Patch-level based vegetation change and environmental drivers in Tarim River drainage area of West China

Information on vegetation-related land cover change and the principle drivers is critical for environmental management and assessment of desertification processes in arid environments. In this study, we investigated patch-level based changes in vegetation and other major land cover types in lower Tarim River drainage area in Xinjiang, West China, and examined the impacts of environmental factors on those changes. Patterns of land cover change were analyzed for the time sequence of 1987-1999-2004 based on satellite-derived land classification maps, and their relationships with environmental factors were determined using Redundancy Analysis (RDA). Environmental variables used in the analysis included altitude, slope, aspect, patch shape index (fractal dimension), patch area, distance to water body, distance to settlements, and distance to main roads. We found that during the study period, 26% of the land experienced cover changes, much of which were the types from the natural riparian and upland vegetation to other land covers. The natural riparian and upland vegetation patches were transformed mostly to desert and some to farmlands, indicating expanding desertification processes of the region. A significant fraction of the natural riparian and upland vegetation experienced a phase of alkalinity before becoming desert, suggesting that drought is not the exclusive environmental driver of desertification in the study area. Overall, only a small proportion of the variance in vegetation-related land cover change is explainable by environmental variables included in this study, especially during 1987-1999, indicating that patch-level based vegetation change in this region is partly attributable to environmental perturbations. The apparent transformation from the natural riparian and upland vegetation to desert indicates an on-going process of desertification in the region

A Spacer Cation Assisted Nucleation and Growth Strategy Enables Efficient and High‐Luminance Quasi‐2D Perovskite LEDs

Quasi‐2D Ruddlesden‐Popper perovskites receive tremendous attention for application in light‐emitting diodes (LEDs). However, the role of organic ammonium spacers on perovskite film has not been fully‐understood. Herein, a spacer cation assisted perovskite nucleation and growth strategy, where guanidinium (GA+) spacer is introduced into the perovskite precursor and at the interface between the hole transport layer (HTL) and the perovskite, to achieve dense and uniform perovskite films with enhanced optical and electrical performance is developed. A thin GABr interface pre‐formed on HTL provides more nucleation sites for perovskite crystal; while the added GA+ in perovskite reduces the crystallization rate due to strong hydrogen bonding interacts with intermediates, which promotes the growth of enhanced‐quality quasi‐2D perovskite films. The ionized ammonium group ( NH3+) of GA+ also favors formation of polydisperse domain distribution, and amine or imine ( NH2 or NH) group interact with perovskite defects through coordination bonding. The spacer cation assisted nucleation and growth strategy is advantageous for producing efficient and high‐luminance perovskite LEDs, with a peak external quantum efficiency of over 20% and a luminance up to 100 000 cd m−2. This work can inform and underpin future development of high‐performance perovskite LEDs with concurrent high efficiency and brightness

Influence of Inertia and Low Active Mineral Admixture on Strength and Microstructure of Cement-Based Materials

Cement-based materials were investigated by comparing the strength and microstructure of pastes and mortar containing limestone powder or low quality fly ash. The compressive strength of the mortar at 28 and 90 d was examined whose microstructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis, and differential thermal analysis (TG-DTA). The results indicated that the strength of mortar decreased with increasing mineral admixtures. The limestone powder mainly acted as inert filler and hardly took part in the chemical reaction. Low quality fly ash may accelerate the formation of hydration products in samples with more chemically bonded water. This further resulted in a higher degree of cement hydration and denser microstructure, while the overall heat of hydration was reduced. At the early stage of hydration, low quality fly ash can be considered as an inert material whereas its reactivity at the later stage became high, especially for ground low quality fly ash

SEM Analysis of the Interfacial Transition Zone between Cement-Glass Powder Paste and Aggregate of Mortar under Microwave Curing

In order to investigate the effects of microwave curing on the microstructure of the interfacial transition zone of mortar prepared with a composite binder containing glass powder and to explain the mechanism of microwave curing on the improvement of compressive strength, in this study, the compressive strength of mortar under microwave curing was compared against mortar cured using (a) normal curing at 20 ± 1 °C with relative humidity (RH) > 90%; (b) steam curing at 40 °C for 10 h; and (c) steam curing at 80 °C for 4 h. The microstructure of the interfacial transition zone of mortar under the four curing regimes was analyzed by Scanning electron microscopy (SEM). The results showed that the improvement of the compressive strength of mortar under microwave curing can be attributed to the amelioration of the microstructure of the interfacial transition zone. The hydration degree of cement is accelerated by the thermal effect of microwave curing and Na+ partially dissolved from the fine glass powder to form more reticular calcium silicate hydrate, which connects the aggregate, calcium hydroxide, and non-hydrated cement and glass powder into a denser integral structure. In addition, a more stable triangular structure of calcium hydroxide contributes to the improvement of compressive strength

Strength and Microstructural Evolution of Magnesium Phosphate Cement Mortar in Plateau Environment

Climatic conditions in plateau areas can enormously affect the properties and microstructure of cement-based materials. This research investigates the strength development and microstructural changes in magnesium phosphate cement (MPC) mortars in a plateau environment. Experiments were conducted in parallel in a plateau area (Lhasa) and a plain area (Chengdu) to evaluate the effects of the water-to-binder ratio (w/b = 0.12, 0.14 and 0.16) and sand-to-binder ratio (s/b = 0.5, 0.75 and 1) on the compressive and flexural strength of MPC mortars. At the same time, hydration products were characterized via XRD, TGA, and SEM/EDX micro-analyses, and the porosity of the materials was also analyzed via MIP. The results demonstrated that curing in a plateau environment resulted in a decrease in workability and yielded higher strength at an early age (before 1 day) but degraded the long-term (180-day) strength of MPC mortars when compared with curing in a plain environment, irrespective of w/b and s/b ratios. Unlike the plain group, the plateau group revealed the deterioration of microstructures over time, including the decrease in struvite content, the morphology change in struvite crystals, and the increase in porosity, which resulted in the degradation of mechanical properties between 1 and 180 days. The strength loss can be effectively alleviated at lower w/b and s/b ratios

Effects of HPMC on Workability and Mechanical Properties of Concrete Using Iron Tailings as Aggregates

Iron ore tailings (IOTs) are gradually used as building materials to solve the severe ecological and environmental problems caused by their massive accumulation. However, the bulk density of IOT as aggregate is too large, which seriously affects the concrete properties. Therefore, in this paper, the effect of hydroxypropyl methylcellulose (HPMC) on the workability, mechanical properties, and durability of concrete prepared from IOT recycled aggregate was studied. The action mechanism of HPMC on the workability and the mechanical properties of the IOT concrete was analyzed by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM). The results show that HPMC can effectively improve the segregation problem caused by the sinking and air entrainment of IOT aggregate and improve the crack resistance of concrete with little effect on its compressive strength and electric flux. These results are due to the air-entraining thickening effect of HPMC, which improves the slurry viscosity, hinders the sinking of aggregate, and improves the workability. At the same time, HPMC film, after concrete hardening, will bridge the slurry and aggregate through physical and chemical effects, hinder the propagation of microcracks, and improve the crack resistance

EFFECT OF CARBONISATION ON THE MECHANICAL PROPERTIES AND HYDRATION OF MAGNESIUM AMMONIUM PHOSPHATE CEMENT

The flexural strength and compressive strength of a magnesium ammonium phosphate cement and a magnesium ammonium phosphate cement-fly ash composite binder before and after carbonisation were compared, and the effects of carbonisation on its hydration and microstructure were studied. The results show that carbonisation can transform the amorphous phase and dittmarite into crystalline struvite, and refine the pore size of more than 1000 nm, so as to improve the strength of the magnesium ammonium phosphate cement. Fly ash inhibits the hydration of magnesium oxide in the magnesium ammonium phosphate cement, thus greatly increasing the porosity, resulting in the reduction of strength; however, carbonisation curing promotes the hydration of magnesium oxide in the magnesium ammonium phosphate cement-fly ash composite binder, and increases the content of the crystalline struvite. Carbonisation reduces the porosity by refining the pore structure, and consequently improves the flexural and compressive strength of the magnesium ammonium phosphate cement-fly ash composite binder

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects

Stable Isotope Composition in Surface Water in the Upper Yellow River in Northwest China

Although stable isotopes of hydrogen and oxygen in surface waters (especially in river waters) are useful tools to understand regional hydrological processes, relevant information at some upper reaches of large rivers in western China is still limited. During 2016−2017, we focused on the Liujiaxia Reservoir along the upper Yellow River, where we collected surface water samples at two locations, above and below the dam (identified as “lake water” and “river water”). The results show that the heavy isotopes in lake and river waters are enriched during the warm months, when the river discharge is large, and depleted during the cold months. The slopes of the water line (δ2H versus δ18O) for both the lake and river waters were lower than that of the global mean, due to evaporation. The different d values of the lake and river water reflect the regional evaporation and water sources