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排序方式: 共有9171条查询结果,搜索用时 15 毫秒
991.
本文主要综述了纳米CaCO3改性PVC、蒙脱土纳米复合材料改性PVC以及其他纳米复合材料改性PVC的方法,研究结果表明:纳米复合技术在聚氯乙烯增韧改性中具有同时提高材料韧性和强度的特点,纳米复合技术将成为聚氯乙烯增韧改性的一种重要方法。 相似文献
992.
Xiang-Yang Wang Pan-Pan Niu Hong-Ying Yang 《Multimedia, IEEE》2009,16(3):60-69
Most of the previous audio-watermarking schemes are robust to common signal-processing attacks, but show severe problems when faced with desynchronization attacks. To solve the problems associated with these approaches, we propose an audio-watermarking scheme based on support-vector-machine (SVM) theory to protect against desynchronization attacks by using audio statistics characteristics and a synchronization code technique. Experimental results with SVM show that our proposed scheme is inaudible, robust against common signal processing, and robust against desynchronization attacks. 相似文献
993.
Yanfeng Zhu Qian Zhang Zhisheng Niu Jing Zhu 《Wireless Communications, IEEE Transactions on》2009,8(10):5030-5040
With the increasing development of IEEE 802.11 based wireless local area network (WLAN) devices, large-scale multi-cell WLANs with a high density of users and access points (APs) have emerged widely in various hotspots. Providing resilient data transmission has been a primary challenge for scaling the WLANs because the high density of users and APs results in too many collisions. In this paper, we analyze and point out the defect of the single association mechanism defined in IEEE 802.11 on transmission reliability from a network perspective. Then, we propose a "multi-AP" architecture with which a MAC layer device called an AP controller (AC) is employed to enable each user to associate and cooperate with multiple APs. In this way, the users can benefit from the diversity effect of multipaths with independent collisions and transmission errors. This paper concentrates on the theoretical analysis of performance comparison between the proposed ldquoMulti-APrdquo architecture and that in IEEE 802.11. Extensive simulation results show that the proposed ldquomulti-APrdquo architecture can obtain much better performance in terms of the throughput per user and the total throughput, and the performance gain is position dependent. Moreover, the unfairness issue in traditional WLANs due to capture effect can be alleviated properly in the ldquomulti-APrdquo framework. 相似文献
994.
义马煤业集团水泥有限责任公司技改一期5000t/d熟料水泥生产线2006年7月点火投产,当年实现达产达标.在二年多运行经验基础上,就生产线的选址、原燃料、配料、生料、水泥磨、包装、供配电等系统的细节设计不足之处作了简单分析. 相似文献
995.
In this study, we demonstrated the needleless electrospinning of poly(vinyl alcohol) (PVA) nanofibers with two nozzles, a rotating disk and a cylinder, and examined the effect of the nozzle shape on the electrospinning process and resultant fiber morphology. The disk nozzle needed a relatively low applied voltage to initiate fiber formation, and the fibers were mainly formed on the top disk edge. Also, the PVA concentration had little influence on the disk electrospinning process (up to 11 wt %). In comparison, the cylinder electrospinning showed a higher dependence on the applied voltage and polymer concentration. The fibers were initiated from the cylinder ends first and then from the entire cylinder surface only if the applied voltage were increased to a certain level. With the same polymer solution, the critical voltage needed to generate nanofibers from the disk nozzle was lower than that needed to generate nanofibers from the cylinder. Both electrospinning systems could produce uniform nanofibers, but the fibers produced from the disk nozzle were finer than those from the cylinder when the operating conditions were the same. A thin disk (8 cm in diameter and 2 mm thick) could produce nanofibers at a rate similar to that of a cylinder of the same diameter but 100 times wider (i.e., 20 cm long). Finite element analysis of electric field profiles of the nozzles revealed a concentrated electric field on the disk edge. For the cylinder nozzle, an uneven distribution of the electric field intensity profile along the nozzle surface was observed. The field lines were mainly concentrated on the cylinder ends, with a much lower electric field intensity formed in the middle surface area. At the same applied voltage, the electric field intensity on the disk edge was much higher than that on the cylinder end. These differences in the electric field intensity profiles could explain the differences in the fiber fineness and rate of the nanofibers produced from these two nozzles. These findings will benefit the design and further development of large‐scale electrospinning systems for the mass production of nanofibers for advanced applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 相似文献
996.
In this paper, the pristine basic morphology of high impact polypropylene (hiPP) particles prepared with an industrial MgCl2/TiCl4 Ziegler–Natta catalyst undergoing sequentially occurred propylene (P) homopolymerization and ethylene (E)/propylene copolymerization has been probed mainly using transmission electron microscopy (TEM) techniques including plain TEM and the advanced transmission electron microtomography (TEMT). It is revealed that the basic structure units comprising a whole hiPP particle are the submicron PP (polypropylene) globule and nano-sized EP (ethylene-co-propylene) droplet. EP rubber (EPR) domain is formed by the agglomeration of EP droplets. Continually formed EP droplets turn to fill, from inside out, the micro- and macro-pores inside the preformed PP skeleton, affording different-sized EPR domains. Taking the two basic structure units into account, new quaternary structure model describing the manifold structures of hiPP particles has been proposed. From these findings, it is suggested that, to gain hiPP polymers with excellent stiffness/toughness-balanced properties, it is crucial to control the first-staged propylene homopolymerization alongside a rational design of the catalyst architecture to accomplish desired EPR dispersion morphologies that dictate hiPP properties. 相似文献
997.
In this article, we have demonstrated a novel needleless electrospinning of PVA nanofibers by using a conical metal wire‐coil as spinneret. Multiple polymer jets were observed to generate on the coil surface. Up to 70 kV electric voltage can be applied to this needleless electrospinning nozzle without causing “corona discharge.” Compared with conventional needle electrospinning, this needleless electrospinning system produced finer nanofibers on a much larger scale, and the fiber processing ability showed a much greater dependence on the applied voltage. Finite element calculation indicates that the electric field intensity profiles for the two systems are also quite different. This novel concept of using wire coil as the electrospinning nozzle will contribute to the further development of new large‐scale needleless electrospinning system for nanofiber production. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers 相似文献
998.
999.
1000.
随着国家对基础建设投资的加大,审计范围逐渐拓宽。文章从审查项目招投标条件、范围、方式、代理机构、评标过程、合同签订、招投标结果的履行等7个方面,详细阐述了进行基建项目招投标审计的办法和重点。 相似文献