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Waste heat management holds great promise to create a sustainable and energy-efficient society as well as contributes to the alleviation of global warming. Harvesting and converting this waste heat in order to improve the efficiency is a major challenge. Here we report biomimetic nacre-like hydroxyl-functionalized boron nitride (BN)-polyimide (PI) nanocomposite membranes as efficient 2D in-plane heat conductor to dissipate and convert waste heat at high temperature. The hierarchically layered nanostructured membrane with oriented BN nanosheets gives rise to a very large anisotropy in heat transport properties, with a high in-plane thermal conductivity (TC) of 51 W m−1 K−1 at a temperature of ∼300 °C, 7314% higher than that of the pure polymer. The membrane also exhibits superior thermal stability and fire resistance, enabling its workability in a hot environment. In addition to cooling conventional exothermic electronics, the large TC enables the membrane as a thin and 2D anisotropic heat sink to generate a large temperature gradient in a thermoelectric module (ΔT = 23 °C) through effective heat diffusion on the cold side under 220 °C heating. The waste heat under high temperature is therefore efficiently harvested and converted to power electronics, thus saving more thermal energy by largely decreasing consumption.

Thanks to their versatile redox behaviors, octahedral molecular sieves show promise in a range of electrochemical applications. Here the authors report a hexagonal polymorph of tungsten trioxide, an octahedral molecular sieve that exhibits fast proton (de)insertion for electrochromic devices.

Abstract Boron carbide/hafnium diboride composites were prepared by spark plasma sintering of a mixture of hafnium diboride and boron carbide powders. Boron carbide was prepared with a 13.3 at.% composition of carbon, known as the ideal carbon content to maximize the dimensionless figure of merit. The hafnium diboride content was varied between 0 and 20% by weight, and the effect on the thermoelectric properties was studied. Addition of HfB2 generally yielded an increase in the electrical conductivity and simultaneously a reduction in thermal conductivity, indicating it has potential as an enhancer of thermoelectric properties. However, the increase in electrical conductivity was not as large as observed in some composite systems, since HfB2 turned out to be a poor sintering additive leading to lower relative densities, and was furthermore offset by a moderate decrease in Seebeck coefficient. For future composite design, the sintering characteristics of the additives can be concluded as an important additional parameter to be taken into account. The optimal hafnium diboride content for relatively dense samples was found to be 10 wt%, resulting in an improvement in the maximum figure of merit, up to ZT = 0.20 at 730 °C.

Manganese (oxyhydr)oxides are synthesized at low temperature (60 or 95 °C) from MnO4−reduction by S2O32−in water. Particles are characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, and electron diffraction. The reactivity of S2O32−as a reducing agent is tuned by the temperature in order to selectively synthesize manganite γ-MnOOH at 60 °C and hausmannite Mn3O4at 95 °C. Feitknechtite β-MnOOH is identified as an intermediate phase during the formation of γ-MnOOH. Twinned triangle-shaped feitknechtite particles are obtained and proposed to grow through nanorod self-assembly. The transformation of feitknechtite nanotriangles into manganite nanowires occurs through dissolution of the feitknechtite particle core and recrystallization on the edge of the feitknechtite nanotriangles. [ABSTRACT FROM AUTHOR]

MnOOH and MnO2polymorphs are synthesized using a reaction between Mn2+and MnO4−in water at low temperature (95 °C). Nanoparticles are characterized using powder XRD, TEM and electron diffraction. Depending on the acidity of the aging medium, γ-MnO2compounds are shown to be obtained as nanorods (final pH = 2.0) or hollow nanocones (3.6 ≤ final pH ≤ 4.5). The external faces of the cones originate from heterogeneous oriented attachment of α-MnOOH nanorods on early cones. The selective attachment between (−110)γ–MnO2and (101)α–MnOOHfaces is discussed by taking into account the surface charge of the primary particles and the reactivity of the Mn–OHxsurface groups. This study underlines the role of the electrostatic repulsions and the surface reactivity in the oriented attachment mechanism in water for oxides. [ABSTRACT FROM AUTHOR]

Hollow γ-MnO2nanocones are synthesized by precipitation in water at 95 °C through a “one-pot” procedure. Characterization of the particles is carried out using powder XRD, electron diffraction, FESEM, and TEM. The hexagonal-shaped base is shown to originate from specific {021} twinning. The growth mechanism is investigated and proposed to proceed through two steps. The first stage involves the growth of pyramidal seeds that direct the final shape of the particles. The second stage proceeds through oriented attachment of primary α-MnOOH nanorods on previously formed γ-MnO2cones. Attachment is accompanied by phase transformation of the oxyhydroxide to the oxide by a topotactic reaction, because of the structural relations between both structures. This study reports on the first demonstration of specific twinning from oriented attachment. This also provides a deep insight into the role of structural defects for further design of complex and novel architectures. Additionally, the procedure developed herein occurs in water without any surfactant. It is therefore low cost and environmentally friendly. [ABSTRACT FROM AUTHOR]

A low-temperature (60 or 95 °C) “one-pot” procedure for the aqueous precipitation of manganese oxide nanoparticles is developed through the MnO4−/Mn2+reaction. Characterization of the particles is carried out using elemental analysis, mean oxidation state determination, powder XRD, FESEM, TEM and electron diffraction. Many synthesis parameters are investigated, such as acidity, reactant ratio, concentration and nature of the counter-cation, temperature and aging time. Speciation diagrams are drawn for the synthesis of five different pure phases (spinel-type Mn3O4, layered birnessite-type δ-MnO2, tunnel-based frameworks cryptomelane-type α-MnO2, γ-MnO2and pyrolusite β-MnO2). The influence of synthetic parameters is rationalized and reaction pathways toward various structures and morphologies are discussed. [ABSTRACT FROM AUTHOR]

A low temperature (60 or 95 °C), “one-pot” procedure for aqueous precipitation of layered manganese oxide core@shell particles is developed, based on the reaction between MnO 4−and Mn 2+. Characterization of the particles was carried out using powder XRD, nitrogen adsorption−desorption, FESEM, TEM, and electron diffraction. Textural and morphological tailoring is achieved through various experimental parameters, with the specific surface area adjusted between 15 and 110 m 2g −1while a ball-like or sheetlike shape is selectively obtained. The synthesis is based on a two-step precipitation process, with the first stage involving in situ seeding and enabling shape control via adjustment of the initial acidity. Hierarchical core−corona particles are obtained through a second slower heterogeneous nucleation stage, controlled by reactant ratio, aging time, temperature, and acidity of the aging medium. [ABSTRACT FROM AUTHOR]

Nanowires of one-dimensional manganese oxide (cryptomelane) have been synthesized using reaction between MnII(MnSO4) and MnVII(KMnO4) in water by a low-temperature route at 60 °C and 95 °C. Characterization of the nanoparticles was carried out using powder X-ray diffraction, transmission electron microscopy, electron diffraction, and nitrogen adsorption−desorption. Two synthesis methods were developed depending whether the pH was initially adjusted or fixed all along the solid formation. Both methods exhibited variation of the nanowire morphology with diameter, length, and specific area varying in the 15−40 nm, 0.1−1.1 m, and 35−110 m2·g-1ranges, respectively, depending on the growth conditions (acidity and temperature). The growth mechanism of the nanowires was proved to proceed by oriented attachment of primary 10 nm width nanorods for the lateral direction and dissolution−crystallization for the longitudinal direction. The diameter variation with acidic conditions was interpreted in terms of electrostatic interactions between primary nanorods. [ABSTRACT FROM AUTHOR]

A microporous manganese oxide nanocomposite is synthesized using heteroepitaxy in an aqueous medium at 95 °C. [ABSTRACT FROM AUTHOR]