首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   98362篇
  免费   10142篇
  国内免费   7933篇
化学   47933篇
晶体学   602篇
力学   8171篇
综合类   1064篇
数学   20347篇
物理学   38320篇
  2023年   1130篇
  2022年   2100篇
  2021年   2923篇
  2020年   3287篇
  2019年   2775篇
  2018年   2303篇
  2017年   2738篇
  2016年   3099篇
  2015年   2669篇
  2014年   4523篇
  2013年   6870篇
  2012年   4851篇
  2011年   5300篇
  2010年   4318篇
  2009年   5906篇
  2008年   6146篇
  2007年   6625篇
  2006年   5529篇
  2005年   4475篇
  2004年   3831篇
  2003年   3986篇
  2002年   3525篇
  2001年   3094篇
  2000年   2823篇
  1999年   2401篇
  1998年   2288篇
  1997年   1558篇
  1996年   1547篇
  1995年   1401篇
  1994年   1390篇
  1993年   1126篇
  1992年   1115篇
  1991年   767篇
  1990年   704篇
  1989年   579篇
  1988年   544篇
  1987年   528篇
  1986年   477篇
  1985年   554篇
  1984年   458篇
  1983年   286篇
  1982年   469篇
  1981年   636篇
  1980年   513篇
  1979年   570篇
  1978年   444篇
  1977年   363篇
  1976年   280篇
  1974年   105篇
  1973年   187篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
According to the Food and Agriculture Organization of the United Nations, approximately 1.3 billion tons of food is wasted each year, equivalent to approximately one-third of world production. Agri-food wastes are the source of proteins, carbohydrates, lipids, and other essential minerals that have been exploited for value-added products by the development of biorefineries and sustainable business as important elements of circular economies. The innovation and materialization of these types of processes, including the use of disruptive technologies on microbial bioconversion and enzyme technology, such as nanotechnology, metabolic engineering, and multi-omics platforms, increase the perspectives on the waste valorization process. Lignocellulolytic enzymes, pectinases, and proteases are mainly used as catalyzers on agri-food waste treatment, and their production in house might be the trend in near future for agro-industrial countries. Another way to transform the agri-food wastes is via aerobic or anaerobic microbial process from fungal or bacterial cultures; these processes are the key to produce waste enzymes.  相似文献   
3.
Different strategies for the preparation of efficient and robust immobilized biocatalysts are here reviewed. Different physico-chemical approaches are discussed.i.- The stabilization of enzyme by any kind of immobilization on pre-existing porous supports.ii.- The stabilization of enzymes by multipoint covalent attachment on support surfaces.iii.- Additional stabilization of immobilized-stabilized enzyme by physical or chemical modification with polymers.These three strategies can be easily developed when enzymes are immobilized in pre-existing porous supports. In addition to that, these immobilized-stabilized derivatives are optimal to develop enzyme reaction engineering and reactor engineering. Stabilizations ranging between 1000 and 100,000 folds regarding diluted soluble enzymes are here reported.  相似文献   
4.
Employing nanocrystals (NCs) as building blocks of porous aerogel network structures allows the conversion of NC materials into macroscopic solid structures while conserving their unique nanoscopic properties. Understanding the interplay of the network formation and its influence on these properties like size-dependent emission is a key to apply techniques for the fabrication of novel nanocrystal aerogels. In this work, CdSe/CdS dot/rod NCs possessing two different CdSe core sizes were synthesized and converted into porous aerogel network structures. Temperature-dependent steady-state and time-resolved photoluminescence measurements were performed to expand the understanding of the optical and electronic properties of these network structures generated from these two different building blocks and correlate their optical with the structural properties. These investigations reveal the influence of network formation and aerogel production on the network-forming nanocrystals. Based on the two investigated NC building blocks and their aerogel networks, mixed network structures with various ratios of the two building blocks were produced and likewise optically characterized. Since the different building blocks show diverse optical response, this technique presents a straightforward way to color-tune the resulting networks simply by choosing the building block ratio in connection with their quantum yield.  相似文献   
5.
孔帅  吴敏  聂凡  曾冬梅 《人工晶体学报》2022,51(11):1878-1883
采用磁控溅射法在ITO玻璃上制备了CdZnTe薄膜,探究机械磨抛对CdZnTe薄膜阻变特性的影响。通过对XRD图谱、Raman光谱、AFM显微照片等实验结果分析阐明了机械磨抛影响CdZnTe薄膜阻变特性的物理机制。研究结果表明,磁控溅射制备的薄膜为闪锌矿结构,F43m空间群。机械磨抛提高了CdZnTe薄膜的结晶质量;CdZnTe薄膜粗糙度(Ra)由磨抛前的3.42 nm下降至磨抛后的1.73 nm;磨抛后CdZnTe薄膜透过率和162 cm-1处的类CdTe声子峰振动峰增强;CdZnTe薄膜的阻变开关比由磨抛前的1.2增加到磨抛后的4.9。机械磨抛提高CdZnTe薄膜质量及阻变特性的原因可能是CdZnTe薄膜在磨抛过程中发生了再结晶。  相似文献   
6.
针对考虑几何和材料非线性的石英晶体板厚度剪切振动和弯曲振动的方程组,利用扩展伽辽金法对该方程组进行转化和求解,分别获得了强烈耦合的厚度剪切振动模态和弯曲振动模态的频率响应关系,绘制了不同振幅比和不同驱动电压影响下的频率响应曲线图。数值计算结果表明可以选取石英晶片的最佳长厚比尺寸来避免两种模态的强烈耦合。驱动电压的变化将引起石英晶体谐振器厚度剪切振动频率的明显改变,必须将振动频率的漂移值控制在常用压电声波器件的允许值之内。扩展伽辽金法对石英晶体板非线性振动方程组的求解为非线性有限元分析和偏场效应分析奠定了基础。  相似文献   
7.
氮和磷是水环境生物生长和繁殖必须的营养成分,影响水体初级生产力的水平,且水体富营养化水平与氮磷形态密切相关,随着水体环境的改变,沉积物会向水体释放氮磷,造成二次污染。同时,对外源氮磷污染来源的贡献进行定量识别,可有效管理和控制水体氮磷污染负荷。毗河和石亭江是沱江的重要支流,影响着长江母亲河的水质。采用钼锑抗分光光度法和连续提取法研究在枯水期和丰水期毗河和石亭江水体和表层沉积物中总氮(TN)、总磷(TP)及各形态氮磷的分布特性,对比不同土地利用类型河流氮磷行为特性及释放风险,并采用Multiple Linear Regression of the Absolute Principal Component Scores(APCS-MLR)受体模型进行氮磷污染源的识别和量化。研究结果表明:① 研究区水体和表层沉积物中氮磷均处于不同程度的污染水平,水体枯水期TP的主要贡献者是颗粒态无机磷(PIP)和颗粒态有机磷(POP),而丰水期却是颗粒态无机磷(PIP)和溶解态无机磷(DIP),两水期水体中TN的主要贡献者是硝态氮(NO-3-N)和有机氮(ON)。而在表层沉积物中,TP的主要贡献者是钙结合态磷(HCl-P),TN的主要贡献者是酸解态氮(HN)。在枯水期和丰水期,毗河表层沉积物生物有效磷(BAP)占TP的平均值(19.7%和23.0%)比石亭江的平均值(11.0%和12.5%)占比更高,具有较高的磷释放风险。研究发现,枯水期氮磷污染程度高于丰水期,而且石亭江的氮磷污染程度高于毗河。②APCS-MLR模型在毗河提取了城镇生活污水、生活垃圾堆积产生的渗滤液、动植物残体分解和养殖业废水4个污染源因子,其中城镇生活污水对毗河氮磷污染的贡献最大(枯水期50.9%,丰水期54.8%),而在石亭江提取了工业生产中产生的废水等、动植物残体的降解、农业废弃物的风化、农田排水渠的农业废水和农药化肥的不合理施用5个污染源因子,其中工业生产中产生的废水等对石亭江氮磷污染的贡献最大(枯水期58.7%,丰水期55.8%)。因此,当地相关部门应加强对高贡献污染源的管控,从而降低流域氮磷污染负荷。  相似文献   
8.
高端飞行器的可靠性往往受限于其发动机内部盘、板等构的结构强度和使用寿命,因此对这类部件的无损检测在制造业上具有巨大需求。超声检测作为一种应用广泛、高效、环保的检测方法,常常被应用于这类构件的检测中。但是,大厚度盘、板类构件内的微小缺陷反射能力弱,常规超声脉冲反射法无法进行有效检测。为实现大厚度盘、板类构件内部微小缺陷的识别和定位,采用共线异侧纵波混频法,通过和差频信号特征识别微小缺陷;研究缺陷埋深变化对混频效果的影响,通过测量和差频信号幅值变化,实现微小缺陷的深度定位。结果表明:该方法可有效识别7075铝合金中埋深80mm的φ0.2mm横孔微缺陷,且可实现微小缺陷的深度定位。  相似文献   
9.
The reaction of N2 with trinuclear niobium and tungsten sulfide clusters Nb3Sn and W3Sn (n=0–3) was systematically studied by density functional theory calculations with TPSS functional and Def2-TZVP basis sets. Dissociations of N−N bonds on these clusters are all thermodynamically allowed but with different reactivity in kinetics. The reactivity of Nb3Sn is generally higher than that of W3Sn. In the favorite reaction pathways, the adsorbed N2 changes the adsorption sites from one metal atom to the bridge site of two metal atoms, then on the hollow site of three metal atoms, and at that place, the N−N bond dissociates. As the number of ligand S atoms increases, the reactivity of Nb3Sn decreases because of the hindering effect of S atoms, while W3S and W3S2 have the highest reactivity among four W3Sn clusters. The Mayer bond order, bond length, vibrational frequency, and electronic charges of the adsorbed N2 are analyzed along the reaction pathways to show the activation process of the N−N bond in reactions. The charge transfer from the clusters to the N2 antibonding orbitals plays an essential role in N−N bond activation, which is more significant in Nb3Sn than in W3Sn, leading to the higher reactivity of Nb3Sn. The reaction mechanisms found in this work may provide important theoretical guidance for the further rational design of related catalytic systems for nitrogen reduction reactions (NRR).  相似文献   
10.
One of the most common problems in wounds is delayed healing and complications such as infection. Therefore, the need for novel materials accelerates the healing of wounds especially abdominal wounds after surgery besides high efficiency and safety is mandatory. The rate of wound healing, anti-inflammatory and biocompatibility of Zn-Al LDH (Zn-Al layer double hydroxide) alone and loaded with Curcumin (Zn-Al LDH/Curcumin) was screened via in-vivo assays through intramuscular implantation in rat abdominal wall with intact peritoneum cavity. The implanted drugs were formed through Curcumin loaded into LDH of Zn-Al with drug release of 56.78 ± 1.51% within 24 h. The synthesized nanocomposite was characterized by (TGA/DTA) thermal analysis, (XRD) X-ray diffraction, (FESEM) Field emission scanning electron microscopy, (HRTEM) high resolution transmission electron microscope, energy dispersive X-ray (EDX) and low-temperature N2 adsorption, pore volume and average pore size distribution. The integrity of blood circulation, inflammatory signs, wound healing rate, capacity of tissue integration, antigenicity and composite biocompatibility, auto fluorescence ability of collagen bundles and the tensile strength of the muscle were assessed histopathologically after 7 and 30 days’ post-implantation. Excellent wound healing ability was achieved with shortest length between the wound gap edges and higher tensile strength of the muscle. Besides emit florescence very well followed by good healing and tensile muscles strength in Curcumin while very low strength with scar formation in Zn-Al LDH/Curcumin in both acute and chronic wound. No signs of inflammation in Curcumin & Zn-Al LDH. No vessels obstruction or bleeding observed in both Zn-Al LDH and Curcumin more than Zn-Al LDH/Curcumin and control which examined through candling. Good healing & infiltrated immune cells in same groups through histopathological examination. This work supports the anti-inflammatory, wound healing and biocompatibility of both LDH and Curcumin with living matter, increasing their biomedical applications in this era with safety and increasing efficacy with prolonged drug release.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号