FAP positive cancer-associated fibroblasts promote tumor progression and radioresistance in esophageal squamous cell carcinoma by transferring exosomal lncRNA AFAP1-AS1.

Cancer-associated fibroblasts (CAFs) are abundant and heterogeneous stromal cells in the tumor microenvironment, which play important roles in regulating tumor progression and therapy resistance by transferring exosomes to cancer cells. However, how CAFs modulate esophageal squamous cell carcinoma (ESCC) progression and radioresistance remains incompletely understood. The expression of fibroblast activation protein (FAP) in CAFs was evaluated by immunohistochemistry in 174 ESCC patients who underwent surgery and 78 pretreatment biopsy specimens of ESCC patients who underwent definitive chemoradiotherapy. We sorted CAFs according to FAP expression, and the conditioned medium (CM) was collected to culture ESCC cells. The expression levels of several lncRNAs that were considered to regulate ESCC progression and/or radioresistance were measured in exosomes derived from FAP+ CAFs and FAP- CAFs. Subsequently, cell counting kit-8, 5-ethynyl-2'-deoxyuridine, transwell, colony formation, and xenograft assays were performed to investigate the functional differences between FAP+ CAFs and FAP- CAFs. Finally, a series of in vitro and in vivo assays were used to evaluate the effect of AFAP1-AS1 on radiosensitivity of ESCC cells. FAP expression in stromal CAFs was positively correlated with nerve invasion, vascular invasion, depth of invasion, lymph node metastasis, lack of clinical complete response and poor survival. Culture of ESCC cells with CM/FAP+ CAFs significantly increased cancer proliferation, migration, invasion and radioresistance, compared with culture with CM/FAP- CAFs. Importantly, FAP+ CAFs exert their roles by directly transferring the functional lncRNA AFAP1-AS1 to ESCC cells via exosomes. Functional studies showed that AFAP1-AS1 promoted radioresistance by enhancing DNA damage repair in ESCC cells. Clinically, high levels of plasma AFAP1-AS1 correlated with poor responses to dCRT in ESCC patients. Our findings demonstrated that FAP+ CAFs promoted radioresistance in ESCC cells through transferring exosomal lncRNA AFAP1-AS1; and may be a potential therapeutic target for ESCC treatment.

Linkage and association mapping in multi-parental populations reveal the genetic basis of carotenoid variation in maize kernels.

Carotenoids are indispensable to plants and critical components of the human diet. The carotenoid metabolic pathway is conserved across plant species, but our understanding of the genetic basis of carotenoid variation remains limited for the seeds of most cereal crops. To address this issue, we systematically performed linkage and association mapping for eight carotenoid traits using six recombinant inbred line (RIL) populations. Single linkage mapping (SLM) and joint linkage mapping (JLM) identified 77 unique additive QTLs and 104 pairs of epistatic QTLs. Among these QTLs, we identified 22 overlapping hotspots of additive and epistatic loci, highlighting the important contributions of some QTLs to carotenoid levels through additive or epistatic mechanisms. A genome-wide association study based on all RILs detected 244 candidate genes significantly associated with carotenoid traits, 23 of which were annotated as carotenoid pathway genes. Effect comparisons suggested that a small number of loci linked to pathway genes have substantial effects on carotenoid variation in our tested populations, but many loci not associated with pathway genes also make important contributions to carotenoid variation. We identified ZmPTOX as the causal gene for a QTL hotspot (Q10/JLM10/GWAS019); this gene encodes a putative plastid terminal oxidase that produces plastoquinone-9 used by two enzymes in the carotenoid pathway. Natural variants in the promoter and second exon of ZmPTOX were found to alter carotenoid levels. This comprehensive assessment of the genetic mechanisms underlying carotenoid variation establishes a foundation for rewiring carotenoid metabolism and accumulation for efficient carotenoid biofortification.

Color Printing and Encryption with Polarization-Switchable Structural Colors on All-Dielectric Metasurfaces.

Metasurface-based structural color with high resolution is promising for color printing and encryption. However, achieving tunable structural colors in practical applications is challenging owing to the immutability after the fabrication of metasurfaces. Herein, we proposed the polarization-switchable dielectric metasurfaces with full colors. The colorful images can be switched on/off by controlling the polarization of incident light. For the nanorods metasurfaces, all colors turned to black in the "off" mode because of the near-zero reflection, and the uniform black was advantageous for designing encryption applications. For the nanocrosses metasurfaces, colors reversed in two different "on" modes and images hidden in the "off" mode. With the polarization-sensitive metasurfaces, a fish-bird image, an overlapped dual-channel image, and a green-red heart image were obtained, respectively. The demonstrations can be applied to dynamic displays, optical cryptography, multichannel imaging, and optical data storage.

Plasmonic Nanostructure Biosensors: A Review.

Plasmonic nanostructure biosensors based on metal are a powerful tool in the biosensing field. Surface plasmon resonance (SPR) can be classified into localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (PSPP), based on the transmission mode. Initially, the physical principles of LSPR and PSPP are elaborated. In what follows, the recent development of the biosensors related to SPR principle is summarized. For clarity, they are categorized into three groups according to the sensing principle: (i) inherent resonance-based biosensors, which are sensitive to the refractive index changes of the surroundings; (ii) plasmon nanoruler biosensors in which the distances of the nanostructure can be changed by biomolecules at the nanoscale; and (iii) surface-enhanced Raman scattering biosensors in which the nanostructure serves as an amplifier for Raman scattering signals. Moreover, the advanced application of single-molecule detection is discussed in terms of metal nanoparticle and nanopore structures. The review concludes by providing perspectives on the future development of plasmonic nanostructure biosensors.

Influential factors and transcriptome analyses of immature diploid embryo anthocyanin accumulation in maize.

BACKGROUND: Anthocyanins are widely applied as a marker for haploid identification after haploid induction in maize. However, the factors affecting anthocyanin biosynthesis in immature embryos and the genes regulating this process remain unclear. RESULTS: In this study, we analyzed the influence of genetic background of the male and female parents, embryo age and light exposure on anthocyanin accumulation in embryos. The results showed that light exposure was the most crucial factor enhancing the pigmentation of immature embryos. The identification accuracy of haploid embryos reached 96.4% after light exposure, but was only 11.0% following dark treatment. The total anthocyanin content was 7-fold higher in immature embryos cultured for 24 h under light conditions compared to embryos cultured in the dark. Transcriptome analysis revealed that the differentially expressed genes between immature embryos cultured for 24 h in dark and light chambers were significantly enriched in the pathways of flavonoid, flavone, flavonol and anthocyanin biosynthesis. Among the genes involved in anthocyanin biosynthesis, five up-regulated genes were identified: F3H, DFR, ANS, F3'H and the MYB transcription factor-encoding gene C1. The expression patterns of 14 selected genes were confirmed using quantitative reverse transcription-polymerase chain reaction. CONCLUSION: Light is the most important factor facilitating anthocyanin accumulation in immature embryos. After 24 h of exposure to light, the expression levels of the structural genes F3H, DFR, ANS, F3'H and transcription factor gene C1 were significantly up-regulated. This study provides new insight into the factors and key genes regulating anthocyanin biosynthesis in immature embryos, and supports improved efficiency of immature haploid embryo selection during doubled haploid breeding of maize.

Tunable structural colors generated by hybrid Si3N4 and Al metasurfaces.

Metasurfaces with the capability of spectrum manipulation at subwavelength can generate structural colors. However, their practical applications in dynamic displays are limited because their optical performance is immutable after the fabrication of the metasurfaces. In this study, we demonstrate a color-tunable metasurface using numerical analysis. Moreover, we select a low-refractive-index dielectric material, Si3N4, which leaks the electric field to its surroundings. We investigate the potencial of these metasurfaces by simulations to achieve color-tuneable devices with encrypted watermarks. This modulation of colors can be applied to encrypted watermarks, anti-counterfeiting, and dynamic displays.

First-Principles Design of Na-ion Superionic Conductors: Interstitial-Based Na Diffusion in NaCuZrS3.

In recent years all-solid-state sodium-ion batteries (SS-SIBs) have drawn significant attention due to their potential to be safer and lower cost than lithium-ion batteries. However, the lack of sodium solid-state electrolytes with high ionic conductivity has become one of the major challenges. Here, with first-principles computation we took NaCuZrS3 , consisting of earth-abundant and environmentally benign elements only, as an example to study Na-ion transport in the post-perovskite-like structure and used computation-guided design to improve its potential as a solid-state electrolyte. With ab initio molecular dynamics simulation and nudged elastic band calculation, we studied possible diffusion mechanisms in this material and found that Na ion interstitials have a favorable migration barrier of 0.22 eV, which is among the smallest in the literature reported values. Considering the large formation energy of Frenkel defects, we proposed doping strategy to introduce extra Na interstitials in the material. Our study suggests that the post-perovskite-like sulfides are worth of exploration for applications in SS-SIBs.

A newly characterized allele of ZmR1 increases anthocyanin content in whole maize plant and the regulation mechanism of different ZmR1 alleles.

KEY MESSAGE: The novel ZmR1CQ01 allele for maize anthocyanin synthesis was identified, and the biological function and regulatory molecular mechanisms of three ZmR1 alleles were unveiled. Anthocyanins in maize are valuable to human health. The R1 gene family is one of the important regulatory genes for the tissue-specific distribution of anthocyanins. R1 gene allelic variations are abundant and its biological function and regulatory molecular mechanisms are not fully understood. By exploiting genetic mapping and transgenic verification, we found that anthocyanin pigmentation in maize leaf midrib was controlled by ZmR1 on chromosome 10. Allelism test of maize zmr1 EMS mutants confirmed that anthocyanin pigmentation in leaf sheath was also controlled by ZmR1. ZmR1CQ01 was a novel ZmR1 allelic variation obtained from purple maize. Its overexpression caused the whole maize plant to turn purple. ZmR1B73 allele confers anthocyanin accumulation in near ground leaf sheath rather than in leaf midribs. The mRNA expression level of ZmR1B73 was low in leaf midribs, resulting in no anthocyanin accumulation. ZmR1B73 overexpression promoted anthocyanin accumulation in leaf midribs. Loss of exon 5 resulted in ZmR1ZN3 allele function destruction and no anthocyanin accumulation in leaf midrib and leaf sheath. DNA affinity purification sequencing revealed 1010 genes targeted by ZmR1CQ01, including the bz2 in anthocyanin synthesis pathway. RNA-seq analysis showed 55 genes targeted by ZmR1CQ01 changed the expression level significantly, and the expression of genes encoding key enzymes in flavonoid and phenylpropanoid biosynthesis pathways were significantly up-regulated. ZmR1 functional molecular marker was developed. These results revealed the effects of transcriptional regulation and sequence variation on ZmR1 function and identified the genes targeted by ZmR1CQ01 at the genome-wide level.

Coupling plasmon-waveguide resonance and multiple plasma modes in hyperbolic metamaterials for high-performance sensing.

A sensor based on plasmon-waveguide resonance (PWR) exhibits an impressive narrow linewidth and has attracted extensive attention in plasmon label-free sensing. However, the low surface electric field intensity limits the detection ability of biomolecules, where the refractive index changes are restricted at the sensor surface. In this study, we study the coupling of PWR and multiple plasma modes in a hyperbolic metamaterial (HMM), combining narrow linewidth and electric field enhancement advantages. The PWR-HMM sensor includes a gold film, lossless dielectric layer, and metal/dielectric multilayer HMM array composed of 2-layer Au/Al2O3stacks. The evanescent field of PWR is used to excite multiple plasma modes in the HMM. The figure of merit of the proposed structure reaches 5417/RIU owing to the existence of lossless dielectric layer, which is 11.7 times than the conventional gold film structure. The maximum bulk sensitivity of the PWR-HMM sensor was 43 000 nm/RIU. In comparison with PWR, the surface electric field intensity and the surface sensitivity of PWR-HMM increase by four and two times, respectively. Furthermore, comparing the sensing performance of the PWR-HMM sensor and PWR-nanoparticle (NP) sensor (coupling PWR and localized surface plasmon resonance), it was found that PWR-HMM has 20% higher surface sensitivity than the PWR-NP. A sensing mechanism coupling PWR and multiple plasma modes in the HMMs opens a gate to significantly improve the PWR sensors performance, which is expected to be used to resolve urgent issues in biological, medical and clinical applications.

Genome-wide identification, characterization, and expression analysis of the monovalent cation-proton antiporter superfamily in maize, and functional analysis of its role in salt tolerance.

Na+, K+ and pH homeostasis are important for plant life and they are controlled by the monovalent cation proton antiporter (CPA) superfamily. The roles of ZmCPAs in salt tolerance are not fully elucidated. In this study, we identified 35 ZmCPAs comprising 13 Na+/H+ exchangers (ZmNHXs), 16 cation/H+ exchanger (ZmCHXs), and 6 K+ efflux antiporters (ZmKEAs). All ZmCPAs have transmembrane domains and most of them were localized to plasma membrane or tonoplast. ZmCHXs were specifically highly expressed in anthers, while ZmNHXs and ZmKEAs showed high expression in various tissues. ZmNHX5 and ZmKEA2 were up-regulated in maize seedlings under both NaCl and KCl stresses. Yeast complementation experiments revealed the roles of ZmNHX5, ZmKEA2 in NaCl tolerance. Analysis of the maize mutants further validated the salt tolerance functions of ZmNHX5 and ZmKEA2. Our study highlights comprehensive information of ZmCPAs and provides new gene targets for salt tolerance maize breeding.

Genetic basis of maize kernel oil-related traits revealed by high-density SNP markers in a recombinant inbred line population.

BACKGROUND: Maize (Zea mays ssp. mays) is the most abundantly cultivated and highly valued food commodity in the world. Oil from maize kernels is highly nutritious and important for the diet and health of humans, and it can be used as a source of bioenergy. A better understanding of genetic basis for maize kernel oil can help improve the oil content and quality when applied in breeding. RESULTS: In this study, a KUI3/SC55 recombinant inbred line (RIL) population, consisting of 180 individuals was constructed from a cross between inbred lines KUI3 and SC55. We phenotyped 19 oil-related traits and subsequently dissected the genetic architecture of oil-related traits in maize kernels based on a high-density genetic map. In total, 62 quantitative trait loci (QTLs), with 2 to 5 QTLs per trait, were detected in the KUI3/SC55 RIL population. Each QTL accounted for 6.7% (qSTOL1) to 31.02% (qBELI6) of phenotypic variation and the total phenotypic variation explained (PVE) of all detected QTLs for each trait ranged from 12.5% (OIL) to 52.5% (C16:0/C16:1). Of all these identified QTLs, only 5 were major QTLs located in three genomic regions on chromosome 6 and 9. In addition, two pairs of epistatic QTLs with additive effects were detected and they explained 3.3 and 2.4% of the phenotypic variation, respectively. Colocalization with a previous GWAS on oil-related traits, identified 19 genes. Of these genes, two important candidate genes, GRMZM2G101515 and GRMZM2G022558, were further verified to be associated with C20:0/C22:0 and C18:0/C20:0, respectively, according to a gene-based association analysis. The first gene encodes a kinase-related protein with unknown function, while the second gene encodes fatty acid elongase 2 (fae2) and directly participates in the biosynthesis of very long chain fatty acids in Arabidopsis. CONCLUSIONS: Our results provide insights on the genetic basis of oil-related traits and a theoretical basis for improving maize quality by marker-assisted selection.

Ultra-wideband tunable metamaterial perfect absorber based on vanadium dioxide.

A dynamically adjustable ultra-wideband metamaterial perfect absorber (MPA) is proposed which consists of three resonance rings based on vanadium dioxide (VO2) and a metal ground layer separated by a dielectric spacer. The simulation results show that the terahertz (THz) absorption bandwidth of more than 90% absorptance reaches 3.30 THz, which covers from 2.34 to 5.64 THz, under different incident polarization angles. The range is better than that of previous VO2-based reports. Moreover, when the conductivity of VO2 changes from 200 S/m to 2×105 S/m, the absorption peak intensity can be adjusted continuously from 4% to 100%. The key is to optimize the geometric structure through interference cancellation and impedance matching theory, to achieve better absorption bandwidth and efficiency. Besides, the terahertz absorber has a wide-angle absorption effect both in TE and TM waves. Thus, the designed absorber may have many potential applications in modulating, sensing and imaging technology.

Natural variations in the P-type ATPase heavy metal transporter gene ZmHMA3 control cadmium accumulation in maize grains.

Cadmium (Cd) accumulation in maize grains is detrimental to human health. Developing maize varieties with low Cd content is important for safe consumption of maize grains. However, the key genes controlling maize grain Cd accumulation have not been cloned. Here, we identified one major locus for maize grain Cd accumulation (qCd1) using a genome-wide association study (GWAS) and bulked segregant RNA-seq analysis with a biparental segregating population of Jing724 (low-Cd line) and Mo17 (high-Cd line). The candidate gene ZmHMA3 was identified by fine mapping and encodes a tonoplast-localized heavy metal P-type ATPase transporter. An ethyl methane sulfonate mutant analysis and an allelism test confirmed that ZmHMA3 influences maize grain Cd accumulation. A transposon in intron 1 of ZmHMA3 is responsible for the abnormal amino acid sequence in Mo17. Based on the natural sequence variations in the ZmHMA3 gene of diverse maize lines, four PCR-based molecular markers were developed, and these were successfully used to distinguish five haplotypes with different grain Cd contents in the GWAS panel and to predict grain Cd contents of widely used maize inbred lines and hybrids. These molecular markers can be used to breed elite maize varieties with low grain Cd contents.

Efficient inverse design and spectrum prediction for nanophotonic devices based on deep recurrent neural networks.

The development of nanophotonic devices has presented a revolutionary means to manipulate light at nanoscale. How to efficiently design these devices is an active area of research. Recently, artificial neural networks (ANNs) have displayed powerful ability in the inverse design of nanophotonic devices. However, there is limited research on the inverse design for modeling and learning the sequence characteristics of a spectrum. In this work, we propose a deep learning method based on an improved recurrent neural network to extract the sequence characteristics of a spectrum and achieve inverse design and spectrum prediction. A key feature of the network is that the memory or feedback loops it comprises allow it to effectively recognize time series data. In the context of nanorods hyperbolic metamaterials, we demonstrated the high consistency between the target spectrum and the predicted spectrum, and the network learned the deep physical relationship concerning the structural parameter changes reflected on the spectrum. The effectiveness of our approach is also tested by user-drawn spectra. Moreover, the proposed model is capable of predicting an unknown spectrum based on a known spectrum with only 0.32% mean relative error. The prediction model may be helpful to predict data beyond the detection limit. We propose this versatile method as an effective and accurate alternative to the application of ANNs in nanophotonics, paving way for fast and accurate design of desired devices.

Stalk architecture, cell wall composition, and QTL underlying high stalk flexibility for improved lodging resistance in maize.

BACKGROUND: Stalk fracture caused by strong wind can severely reduce yields in maize. Stalks with higher stiffness and flexibility will exhibit stronger lodging resistance. However, stalk flexibility is rarely studied in maize. Stalk fracture of the internode above the ear before tasseling will result in the lack of tassel and pollen, which is devastating for pollination in seed production. In this study, we focused on stalk lodging before tasseling in two maize inbred lines, JING724 and its improved line JING724A1 and their F2:3 population. RESULTS: JING724A1 showed a larger stalk fracture angle than JING724, indicating higher flexibility. In addition, compared to JING724, JING724A1 also had longer and thicker stalks, with a conical, frustum-shaped internode above the ear. Microscopy and X-ray microcomputed tomography of the internal stalk architecture revealed that JING724A1 had more vascular bundles and thicker sclerenchyma tissue. Furthermore, total soluble sugar content of JING724A1, especially the glucose component, was substantially higher than in JING724. Using an F2:3 population derived from a JING724 and JING724A1 cross, we performed bulk segregant analysis for stalk fracture angle and detected one QTL located on Chr3: 14.00-19.28 Mb. Through transcriptome data analysis and ∆ (SNP-index), we identified two candidate genes significantly associated with high stalk fracture angle, which encode a RING/U-box superfamily protein (Zm00001d039769) and a MADS-box transcription factor 54 (Zm00001d039913), respectively. Two KASP markers designed from these two candidate genes also showed significant correlations with stalk fracture angle. CONCLUSIONS: The internode shape and glucose content are possibly correlated with stalk flexibility in maize. Two genes in the detected QTL are potentially associated with stalk fracture angle. These novel phenotypes and associated loci will provide a theoretical foundation for understanding the genetic mechanisms of lodging, and facilitate the selection of maize varieties with improved flexibility and robust lodging resistance.

Current-induced effective Dzyaloshinskii-Moriya interaction and its Kondo enhancement in double quantum dot.

We studied the nonequilibrium transport of serially coupled double quantum dots connected to ferromagnetic electrodes. We demonstrated that the nonadiabatic part of the spin gauge field resulted in a current-induced Dzyaloshinskii-Moriya (DM) interaction effect in a double quantum dot and numerically confirmed this observation through the hierarchical equations of motion approach. We report that the spin current and the effective DM interaction are enhanced in the Kondo regime. We demonstrate that this enhancement occurs because the Kondo resonance, which is supposed to be suppressed by the local ferromagnetic exchange, is enhanced by the inter-dot coupling. This additional Kondo resonance channel increases the spin current. In addition, the impact of the spin-spin interaction and the Kondo effect on tunnel magnetoresistance is discussed. Our results offer a new approach for controlling the non-collinear spin interaction in double quantum dot devices.

Comparative Proteomics of Contrasting Maize Genotypes Provides Insights into Salt-Stress Tolerance Mechanisms.

Salt stress is a major abiotic factor limiting maize yield. To characterize the mechanism underlying maize salt tolerance, we compared the seedling root proteomes of salt-tolerant Jing724 and salt-sensitive D9H. The germination rate and growth parameter values (weight and length) were higher for Jing724 than for D9H under saline conditions. Using an iTRAQ-based method, we identified 513 differentially regulated proteins (DRPs), with 83 and 386 DRPs specific to Jing724 and D9H, respectively. In salt-stressed Jing724, the DRPs were primarily associated with the pentose phosphate pathway, glutathione metabolism, and nitrogen metabolism. Key DRPs, such as glucose-6-phosphate 1-dehydrogenase, NADPH-producing dehydrogenase, glutamate synthase, and glutamine synthetase, were identified based on pathway enrichment and protein-protein interaction analyses. Moreover, salt-responsive proteins in Jing724 seedlings were implicated in energy management, maintenance of redox homeostasis, detoxification of ammonia, regulation of osmotic homeostasis, stress defense and adaptation, biotic cross-tolerance, and regulation of gene expression. Quantitative analyses of superoxide dismutase activity, malondialdehyde content, relative electrolyte leakage, and proline content were consistent with the predicted changes based on DRP functions. Furthermore, changes in the abundance of eight representative DRPs were correlated with the corresponding mRNA levels. Our results may be useful for elucidating the molecular networks mediating salt tolerance.

Manipulation of Pauli spin blockade in double quantum dot systems.

Pauli spin blockade (PSB) is a significant physical effect in double quantum dot (DQD) systems. In this paper, we start from the fundamental quantum model of the DQD with the electron-electron interaction being considered and then systematically study the PSB effect in DQD by using a recently developed nonperturbative method, the hierarchical equations of motion approach. By checking the current-voltage and nonequilibrium spectral function features, the physical picture of the PSB is explicitly elucidated. Then, various kinds of manipulation of PSBs are discussed, including gate voltage, exchange interaction, and electron spin resonance. Three main characteristics beyond low-order perturbation theory are demonstrated in detail as follows: (1) the finite leakage current in the strongly correlated limit; (2) the enhancement and lifting of PSB by exchange interaction; and (3) the ON-and-OFF switch of PSB by real-time modulation.

Dynamics of urban expansion and form changes impacting carbon emissions in the Guangdong-Hong Kong-Macao Greater Bay Area counties.

Cities are the main carriers of social and economic development, and they are also important sources of carbon emissions. Therefore, it is essential to explore the impact of urban expansion and form changes on carbon emissions. Here, we attempted to analyzes the relationship between urban expansion and carbon emissions at the county level in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) from 1997 to 2017. It further decomposes the driving effects of carbon emissions from multiple factors, and considers the spatial heterogeneity between different urban form changes and driving effects. The results show that: The relationship between urban expansion and carbon emissions in the GBA has gone through three stages from 1997 to 2017, with 2012 as a turning point. Optimization of economic development models and strict protection of the ecological environment can effectively control carbon emissions. After 2012, the economic development effect (GE) and population scale effect (PE) are the driving factors of carbon emissions, while the carbon emission intensity effect (CE) and urban land intensity effect (UE) are the inhibitory factors of carbon emissions. The contribution rate of UE to carbon emission reduction can reach 86 %. The impact of urban form changes on carbon emissions has spatial heterogeneity. The changes in urban form have a significant impact on the carbon emissions of counties in Dongguan and Shenzhen. The increase in fragmentation indirectly promotes carbon emissions. In 2007-2012, the increase in centrality significantly weakened the economic development effect, which is conducive to emission reduction. After 2007, the increase in compactness in counties in the eastern part of the GBA, including Zhongshan and Zhuhai, is not conducive to emission reduction.

Molecular dynamics study on the influence of thermal aging on the mechanical properties of epoxy resins for high voltage bushing.

CONTEXT: Thermal aging significantly deteriorates the mechanical properties and service performance of epoxy resins used for the high-voltage bushing. Current studies on the thermal aging behavior of epoxy resins mainly focus on experimental observations. However, an in-depth understanding of the mechanism of thermal aging of epoxy resins requires the monitoring of structural evolution of epoxy resins during thermal aging at the molecular level. To thoroughly analyze the intrinsic factors affecting structural evolution and the effect of thermal aging on the mechanical properties of epoxy resin for high-voltage bushing, epoxy resin models with different crosslinking degrees were established and thermal aging treatments at various temperatures and time periods were carried out by molecular dynamics simulation. It was found that the tensile strength of the epoxy resin was enhanced with the increase of the crosslinking degree, which was related to the elevation of the proportion of C-N and O-H bonds in its structure. With the increase of thermal aging temperature, the tensile strength of the epoxy resin decreased, which was related to the formation of weak bonds. At the early stage of thermal aging and after a period of high-temperature thermal aging, the strength of epoxy resin significantly decreases. The thermal aging of the epoxy resin is accelerated under external loading. In addition, the crosslinking degree and curing agent also affect the thermal aging resistance of epoxy resins. The results of this study can provide guidance for predicting and improving the thermal aging resistance of epoxy resins. METHODS: Materials Studio was used to construct molecular models and complete crosslinking reactions. DGEBA and 44DDS (or 33DDS) were mixed at a ratio of 2:1, followed by crosslinking reaction. During this process, the Nose method was used to control temperature, the Berendsen method was used to control pressure, and the polymer consistent force field (PCFF) was used to control the motion and force of atoms. Isobaric-isothermal ensemble (NPT ensemble) was used to heat up epoxy resin models to various thermal aging temperatures of 400 K, 500 K, 600 K and 700 K. The models were maintained at these temperatures for different thermal aging times of 100 ps, 200 ps, 300 ps, 400 ps, 500 ps, 600 ps, 700 ps and 800 ps. Afterwards, the models were cooled down to 300 K and subjected to uniaxial tensile testing at this temperature with a strain rate of 1 × 109 s-1. The structural configurations and stress-strain data during the tensile process were recorded. The flow stress of the material was derived by counting the average stress in the 20-50% strain interval.