Enhanced microwave absorption property of silver decorated biomass ordered porous carbon composite materials with frequency selective surface incorporation

Porous carbon(PC)is a promising electromagnetic(EM)wave absorbing material thanks to its light weight,large specific surface area as well as good dissipating capacity.To further improve its microwave absorbing performance,silver coated porous carbon(Ag@PC)is synthesized by one-step hydro-thermal synthesis process making use of fir as a biomass formwork.Phase compositions,morphological structure,and microwave absorption capability of the Ag@PC has been explored.Research results show that the metallic Ag was successfully reduced and the particles are evenly distributed inward the pores of the carbon formwork,which accelerates graphitization process of the amorphous carbon.The Ag@PC composite without adding polyvinyl pyrrolidone(PVP)exhibits higher dielectric constant and better EM wave dissipating capability.This is because the larger particles of Ag give rise to higher electric conductivity.After combing with frequency selective surface(FSS),the EM wave absorbing performance is further improved and the frequency region below-10 d B is located in8.20-11.75 GHz,and the minimal reflection loss value is-22.5 dB.This work indicates that incorporating metallic Ag particles and FSS provides a valid way to strengthen EM wave absorbing capacity of PC material.

Integrated design optimization of composite frames and materials for maximum fundamental frequency with continuous fiber winding angles

Fiber reinforced composite frame structure is an ideal lightweight and large-span structure in the fields of aerospace,satellite and wind turbine.Natural fundamental frequency is one of key indicators in the design requirement of the composite frame since structural resonance can be effectively avoided with the increase of the fundamental frequency.Inspired by the concept of integrated design optmization of composite frame structures and materials,the design optimization for the maximum structural fundamental frequency of fiber reinforced frame structures is proposed.An optimization model oriented at the maximum structural fundamental frequency under a composite material volume constraint is established.Two kinds of independent design variables are optimized,in which one is variables represented structural topology,the other is variables of continuous fiber winding angles.Sensitivity analysis of the frequency with respect to the two kinds of independent design variables is implemented with the semi-analytical sensitivity method.Some representative examples in the manuscript demonstrate that the integrated design optimization of composite structures can effectively explore coupled effects between structural configurations and material properties to increase the structural fundamental frequency.The proposed integrated optimization model has great potential to improve composite frames structural dynamic performance in aerospace industries.

High-frequency magnetic properties and core loss of carbonyl iron composites with easy plane-like structures

To fully release the potential of wide bandgap(WBG)semiconductors and achieve high energy density and efficiency,a carbonyl iron soft magnetic composite(SMC)with an easy plane-like structure is prepared.Due to this structure,the permeability of the composite increases by 3 times(from 7.5 to 21.5)at 100 MHz compared with to the spherical carbonyl iron SMC,and the permeability changes little at frequencies below 100 MHz.In addition,the natural resonance frequency of the composite shifts to higher frequencies at 1.7 GHz.The total core losses of the composites at 10,20,and 30 m T are80.0,355.3,and 810.7 m W/cm^(3),respectively,at 500 k Hz.Compared with the spherical carbonyl iron SMC,the core loss at500 k Hz is reduced by more than 60%.Therefore,this kind of soft magnetic composite with an easy plane-like structure is a good candidate for unlocking the potential of WBG semiconductors and developing the next-generation power electronics.

Low frequency noise reduction using stiff light composite panels

The experiment presented in this paper is to investigate and analyze the noise reduction at low frequency using stiff light composite panels. Since these composite panels are made of lightweight and stiff materials, this actuation strategy will enable the creation of composite panels for duct noise control without using traditional heavy structural mass. The results suggest that the mass-spring resonance absorption in the case of a comparatively stiff thick panel with a thin flexible plate is more efficient with minimum weight, when subjected to low-frequency (<500 Hz). The efficiency of the panel absorber depends on the mass of the thin flexible plate and the stiffness of the panel.

Composite Type Wind Wave Frequency Spectrum

Based on the P-M spectrum and improved theoretical wind wave frequency spectrum, this paper presents the composite type wind wave frequency spectrum, which is given by 2 pieces and satisfies 4 conditions: the spectrum peak passes through a given point; spectral area m0 and significant wave height H have the relationship H = 4.0 ; at the piecewise point, the values of spectrum and slope are contin-

Tunable characteristics of bending resonance frequency in magnetoelectric laminated composites

As the magnetoelectric （ME） effect in piezoelectric/magnetostrictive laminated composites is mediated by mechanical deformation, the ME effect is significantly enhanced in the vicinity of resonance frequency. The bending resonance frequency （fr） of bilayered Terfenol-D/PZT （MP） laminated composites is studied, and our analysis predicts that （i） the bending resonance frequency of an MP laminated composite can be tuned by an applied dc magnetic bias （Hdc） due to the E effect; （ii） the bending resonance frequency of the MP laminated composite can be controlled by incorporating FeCuNbSiB layers with different thicknesses. The experimental results show that with H dc increasing from 0 Oe （1 Oe=79.5775 A/m） to 700 Oe, the bending resonance frequency can be shifted in a range of 32.68 kHz≤fr≤33.96 kHz. In addition, with the thickness of the FeCuNbSiB layer increasing from 0 μm to 90 μm, the bending resonance frequency of the MP laminated composite gradually increases from 33.66 kHz to 39.18 kHz. This study offers a method of adjusting the strength of dc magnetic bias or the thicknesses of the FeCuNbSiB layer to tune the bending resonance frequency for ME composite, which plays a guiding role in the ME composite design for real applications.

Wider frequency domain for negative refraction index in a quantized composite right–left handed transmission line

The refraction index of the quantized lossy composite right-left handed transmission line（CRLH-TL） is deduced in the thermal coherence state. The results show that the negative refraction index（herein the left-handedness） can be implemented by the electric circuit dissipative factors（i.e., the resistances R and conductances G） in a higher frequency band（1.446 GHz≤ ω ≤ 15 GHz）, and flexibly adjusted by the left-handed circuit components（Cl, Ll） and the right-handed circuit components（Cr, Lr） at a lower frequency（ω = 0.995 GHz）. The flexible adjustment for left-handedness in a wider bandwidth will be significant for the microscale circuit design of the CRLH-TL and may make the theoretical preparation for its compact applications.

Theoretical study on dynamic effective electroelastic properties of random piezoelectric composites with aligned inhomogeneities

The closed-form solutions of the dynamic problem of heterogeneous piezoelectric materials are formulated by introducing polarizations into a reference medium and using the generalized reciprocity theorem.These solutions can be reduced to the ones of an elastodynamic problem.Based on the effective medium method,these closedform solutions can be used to establish the self-consistent equations about the frequencydependent effective parameters,which can be numerically solved by iteration.Theoretical predictions are compared with the experimental results,and good agreement can be found.Furthermore,the analyses on the effects of microstructure and wavelength on the effective properties,resonance frequencies,and attenuation are also presented,which may provide some guidance for the microstructure design and analysis of piezoelectric composites.

Nonlinear vibrations of a composite circular plate with a rigid body

The influence of weights is usually ignored in the study of nonlinear vibrations of plates.In this paper,the effect of structure weights on the nonlinear vibration of a composite circular plate with a rigid body is presented.The nonlinear governing equations are derived from the generalized Hamilton's principle and the von Kármán plate theory.The equilibrium configurations due to weights are determined and validated by the finite element method(FEM).A nonlinear model for the vibration around the equilibrium configuration is established.Moreover,the natural frequencies and amplitude-frequency responses of harmonically forced vibrations are calculated.The study shows that the structure weights introduce additional linear and quadratic nonlinear terms into the dynamical model.This leads to interesting phenomena.For example,considering weights increases the natural frequency.Furthermore,when the influence of weights is considered,the vibration response of the plate becomes asymmetrical.

Eigen value analysis of composite hollow shafts using modified EMBT formulation considering the shear deformation along the thickness direction

Composite hollow shafts are used in power transmission applications due to their high specific stiffness and high specific strength.The dynamic characteristics of these shafts are important for transmission applications.Dynamic modelling of these shafts is generally carried out using Equivalent Modulus Beam Theory(EMBT)and Layerwise Beam Theory(LBT)formulations.The EMBT formulation is modified by considering stacking sequence,shear normal coupling,bending twisting coupling and bending stretching coupling.It is observed that modified EMBT formulation is underestimating the shafts stiffness at lower length/mean diameter(l/dm)ratios.In the present work,a new formulation is developed by adding shear deformation along the thickness direction to the existing modified EMBT formulation.The variation of shear deformation along the thickness direction is found using different shear deformation theories,i.e.,first-order shear deformation theory(FSDBT),parabolic shear deformation theory(PSDBT),trigonometric shear deformation theory(TSDBT),and hyperbolic shear deformation theory(HSDBT).The analysis is performed at l/d_(m) ratios of 5,10,15,20,25,30,35,and 40 for carbon/epoxy composites,E-glass/epoxy composites,and boron/epoxy composite shafts.The results show that new formulation has improved the bending natural frequency of the composite shafts for l/d_(m)<15 in comparison with modified EMBT.The effect of new formulation is more significant for the second and third bending modes of natural frequencies.

《西北农林科技大学学报(社会科学版)》2013-2022年高被引论文特征分析

利用中国知网引文数据库的资料,以2013-2022年《西北农林科技大学学报(社会科学版)》(以下简称《学报》)刊发文章总量的5%为标准遴选出的高被引论文作为研究对象,对高被引论文的栏目/专题、基金项目、高被引论文作者、第一作者机构和排名前20的高被引论文进行了分析,为组稿、约稿和筛选优质稿源提供依据。结果显示,《学报》2013-2022年共刊发论文1266篇,总被引频次为32051次,从中遴选出高被引论文63篇,高被引论文总被引频次为8308次,占总被引频次的25.92%。63篇高被引论文均属学报特色栏目——“三农”问题研究的下设专题论文,此栏目的高被引论文占到了73.02%,“精准扶贫”“乡村振兴”“农村电商”3个专题高被引论文表现抢眼。在高被引论文获助的基金项目中,国家社科基金资助20项,占31.75%,是获助的主体;国家自然科学基金资助论文居于第2位。高被引论文中作者的职称(学历)居前3位的分别是博士研究生、教授(研究员)、副教授(副研究员),其中博士研究生的高被引论文占到了总数的38.10%,表明其已经成为学术研究的主力军;发表2篇以上高被引论文的作者有3位。来源论文机构总被引频次的排序前5名为西北农林科技大学、中国农业大学、华中农业大学、华中科技大学和西安交通大学。被引频次最多的前20篇论文的统计结果与前面的统计情况相吻合,值得注意的是“精准扶贫”和“乡村振兴”两个专题的高被引论文占了几近一半。鉴于以上结果,今后应继续做好选题策划,持续加强“三农”问题研究特色栏目建设。组稿约稿中要扩大核心作者群,积极关注青年科技工作者研究成果,打造高水平服务团队,提升论文学术水平;大力推进新媒体融合出版,提升学术成果传播速度。

声波式机械天线的谐振频率调控方法

声波式机械天线作为机械天线的重要类型之一,其谐振频率与固有频率高度耦合,而目前声波式机械天线受到材料尺寸限制,谐振频率较高,无法满足甚低频或更低频段的需求。为调控声波式机械天线的谐振频率,对声波式机械天线进行了振动建模,提出了增加单边固定约束、增加自由端质量和去除天线部分材料三种方法,并通过有限元仿真对所提方法进行了验证。结果表明,三种方法均能有效调控声波式机械天线的谐振频率,且与解析模型具有良好的一致性。通过应力分布研究了三种调控谐振频率方法对天线辐射的电场强度或磁场强度的影响,对声波式机械天线的设计具有重要参考意义。

异形平面组合空腹夹层板楼盖舒适度实测研究

利用环境激励,采用时域法模态拟合方法对异形平面组合空腹夹层板楼盖结构进行了现场竖向振动模态测试,得到组合楼盖结构前4阶竖向振型、自振频率、阻尼比等参数,并与有限元软件计算出的振型对比分析;对异形平面组合空腹夹层版楼盖进行6种工况作用下的人致振动加速度时程测试,分别得到各工况下不同测点的加速度时程曲线及相应各测点的加速度峰值。结果表明,现场实测和有限元分析得到的组合空腹夹层板楼盖自振频率均远大于3Hz,且两者误差在5%之内;楼盖阻尼比均小于005;最不利工况下异形组合空腹夹层板楼盖竖向最大加速度峰值为0036m/s2,其自振频率和加速度峰值均满足《高层建筑混凝土结构技术规程》(JGJ 3—2010)和《组合楼板设计与施工规范》(CECS 273—2010)要求,楼盖应用在大跨度异形平面中具有较好的舒适度。

CIPs@Mn_(0.8)Zn_(0.2)Fe_(2)O_(4)-CNTs复合材料低频吸波性能研究

随着5G无线通信与低频雷达侦察技术的飞速发展,低频电磁波辐射已成为当代的严重问题。目前,中高频段吸波材料的研究已趋于成熟,而设计低频段吸波材料仍面临巨大的挑战,亟待研究者们解决。基于四分之一波长相消机制,本研究设计了0.5~3 GHz低频段复合吸波材料。采用简单的一步水热法,诱导铁氧体在羰基铁粉与碳纳米管表面生长,制备出CIPs@Mn_(0.8)Zn_(0.2)Fe_(2)O_(4)-CNTs三元复合材料,对比研究了碳纳米管含量对材料吸收峰频率的影响。实验结果表明,引入碳纳米管,一方面为材料带来了界面极化、偶极极化等额外的损耗机制,增加了材料的衰减系数;另一方面基于四分之一波长相消机制,高介电与高磁导率的耦合,使材料在低频段获得良好的阻抗匹配。最终,在4 mm厚度下,样品分别在2.11与1.75 GHz处,获得了–40.8与–32.1 dB的反射损耗,–10 dB带宽分别为1.70~2.70 GHz和1.40~2.20 GHz。该复合材料制备工艺简单,低频吸收性能良好,具有很大的应用潜力,为开发更有效的低频吸波材料提供了新的思路和方法。

高低频复合励磁磁心损耗量化的研究

针对高低频复合励磁磁元件磁心损耗难以精确测量和建模的问题,在详细分析现有磁心损耗测量方法和磁心损耗模型局限性的基础上,根据直流功率法测量原理搭建高低频复合励磁磁心损耗测量系统,利用损耗能精确测量的空心电感验证其测量精度。最后,基于直流偏置下脉宽调制(pulse width modulation,PWM)波励磁的磁心损耗模型,提出高低频复合励磁磁性元件的磁心损耗预测模型,并实验验证预测模型最大相对误差为-8.83%,具有较高的精度。

介质粗糙面与组合目标宽带电磁散射特性研究

文中对介质粗糙面和组合目标的宽带复合电磁散射特性进行研究,通过数值计算得到了复合散射系数频率响应曲线。分析结果显示,后向复合散射系数在宽频带范围内随着频率的变化而产生振荡,粗糙面的高度均方根、介电常数、组合目标尺度、目标介电常数等因素对复合散射系数有显著的影响,且影响规律较为复杂。相比之下,粗糙面的相关长度对复合散射系数的影响较为微弱。当目标是理想导体时,频率响应曲线近似呈现出准周期性的振荡形态。

中青年2型糖尿病患者人体成分与营养素餐次分布的相关性分析

目的 分析中青年2型糖尿病患者人体成分与膳食能量、营养素餐次分布的相关性。方法 选择102名中青年2型糖尿病患者进行膳食调查及人体成分测定。结果 患者平均体质指数处于超重范围,其上肢脂肪含量与每日加餐次数、晚餐碳水化合物餐次比呈正相关关系,与早餐优质蛋白比呈负相关关系;躯干脂肪含量与早餐碳水化合物餐次比呈正相关关系,与全日碳水化合物供能比呈负相关关系;下肢脂肪含量与每日餐次呈正相关关系,与早餐优质蛋白比呈负相关关系;躯干及上肢、下肢肌肉含量均与晚餐优质蛋白比、全日能量呈正相关关系,下肢、躯干肌肉含量与全日脂肪供能比及全日碳水化合物供能比呈负相关关系(P<0.05)。结论 为使中青年2型糖尿病患者人体成分更合理,在营养干预中,要保证能量适当、营养素科学配比,注重每日适宜的餐次及能量、营养素在各餐次中的合理分布。

基于低频吸声超构材料的复合消声器

针对管道内低频噪声难以抑制的问题,基于亥姆霍兹共振腔阵列吸声板和穿孔管消声器组合,设计了一种复合式宽带消声器。首先利用有限元法仿真分析传统穿孔管消声器,发现中低频消声能力较差,通过嵌入亥姆霍兹共振腔阵列吸声板吸收中低频噪声。采用仿真与实验的方式研究吸声板的声学性能:在400∼1000 Hz频段内的平均吸声系数达到了0.88。然后对复合式消声器进行数值模拟及3D打印阻抗管实验测试对比:复合式消声器在400∼1718 Hz频率范围内的平均传递损失为18.15 dB。最终实现了管道内全频带噪声有效控制。

一元及多元信号分解发展历程与展望

现实世界中,所获得的信号大部分都是非平稳和非线性的,将此类复杂信号分解为多个简单的子信号是重要的信号处理方法.1998年,提出希尔伯特–黄变换(Hilbert-Huang transform,HHT)以来,历经20余年的发展,信号分解已经成为信号处理领域相对独立又具有创新性的重要内容.特别是近10年,多元/多变量/多通道信号分解理论方法方兴未艾,在诸多领域得到了成功应用,但目前尚未见到相关综述报道.为填补这个空缺,从单变量和多变量两个方面系统综述了国内/外学者对主要信号分解方法的研究现状,对这些方法的时频表达性能进行分析和比较,指出这些分解方法的优势和存在的问题.最后,对信号分解研究进行总结和展望.

中文药学领域核心期刊2019—2023年高被引文献计量学分析

目的 通过对中文药学领域核心期刊高被引文献的特征进行分析,明确近5年来药学领域高被引文献的分布特点,把握国内药学领域研究热点。方法 以2023版《中国科技期刊引证报告(核心版)》中发布的中文药学领域核心期刊为期刊来源,检索2019—2023年发表在CNKI数据库上的中文药学领域核心期刊的文献,并根据被引频次,采用历时法确定高被引文献。借助Microsoft Excel及CiteSpace 6.1.R6软件,分析其载文量、作者、机构、基金及关键词等特征。结果 中文药学领域核心期刊43本,近5年共发表高被引文献479篇分布在40种期刊上,其中《中国临床药理学杂志》发文量最多,《中国药房》总被引频次最高,《西北药学杂志》篇均被引频次最高。高被引文献作者以小规模合作为主。发文机构以高校及其附属医院为主,主要包括中国药科大学、中国医学科学院北京协和医学院及山东中医药大学等。有基金项目的高被引文献共计339篇(70.77%)。各年度被引频次最高的文献均为综述类文献,基金支持率100%,且以国家级基金为主。频次较高关键词有“药理作用”“化学成份”“作用机制”及“研究进展”等。结论 文献质量与期刊质量呈正相关,高质量的期刊更能吸引优质稿源。综述类文献更加具有被引优势。近5年中文药学领域的研究热点是“药理作用”“化学成份”“作用机制”等,此外,“安全性”及“质量控制”等也是近年来持续关注的焦点。