The potassium sodium niobate, K0.5Na0.5NbO3, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

Electrode materials with three-dimensional (3D) mesoporous structures possess superior features, such as shortened solid-phase lithium diffusion distance, large pore volume, full lithium ion accessibility, and a high specific area, which can facilitate fast lithium ion transport and electron transfer between solid/electrolyte interfaces. In this work, we introduce a facile synthesis route for the preparation of a 3D nanoarchitecture of Ge coated with carbon (3D-Ge/C) via a carbothermal reduction method in an inert atmosphere. The 3D-Ge/C showed excellent cyclability: almost 86.8% capacity retention, corresponding to a charge capacity of 1216 mAh g -1 even after 1000 cycles at a 2 C-rate. Surprisingly, the high average reversible capacity of 1122 mAh g -1 was maintained at a high charge rate of 100 C (160 A g -1 ). Even at an ultrahigh charge rate of 400 C (640 A g -1 ), an average capacity of 429 mAh g -1 was attained. Further, the full cell composed of 3D-Ge/C anode and LiCoO2 cathode exhibited excellent rate capability and cyclability with 94.7% capacity retention over 50 cycles. 3D-Ge/C, which offers a high energy density like batteries as well as a high power density like supercapacitors, is expected to be used in a wide range of electrochemical devices.A novel, facile synthetic route has been proposed to prepare a 3D nanoarchitecture Ge coated with carbon (3D-Ge/C) via a carbothermal reduction. The GeO 2 /PVP composite was carbonized in an argon atmosphere at 775 °C for 1 h to carbonize the PVP. During carbonization, the carbothermal reduction of GeO 2 occurred and simultaneously formed Ge within a 3D structure.

Carbon release from thawing permafrost soils could significantly exacerbate global warming as the active‐layer deepens, exposing more carbon to decay. Plant community and soil properties provide a major control on this by influencing the maximum depth of thaw each summer (active‐layer thickness; ALT), but a quantitative understanding of the relative importance of plant and soil characteristics, and their interactions in determine ALTs, is currently lacking. To address this, we undertook an extensive survey of multiple vegetation and edaphic characteristics and ALTs across multiple plots in four field sites within boreal forest in the discontinuous permafrost zone (NWT, Canada). Our sites included mature black spruce, burned black spruce and paper birch, allowing us to determine vegetation and edaphic drivers that emerge as the most important and broadly applicable across these key vegetation and disturbance gradients, as well as providing insight into site‐specific differences. Across sites, the most important vegetation characteristics limiting thaw (shallower ALTs) were tree leaf area index (LAI), moss layer thickness and understory LAI in that order. Thicker soil organic layers also reduced ALTs, though were less influential than moss thickness. Surface moisture (0–6 cm) promoted increased ALTs, whereas deeper soil moisture (11–16 cm) acted to modify the impact of the vegetation, in particular increasing the importance of understory or tree canopy shading in reducing thaw. These direct and indirect effects of moisture indicate that future changes in precipitation and evapotranspiration may have large influences on ALTs. Our work also suggests that forest fires cause greater ALTs by simultaneously decreasing multiple ecosystem characteristics which otherwise protect permafrost. Given that vegetation and edaphic characteristics have such clear and large influences on ALTs, our data provide a key benchmark against which to evaluate process models used to predict future impacts of climate warming on permafrost degradation and subsequent feedback to climate.

Methane (CH 4 ) emissions from Arctic tundra are an important feedback to global climate. Currently, modelling and predicting CH 4 fluxes at broader scales are limited by the challenge of upscaling plot-scale measurements in spatially heterogeneous landscapes, and by uncertainties regarding key controls of CH 4 emissions. In this study, CH 4 and CO 2 fluxes were measured together with a range of environmental variables and detailed vegetation analysis at four sites spanning 300 km latitude from Barrow to Ivotuk (Alaska). We used multiple regression modelling to identify drivers of CH 4 flux, and to examine relationships between gross primary productivity (GPP), dissolved organic carbon (DOC) and CH 4 fluxes. We found that a highly simplified vegetation classification consisting of just three vegetation types (wet sedge, tussock sedge and other) explained 54% of the variation in CH 4 fluxes across the entire transect, performing almost as well as a more complex model including water table, sedge height and soil moisture (explaining 58% of the variation in CH 4 fluxes). Substantial CH 4 emissions were recorded from tussock sedges in locations even when the water table was lower than 40 cm below the surface, demonstrating the importance of plant-mediated transport. We also found no relationship between instantaneous GPP and CH 4 fluxes, suggesting that models should be cautious in assuming a direct relationship between primary production and CH 4 emissions. Our findings demonstrate the importance of vegetation as an integrator of processes controlling CH 4 emissions in Arctic ecosystems, and provide a simplified framework for upscaling plot scale CH 4 flux measurements from Arctic ecosystems.

AimsDespite multiple studies investigating the environmental controls on CH4 fluxes from arctic tundra ecosystems, the high spatial variability of CH4 emissions is not fully understood. This makes the upscaling of CH4 fluxes from plot to regional scale, particularly challenging. The goal of this study is to refine our knowledge of the spatial variability and controls on CH4 emission from tundra ecosystems.MethodsCH4 fluxes were measured in four sites across a variety of wet-sedge and tussock tundra ecosystems in Alaska using chambers and a Los Gatos CO2 and CH4 gas analyser.ResultsAll sites were found to be sources of CH4, with northern sites (in Barrow) showing similar CH4 emission rates to the southernmost site (ca. 300 km south, Ivotuk). Gross primary productivity (GPP), water level and soil temperature were the most important environmental controls on CH4 emission. Greater vascular plant cover was linked with higher CH4 emission, but this increased emission with increased vascular plant cover was much higher (86 %) in the drier sites, than the wettest sites (30 %), suggesting that transport and/or substrate availability were crucial limiting factors for CH4 emission in these tundra ecosystems.ConclusionsOverall, this study provides an increased understanding of the fine scale spatial controls on CH4 flux, in particular the key role that plant cover and GPP play in enhancing CH4 emissions from tundra soils.