High temperature structural ceramic materials manufactured by the CNTD process

Controlled Nucleation Thermochemical Deposition (CNTD) has emerged from classical chemical deposition (CVD) technology. This paper describes the techniques of thermochemical grain refinement. The effects of such refinement on mechanical properties of materials at room temperature and at elevated temperatures are outlined. Emphasis is given to high temperature structural ceramic materials such as SiC, Si₃N₄, AiN, and TiB₂ and ZrB₂. An example of grain refinement accompanied by improvements in mechanical properties is SiC. Grain sizes of 500 to 1000 Å have been observed in CNTD SiC with room temperature MOR of 1380 to 2070 MPa (4 pt bending) and MOR of 3450 to 4140 MPa (4 pt bending) at 1350°C. Various applications of these materials to the solution of high temperature structural problems are described.     

Role,effect, and influences of micro and nano‐fillers on various properties of polymer matrix composites for microelectronics: A review

Ever since the discovery of polymer composites, its potential has been anticipated for numerous applications in various fields such as microelectronics, automobiles, and industrial applications. In this paper, we review filler reinforced polymer composites for its enormous potential in microelectronic applications. The interface and compatibility between matrix and filler have a significant role in property alteration of a polymer nanocomposites. Ceramic reinforced polymeric nanocomposites are promising candidate dielectric materials for several micro‐ and nano‐electronic devices. Because of its synergistic effect like high thermal conductivity, low thermal expansion, and dielectric constant of ceramic fillers with the polymer matrix, the resultant nanocomposites have high dielectric breakdown strength. The thermal and dielectric properties are discussed in the view of filler alignment techniques and its effect on the composites. Furthermore, the effect of various surface modified filler materials in polymer matrix, concepts of network forming using filler, and benefits of filler alignment are also discussed in this work. As a whole, this review article addresses the overall view to novice researchers on various properties such as thermal and dielectric properties of polymer matrix composites and direction for future research to be carried out.     

Formation and spectra of clathrate hydrates of methanol and methanol-ether mixtures

Infrared spectra of mixed clathrate hydrates, with either ethylene oxide (EO) or tetrahydrofuran (THF) and methanol molecules as the guest species, have been obtained from thin films prepared by vapor deposition of D₂O mixtures in the 115–130 K range. Although methanol acts as a suppressant to the direct vapor deposition of a type I clathrate with EO, nearly complete conversion of 115 K amorphous codeposits, to the crystalline mixed clathrate, occurs upon warming near 150 K. By contrast, the type II clathrate of THF shows an increased crystalline quality when methanol is included in the vapor deposits of the mixed clathrate hydrate at 130 K. The observation of the O---D stretch-mode band of weakly bonded CD₃OD near 2575 cm⁻¹ is part of the evidence that the methanol molecules are encaged. However, as shown theoretically by Tanaka, the clathrate hydrates of methanol, even when mixed with an ether help gas, are not stable structures but form at low temperatures because of kinetic factors, only to decompose in the 140–160 K range. Attempts to prepare a simple type I or type II clathrate hydrate of methanol have produced mixed results. Limited amounts of clathrate hydrate form during deposition but annealing does not result in complete conversion to crystalline clathrates, particularly for host : guest ratios of 17 : 1.     

Controlled thermal decomposition of NaSi to derive silicon clathrate compounds

Formation conditions of two types of sodium containing silicon clathrate compounds were determined by the controlled thermal decomposition of sodium monosilicide NaSi under vacuum. The decomposition began at 360 °C. Much higher decomposition temperatures and the presence of sodium metal vapor were favorable for the formation of type I clathrate compound Na₈Si₄₆. Type II clathrate compound Na_xSi₁₃₆ was obtained as a single phase at a decomposition temperature <440 °C under the condition without sodium metal vapor. The type I clathrate compound was decomposed to crystalline Si above 520 °C. The type II clathrate compound was thermally more stable, and retained at least up to 550 °C in vacuum.     

An overview of boron nitride based polymer nanocomposites

Recently, boron nitride (BN) based materials have received significant attention in both academic and industrial sectors due to its interesting properties like large energy band gap, good resistance to oxidation, excellent thermal conductivity, thermal stability, chemical inertness, significant mechanical property and widespread applications. This review article deals with the preparation and properties of boron nitride and its nanocomposites with various polymers. Diverse polymers have been explored for the preparation of boron nitride filled polymer nanocomposites by adopting different mixing methods. Properties of the resulting polymer nanocomposites mainly depend up on filler size and dispersion, mixing conditions and type of interaction between polymer matrix and the filler. Herein, the structure, preparation and properties of various boron nitride based polymer nanocomposites are reviewed in detail along with a brief overview of different classes of BN nanomaterials.     

LDPE/Mg-Al layered double hydroxide nanocomposite: Thermal and flammability properties

Layered double hydroxides (LDHs) are new nanofillers which exhibit improved thermal and flammability properties in various kinds of polymer matrices. These materials have certain advantages over conventional metal hydroxides and also layered silicates so far as the flame retardancy is concerned. In this article, flammability and thermal properties of the nanocomposite based on low density polyethylene (LDPE) and Mg-Al based layered double hydroxide (Mg-Al LDH) are reported in detail. The nanocomposites containing different LDH concentrations were prepared by melt-compounding using a tightly intermeshing co-rotating twin-screw extruder. The morphological analysis reveals an exfoliated/intercalated type LDH particle morphology in these nanocomposites. The thermogravimetric analysis (TGA) shows that even a small amount of LDH improves the thermal stability and onset decomposition temperature in comparison with the unfilled LDPE. The heat release rate (HRR) and its maximum (PHRR) during cone-calorimeter investigation are found to be reduced significantly with increasing LDH concentration. The nanocomposites not only exhibit reduced total heat released (measure of propensity to produce long duration fire), but also lower tendency to fast fire growth (measured by the ratio of PHRR and time of ignition). The limited oxygen index (LOI) and the dripping behavior are also improved with increasing LDH concentration.     

Synthesis of soluble and meltable pre‐ceramic polymers for Zr‐containing ceramic nanocomposites

Polymer‐derived methods are one of the most important tools for the synthesis of ceramics with a finely dispersed microstructure. In this study, a soluble and meltable ZrC/C pre‐ceramic polymer, P‐DACZ, (which would later exhibit a high ceramic yield of 71 wt%) was synthesized via radical polymerization. By adding low molecular weight polycarbosilane in any proportion during the radical polymerization process of P‐DACZ, a soluble and meltable ZrC/SiC/C pre‐ceramic precursor, PCS‐DACZ (which would later exhibit a high ceramic yield of >80 wt%) was synthesized. After annealing at 1400 °C under an argon flow, the precursors converted into bulk ZrC/C and ZrC/SiC/C ceramic nanocomposites. The ZrC nanoparticles could resist any grain growth when heat‐treated at temperatures above 1800 °C because the C or SiC matrix prevented long‐range atomic diffusion of zirconium. Such ceramic nanocomposites would be suitable for structural and (multi)functional applications at harsh environments with high temperatures.     

Phase and Defect Engineering of GeTe-based Alloys for High Thermoelectric Performance

The widespread applications of thermoelectric(TE) materials in power generation and solid-state cooling require improving their TE figure of merit(ZT) significantly. Recently, GeTe-based alloys have shown great promise as mid-temperature TE materials with superhigh TE performance, mostly due to their relatively high-degeneracy band structures and low lattice thermal conductivity. In this perspective, we review the most recent progress of the GeTe-based TE alloys from the view of phase and defect engineering. These two strategies are the most widely-used and efficient approaches in GeTe-based alloys to optimize the transport properties of electrons and phonons for high ZT. The phase transition from rhombohedral to cubic structure is believed to improve the band convergence of GeTe-based alloys for higher electrical performance. Typical defects in GeTe-based alloys include the point defects from Ge vacancies and substitutional dopants, linear and planar defects from Ge vacancies. The defect engineering of GeTe-based alloys is important not only for optimizing the carrier density but also for tuning the band structure and phonon-scattering processes. The summarized strategies in this review can also be used as a reference for guiding the further development of GeTe-based alloys and also other TE materials.     

Efficient detection of CO2 by nanocomposites: Environmental and energy technologies

Nanocomposite materials have received much attention from scientists and engineers interested in the detection and photoreduction of CO₂ compounds. Their interest is due in large part to the unique properties of these materials, including their high degree of photoactivity, thermal stability, high surface area, and malleability. In the present review, we focus on several nanocomposite types used for the detection and photochemical reduction of CO₂: titania-based nanocomposites, chalcogenide-based nanocomposites, LDHs-based nanocomposites, and MOFs-based nanocomposites. More specifically, trends in green synthesis nanocomposites, methods for detecting CO₂ compounds, and the photoreduction of those compounds are summarized in this paper. Several modified approaches to nanocomposite materials have been discussed to achieve optimum results. Generally, we find that the presence of functional active groups, doping metal, and other semiconductor materials act as catalysts, significantly enhancing the photoreduction properties of nano-materials. Moreover, we will also discuss additional challenges, especially in regard to large-scale industrial applications. In our discussion, we will highlight the use of nanocomposite-based materials in the detection and photoreduction of CO₂. It is hoped that our findings will serve as a reference and inspiration for academic researchers and industrial professionals.     

Boron Nitride Nanomaterials for Thermal Management Applications

Hexagonal boron nitride nanosheets (BNNs) are analogous to their two‐dimensional carbon counterparts in many materials properties, in particular, ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. Significant recent advances in the production of BNNs, understanding of their properties, and the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and systems are highlighted herein. Major opportunities and challenges for further studies in this rapidly advancing field are also discussed.     

Polymer Composites for Thermoelectric Applications

This review covers recently reported polymer composites that show a thermoelectric (TE) effect and thus have potential application as thermoelectric generators and Peltier coolers. The growing need for CO₂‐minimizing energy sources and thermal management systems makes the development of new TE materials a key challenge for researchers across many fields, particularly in light of the scarcity or toxicity of traditional inorganic TE materials based on Te and Pb. Recent reports of composites with inorganic and organic additives in conjugated and insulating polymer matrices are covered, as well as the techniques needed to fully characterize their TE properties.     

The role of graphene interactions and geometry on thermal and electrical properties of epoxy nanocomposites: A theoretical to experimental approach

The influence of dispersion procedure and nanofiller geometry on thermal and electrical properties of graphene nanoplatelet (GNP) based composites has been investigated. A theoretical model, based on the contacts between adjacent nanoparticles, has been proposed aiming to connect thermal and electrical properties. It has been observed that GNP overlapping (type I) induces a decrease on thermal conductivity. Its effect on electrical conductivity is more complex and depends on the areas of overlap and in-plane contacts (type II). A higher type I area in comparison to type II implies an increase of electrical conductivity with overlapping whereas the opposite effect is found when type II area is higher than type I. The predicted results of the theoretical model fitted experimental measurements at different GNP contents and three roll milling processing conditions, giving a better overview of the influence of GNP geometry and interactions on electrical and thermal properties of nanocomposites.     

Thermal stimuli-responsive behavior of pyrene end-functionalized PDMS through tunable Π–Π interactions

Pyrene end-functionalized, telechelic poly(dimethyl siloxane) (PDMS) materials were synthesized and their response to different thermal stimuli was evaluated. The incorporation of pyrene end groups introduces strong π–π interactions that facilitated a broad range of thermally responsive properties, in some circumstances forming pyrene nanocrystals that serve as physical crosslinks leading to elastic materials. By synthesizing different chain lengths, samples exhibiting a 7 orders of magnitude change in storage modulus in response to thermal stimuli were produced by modifying only the end-groups (0.6 wt % of all polymer segments). Repeated thermal cycling during rheological experiments revealed that π–π interaction and crystallization/melting kinetics of pyrene chain-ends plays a key role in their thermal responsiveness. The properties of these materials were tuned by adding free pyrene, neat PDMS, or graphene oxide (GO) nanoparticles, making them attractive for many applications (e.g., tunable damping materials, heat/light sensors, conductive gels, or light repositionable adhesives). For example, nanocomposites containing 1 wt % GO caused the melting temperature for pyrene crystal domains to more than double, and even induced pyrene end-group crystallization in samples that did not exhibit crystals in neat form. It is hypothesized that these features originate from π–π interactions between pyrene ends and GO surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 159–168     

Fe2O3/rGO纳米复合物的制备及其储锂和储钠性能

金属氧化物可通过电化学转换反应与锂离子及钠离子发生多电子可逆结构转换,是一类极具应用前景的高容量锂离子和钠离子电池负极材料。实验以氧化石墨烯和铁盐为前驱体,采用简单的溶剂法,成功将Fe₂O₃纳米单晶粒子均匀负载于石墨烯的导电片层上,获得Fe₂O₃/rGO(还原氧化石墨烯)纳米复合材料。复合电极在锂离子和钠离子电池中都表现出优异的充放电性能和循环稳定性。实验结果表明石墨烯的包覆不仅能降低Fe₂O₃发生转换反应的电荷传递阻抗,而且能够稳定电极在循环过程中带来的结构转变,极大改善电极大电流充放能力和循环稳定性。本研究为发展高容量的锂离子和钠离子电池负极材料提供了可行的途径。     

All‐Inorganic Nanocrystals as a Glue for BiSbTe Grains: Design of Interfaces in Mesostructured Thermoelectric Materials

Nano‐ and mesostructuring is widely used in thermoelectric (TE) materials. It introduces numerous interfaces and grain boundaries that scatter phonons and decrease thermal conductivity. A new approach has been developed for the rational design of the interfaces in TE materials by using all‐inorganic nanocrystals (NCs) that serve as a “glue” for mesoscopic grains. For example, circa 10 nm Bi NCs capped with (N₂H₅)₄Sb₂Te₇ chalcogenidometallate ligands can be used as an additive to BiSbTe particles. During heat treatment, NCs fill up the voids between particles and act as a “glue”, joining grains in hot‐pressed pellets or solution‐processed films. The chemical design of NC glue allowed the selective enhancement or decrease of the majority‐carrier concentration near the grain boundaries, and thus resulted in doped or de‐doped interfaces in granular TE material. Chemically engineered interfaces can be used as to optimize power factor and thermal conductivity.     

Thermal properties of epoxy resin nanocomposites based on hydrotalcites

Epoxy resin nanocomposites containing home-made hydrotalcites (HTlc) have been prepared and their properties have been studied and compared with those of montmorillonite (MMT)-type layered silicates-based nanocomposites. Nanofiller dispersion in the polymer matrix has been evaluated by transmission (TEM) electron microscopy and wide angle X-ray diffraction (WAXD), while nanocomposite thermal properties have been studied in detail by thermogravimetric analysis (TGA/DTG) and cone calorimeter tests.The morphological studies have shown that the compatibilisation of the above two type of nanofillers allowed us to obtain nanostructured materials. As far as thermal properties are concerned, nanocomposites based on HTlc are found to decompose, both in air and nitrogen, following a trend similar to that of the neat polymer matrix, while in the case of the nanocomposite based on the organophilic MMT a slight improvement was found in air. Conversely, cone calorimetric tests have demonstrated that only the organophilic hydrotalcite was capable of decreasing the peak of the heat release rate in a relevant way.     

Tuning Optimum Temperature Range of Bi2Te3‐Based Thermoelectric Materials by Defect Engineering

Bi₂Te₃‐based solid solutions, which have been widely used as thermoelectric (TE) materials for the room temperature TE refrigeration, are also the potential candidates for the power generators with medium and low‐temperature heat sources. Therefore, depending on the applications, Bi₂Te₃‐based materials are expected to exhibit excellent TE properties in different temperature ranges. Manipulating the point defects in Bi₂Te₃‐based materials is an effective and important method to realize this purpose. In this review, we focus on how to optimize the TE properties of Bi₂Te₃‐based TE materials in different temperature ranges by defect engineering. Our calculation results of two‐band model revel that tuning the carrier concentration and band gap, which is easily realized by defects engineering, can obtain better TE properties at different temperatures. Then, the typical paradigms about optimizing the TE properties at different temperatures for n‐type and p‐type Bi₂Te₃‐based ZM ingots and polycrystals are discussed in the perspective of defects engineering. This review can provide the guidance to improve the TE properties of Bi₂Te₃‐based materials at different temperatures by defects engineering.     

Wet chemical synthesis of Cu/TiO2 nanocomposites with integrated nano-current-collectors as high-rate anode materials in lithium-ion batteries

Using a soft-template assisted method, well-organized Cu/TiO(2) nanoarchitectured electrode materials with copper nanowires as their own current collectors are synthesized by controlled hydrolysis of tetrabutyl titanate in the presence of Cu-based nanowires, and investigated by SEM, TEM, XRD, Raman spectroscopy and electrochemical tests towards lithium storage. Two types of Cu/TiO(2) nanocomposites with different TiO(2) grain sizes are obtained by using different thermal treatments. The two types of Cu/TiO(2) nanocomposites show much enhanced rate performances compared with bare TiO(2). A high-rate capability (reversible capacity at 7500 mA g(-1) still accounts for 58% of its initial capacity at 50 mA g(-1)) is observed for the Cu/TiO(2) nanocomposite with smaller TiO(2) grain size. The improvements can be attributed to the integrated Cu nanowires as mechanical supports and efficient current collectors. A cell made from the Cu/TiO(2) nanoarchitectured electrodes exhibits promise as an energy storage device with both high energy and high power densities.     

Impact of SiC particle size upon the microstructure and characteristics of Ni-SiC nanocomposites

In order to deposit the Ni–SiC nanocomposites on Q235A steel substrates, magnetic pulse electrodeposition was employed in this article. Scanning electron microscopy (SEM), triboindenter in-situ nanomechanical testing, scanning probe microscopy (SPM), and X-ray diffraction (XRD) were used to assess the composites in terms of their compositions, microstructures, and microhardness. There were three specimens in total, with SiC particle sizes of 30, 80, and 200 nm, referred to as SN-30, SN-80, and SN-200 composites. With fine nickel grains and nanoparticles of SiC, SN-30 nanocomposites have fine, compact, and homogeneous architectures. There is a substantial impact of the size of SiC particles on the spectra of XRD for the nanocomposites. For SN-200 nanocomposite, the peaks of diffraction for SiC and Ni seemed to be high and sharp. SiC nanoparticle and Ni grain sizes in SN-30 nanocomposite were measured to be 34.4 and 381.3 nm, respectively. SN-30 and SN-200 composites have final depths of roughly 15.2 μm and 24.6 μm, accordingly. Following a subsequent investigation of the corrosion and wear characteristics of Ni-SiC nano-composites, the weight loss values of SN-30, SN-80, and SN-200 composites were calculated to be 1.01, 1.48, and 1.69 g, respectively, after corrosion testing for 24 h. During wear testing, however, just a few small pits were discovered on the SN-30 nanocomposite surface, indicating optimum wear resistance.     

Colossal Anisotropic Thermal Expansion in a Diazo-Functionalized Compound with Switchable Solid-State Behavior

Achieving substantial anisotropic thermal expansion (TE) in solid-state materials is challenging as most materials undergo volumetric expansion upon heating. Here, we describe colossal, anisotropic TE in crystals of an organic compound functionalized with two azo groups. Interestingly, the material exhibits distinct and switchable TE behaviors within different temperature regions. At high temperature, two-dimensional, area zero TE and colossal, positive linear TE (α=211 MK⁻¹) are attained due to dynamic motion, while at low temperature, moderate positive TE occurs in all directions. Investigation of the solid-state motion showed the change in enthalpy and entropy are quite different in the two temperature regions and solid-state NMR experiments support motion in the solid. Cycling experiments demonstrate that the solid-state motions and TE behaviors are completely reversible. These results reveal strategies for designing significant anisotropic and switchable behaviors in solid-state materials.