategy to promote the electrochemical performance of this material by manipulating the electronic structure through incorporation of boracic polyanions is developed. As-prepared Li[Li0.2Ni0.13Co0.13Mn0.54](BO4)0.015(BO3)0.005O1.925 shows a decreased M-O covalency and a lowered O 2p band top compared with pristine Li[Li0.2Ni0.13Co0.13Mn0.54]O2. As a result, the modified cathode exhibits a superior reversible capacity of 300 mA h g−1 after 80 cycles, excellent cycling stability with a capacity retention of 89% within 300 cycles, higher thermal stability, and enhanced redox couple potentials. The improvements are correlated to the enhanced oxygen stability that originates from the tuned electronic structure. This facile strategy may further be extended to other high capacity electrode systems.

Pillar[5]arene-Based Supramolecular Organic Frameworks for Highly Selective CO2-Capture at Ambient Conditions

Abstract: Low-density, solid-state, porous supramolecular organic frameworks are constructed using pillarenes. The frameworks have a honeycomb-like structure, permanent porosity, high thermal stability, and selective and reversible sorption properties toward CO2. The exceptionally selective CO2-sorption properties (375/1, 339/1) of one framework over N2 and CH4 indicate potential applications in CO2-capture for post-combustion power plants and natural gas sweetening.

Increasing the Stability of Metal-Organic Frameworks

Abstract: Metal-organic frameworks (MOFs) are a new category of advanced porous materials undergoing study by many researchers for their vast variety of both novel structures and potentially useful properties arising from them. Their high porosities, tunable structures, and convenient process of introducing both customizable functional groups and unsaturated metal centers have afforded excellent gas sorption and separation ability, catalytic activity, luminescent properties, and more. However, the robustness and reactivity of a given framework are largely dependent on its metal-ligand interactions, where the metal-containing clusters are often vulnerable to ligand substitution by water or other nucleophiles, meaning that the frameworks may collapse upon exposure even to moist air. Other frameworks may collapse upon thermal or vacuum treatment or simply over time. This instability limits the practical uses of many MOFs. In order to further enhance the stability of the framework, many different approaches, such as the utilization of high-valence metal ions or nitrogen-donor ligands, were recently investigated. This review details the efforts of both our research group and others to synthesize MOFs possessing drastically increased chemical and thermal stability, in addition to exemplary performance for catalysis, gas sorption, and separation.

Mechanical Downsizing of a Gadolinium(III)‐based Metal–Organic Framework for Anticancer Drug Delivery

Abstract: AG d III -based porous metal-organic framework (MOF), Gd-pDBI, has been synthesized using fluorescent linker pDBI (pDBI = (1,4-bis(5-carboxy-1H-benzimidazole-2- yl)benzene)), resulting in a three-dimensional interpenetrat- ed structure with a one-dimensional open channel (1.9 � 1.2 nm) filled with hydrogen-bonded water assemblies. Gd- pDBI exhibits high thermal stability, porosity, excellent water stability, along with organic-solvent and mild acid and base stability with retention of crystallinity. Gd-pDBI was trans- formed to the nanoscale regime (ca. 140 nm) by mechanical grinding to yield MG-Gd-pDBI with excellent water dispersi- bility (> 90 min), maintaining its porosity and crystallinity. In vitro and in vivo studies on MG-Gd-pDBI revealed its low blood toxicity and highest drug loading (12 wt %) of anti- cancer drug doxorubicin in MOFs reported to date with pH- responsive cancer-cell-specific drug release.

From biomass to high performance solar–thermal and electric–thermal energy conversion and storage materials

Abstract: We demonstrate that lightweight, highly electrically conductive, and three-dimensional (3D) carbon aerogels (CAs) can be produced via a hydrothermal carbonization and post pyrolysis process using various melons as raw materials. This two-step process is a totally green synthetic method with cheap and ubiquitous biomass as the only raw material. These black-colored, highly electrically conductive and 3D structured CAs are ideal materials for energy conversion and storage. Paraffin wax was impregnated into the CA scaffold by vacuum infusion. The obtained CA–wax composites show excellent form-stable phase change behavior, with a high melting enthalpy of 115.2 J g−1. The CA–wax composites exhibit very high solar radiation absorption over the whole UV-vis-NIR range, and 96% of light can be absorbed by the phase-change composite and stored as thermal energy. With an electrical conductivity of 3.4 S m−1, the CA–wax composite can be triggered by low electric potential to perform energy storage and release, with an estimated electric–heat conversion efficiency of 71.4%. Furthermore, the CA–wax composites have excellent thermal stability with stable melting–freezing enthalpy and excellent reversibility. With a combination of low-cost biomass as the raw materials, a green preparation process, low density, and excellent electrical conductivity, the 3D CAs are believed to have promising potential applications in many energy-related devices.

Poly(3-hydroxybutyrate)/ZnO Bionanocomposites with Improved Mechanical, Barrier and Antibacterial Properties

Abstract: Poly(3-hydroxybutyrate) (PHB)-based bionanocomposites incorporating different contents of ZnO nanoparticles were prepared via solution casting technique. The nanoparticles were dispersed within the biopolymer without the need for surfactants or coupling agents. The morphology, thermal, mechanical, barrier, migration and antibacterial properties of the nanocomposites were investigated. The nanoparticles acted as nucleating agents, increasing the crystallization temperature and the degree of crystallinity of the matrix, and as mass transport barriers, hindering the diffusion of volatiles generated during the decomposition process, leading to higher thermal stability. The Young’s modulus, tensile and impact strength of the biopolymer were enhanced by up to 43%, 32% and 26%, respectively, due to the strong matrix-nanofiller interfacial adhesion attained via hydrogen bonding interactions, as revealed by the FT-IR spectra. Moreover, the nanocomposites exhibited reduced water uptake and superior gas and vapour barrier properties compared to neat PHB. They also showed antibacterial activity against both Gram-positive and Gram-negative bacteria, which was progressively improved upon increasing ZnO concentration. The migration levels of PHB/ZnO composites in both non-polar and polar simulants decreased with increasing nanoparticle content, and were well below the current legislative limits for food packaging materials. These biodegradable nanocomposites show great potential as an alternative to synthetic plastic packaging materials especially for use in food and beverage containers and disposable applications.

High Shear Homogenization of Lignin to Nanolignin and Thermal Stability of Nanolignin‐Polyvinyl Alcohol Blends

Abstract: A new method to prepare nanolignin using a simple high shear homogenizer is presented. The kraft lignin particles with a broad distribution ranging from large micron- to nano-sized particles were completely homogenized to nanolignin particles with sizes less than 100 nm after 4 h of mechanical shearing. The (13) C nuclear magnetic resonance (NMR) and (31) P NMR analysis showed that there were no major changes in the chemical composition between the starting kraft lignin particles and the nanolignin obtained after 4 h of mechanical treatment. The nanolignin particles did not show any change in molecular weight distribution and polydispersity compared to the original lignin particles. The nanolignin particles when used with polyvinyl alcohol (PVA) increased the thermal stability of nanolignin/PVA blends more effectively compared to the original lignin/PVA blends.

Improving the thermal and mechanical properties of polysulfone by incorporation of graphene oxide

Abstract: Nanocomposites of polysulfone (PSF)-graphene oxide (GO) were prepared by classical phase inversion method. The structural and surface features and the mechanical and thermal performances of the prepared materials were investigated in detail. TEM and X-ray diffraction analysis indicated a good compatibility and excellent dispersability with PSF matrix for the low GO content (0.25, 0.5 and 1 wt.%) composites. It was observed that GO dispersion was reasonably homogeneous for the composite with 2 wt.% GO. The mechanical properties of the prepared materials were found to be greatly enhanced by the addition of GO for some compositions. The thermogravimetrical investigation demonstrated considerable improvements in thermal stability for the composite with low GO content. This novel material offers a feasible candidate for practical membrane application.