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71.
Fused deposition modeling (FDM) produces parts through layer by layer on the top of each other, making it almost impossible to obtain smooth printed parts. Hence, there is a huge demand for the postprocessing of the FDM-printed parts. Laser polishing is a novel technique that can be used to polish products to obtain a smoother surface. The aim of this work was to explore the feasibility of surface-finishing FDM-printed polylactic acid (PLA) parts by laser polishing. The surface roughness, surface morphology, dynamic mechanical analysis (DMA), and tensile properties were investigated. The results indicated that the lower laser power and the bigger laser beam diameter within a certain range could facilitate the formation of smoother surface. With optimized parameters, the surface roughness was reduced by 90.4%. DMA showed that the storage modulus (E’) and glass transition temperature of PLA specimens were significantly improved due to the decrease of molecular mobility of denser structures. Moreover, the tensile strength and Young's modulus of the PLA specimen were also significantly increased after laser polishing. The fracture morphologies were observed, and the possible strengthening mechanism was also discussed. These results indicated that laser polishing could be an efficient method for surface polishing of FDM parts. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48288.  相似文献   
72.
以钢渣颗粒为骨料,分别包裹无机胶凝剂、有机胶凝剂和无机-有机复合胶凝剂制备人造彩砂,研究了胶凝剂掺量对彩砂耐磨性的影响,并利用XRD、SEM及TG等方法测试彩砂的结构组成特性。结果表明,随胶凝剂掺量的增加,彩砂磨损率先快速降低后趋于平缓;单独用无机胶凝剂和有机胶凝剂所得彩砂的磨损率均较高,分别为2.39%~0.58%和1.70%~0.80%;采用在无机胶凝剂中掺加小于10%有机胶凝剂配制的复合胶凝剂,彩砂的磨损率显著降低,最低可降至0.05%。复合胶凝剂中无机组分与有机组分发生协同反应是彩砂磨损率显著降低的原因。  相似文献   
73.
油页岩矿物质催化半焦燃烧特性及机理   总被引:1,自引:0,他引:1  
利用微型流化床反应分析仪(MFBRA)研究了油页岩矿物质催化半焦燃烧特性,重点考察了半焦内部矿物质和外部页岩灰床料对半焦燃烧的催化作用,揭示了流化床反应器中半焦燃烧过程和机理。结果表明:内部矿物质和外部床料对半焦燃烧均具有明显催化作用,而两者共同催化效果最为显著。矿物质中CaO和Fe2O3对半焦燃烧具有催化活性,CaO催化作用强于Fe2O3。油页岩半焦燃烧反应活化能在60.41~78.97 kJ/mol之间,矿物质的催化作用会明显降低反应活化能。流化床反应器中,矿物质对半焦燃烧的催化作用主要表现在四个反应,即:挥发分裂解和燃烧、半焦表面炭燃烧、半焦内部炭燃烧以及一氧化碳燃烧。  相似文献   
74.
在不改变成分的条件下,为了探讨将表面镀覆和表面刻蚀相结合来提高金刚石与金属结合剂把持力的方法和条件,将镀Ti、镀Cr和镀Ni金刚石在1050℃下保温1 h后,用真空热压烧结法制备未镀、高温处理前后镀覆金刚石的铜基结合剂样条。用扫描电子显微镜对金刚石镀层表面形貌进行观测,用金刚石高温热处理前后铜基结合剂样条的抗弯强度来评估金刚石与铜基结合剂的把持力,经1050℃高温处理1 h后,金刚石表面的镍镀层基本上保持完整,而钛镀层和铬镀层则出现脱落;镀覆金刚石的单颗粒抗压强度都下降;镀Ti和镀Cr金刚石铜基结合剂样条的抗弯强度下降,但镀Ni金刚石样条的抗弯强度却大幅度提高,达到833 MPa,增幅为12.1%。结果表明:只有高温处理后镀层保持完整而且镀层能使金刚石表面粗糙度提高的情况下,才能大幅度提高金刚石与金属结合剂之间的把持力。  相似文献   
75.
为诠释柱状晶贵金属兼具优良超细丝加工性能及高电阻率的原因,采用水平连铸、水冷铜模铸造及石墨模铸造,分别获得了具有20 μm×60 μm的柱状晶、10 μm的等轴晶、以及5 μm的表面细晶+长径比约为3的柱状晶+30 μm的芯部等轴晶三晶区的三种Ag-28Cu-0.75Ni合金棒材,研究了晶粒形态、尺寸对Ag-28Cu-0.75Ni合金棒材超细丝加工性能及电阻率的影响。结果表明:与等轴晶和三晶区Ag-28Cu-0.75Ni合金棒材相比,柱状晶棒材由于其较低的位错密度(1.78×1015 m-2)及显微硬度变化率(33.3%)、铸态断口无解理台阶,表现了出最低的加工硬化率,且铸态轴向抗拉强度高达384.6 MPa,可实现100 m以上、直径0.05 mm超细丝高效不断丝加工;加工态及退火态的柱状晶Ag-28Cu-0.75Ni合金超细丝均表现出了较高的电阻率,经两次连续退火后电阻率高达3.68 μΩ.cm,与其较低的位错密度及加工硬化率有关。  相似文献   
76.
The family Cactaceae is the diversified group of angiosperm plants whose pollen statistics has been used for taxonomic identification. In this article, we present the pollen morphology of eight species belong to seven taxonomically complex genera of Cactaceae including Astrophytum, Cylindropuntia, Echinocereus, Echinopsis, Mammillaria, Opuntia, and Thelocactus using light and scanning electron microscopy. The pollen grains were acetolyzed, measured, described, and electron photomicrographs were taken. Cactaceae can be characterized by presenting different palynomorphological features including pollen type, sculpturing, polar and equatorial diameter, aperture orientation, exine thickness, P/E ratio, and echini features. Four types of pollen shapes, that is, prolate spheroidal (three species), subprolate (two species), prolate (two species), and oblate spheroidal in Echinocereus reichenbachii were observed. The polar and equatorial diameter observed maximum in O. ficus indica 116.95 and 112.27 μm while minimum in M. compressa 38.42 and 21.05 μm. Pollen of two types, tricolpate in members of subfamily Cactioideae and pantoporate in the Opuntioideae were examined. The fertility percentage has been observed maximum in Opuntia macrocentra (83.84%) and minimum in Opuntia ficus‐indica (57.89%). Exine sculpturing showing great variations such as granulate, reticulate, granulate perforate and micro‐echinate foveolate ornamentation was examined only in Echinopsis eyriesii. A key to species, based on pollen micromorphological attributes, has been constructed for correct identification of complex cactus species.  相似文献   
77.
Previous studies reported that intrageneric relationships of genus Veratrum of family Melanthiaceae are controversial and hard to delimit. Therefore, we observed the pollen morphological features of six species in the genus Veratrum in detail using both light microscopy and scanning electron microscopy and investigated their significance for Veratrum taxonomy. Among them, five were studied for the first time. The results demonstrated that pollen grains of Veratrum are medium in size with P/E being 0.31–0.60. Three types of shape in polar view have been observed elliptic, long‐elliptic, or wide‐elliptic with blunt or rounded at both ends. Two types of width of colpus reported narrow or wide, two types of depth of colpus reported deep or flat, and two types of length of colpus reported extend almost or do not extend to the ends, whereas two types of colpus membranes reported absent or obvious. One type of surface ornamentation was noted as reticulate. These results support species Veratrum album and Veratrum lobelianum as well as Veratrum grandiflorum and Veratrum oxysepalum as two independent species, respectively, rather than classifying Veratrum into two sections. Overall, we demonstrated that the ratio of polar axis length to equatorial axial length, pollen characteristics at the polar view, the colpus morphology, and the surface ornamentation of pollen grains of genus Veratrum have important systematic significance in identification and delimitation of species.  相似文献   
78.
Due to rapid urbanization around the world, high concentrations of vehicular pollutants have deteriorated the outdoor air quality, which can affect the physical and psychological well-being of humans. Numerous strategies have been proposed to overcome these harmful impacts by improving the dispersion of air pollutants. Consequently, a question arises regarding the potential effects of building morphology on the dispersion of pollutants. Subsequently, transient three-dimensional Computational Fluid Dynamics (CFD) simulations are performed to examine the effect of building morphology on PM10 dispersion. Eleven cases with various prototypes and morphological methods are compared with a simple building form to identify the patterns of PM10 dispersion within a given time sequence under a prevailing inflow condition. The results indicate that the different designs of building morphology with varying Relative compactness (RC) indicator highlight the importance of considering morphological factors to improve outdoor air quality. In addition, the proposed prototypes can reduce PM10 concentrations by approximately 30%–90% at specific points in the studied time sequence. In particular, the vertical, horizontal, and grid folded prototypes can be considered more effective as an approximate decrease between 70% and 90% in PM10 concentrations is observed, which reflects the influence of building morphology on improving outdoor air quality.  相似文献   
79.
随着航空发动机涡轮叶片工作温度的提升,使得一种主要由CaO,MgO,Al2O3和SiO2组成的玻璃态物质(CMAS)对热障涂层的危害越来越严重,从而对热障涂层的性能和耐久性有了更高的要求。本文以电子束物理气相沉积热障涂层为研究对象,利用有限元方法研究了CMAS的渗入对界面裂纹扩展及CMAS对陶瓷层(TC)内部残余应力的影响规律。采用波长固定、振幅变化的正弦曲线表示不同粗糙度的涂层界面,同时考虑了CMAS的弹性模量变化的影响及不同界面形貌与CMAS之间的相互作用。结果表明:CMAS弹性模量的增加对界面裂纹具有抑制作用,并且TGO幅值和厚度越小,抑制作用越明显。CMAS弹性模量对TC层最大残余应力S22的影响存在临界点,在临界点之前,CMAS弹性模量的变化对TC层最大残余应力的影响较大,随着CMAS弹性模量的增加,TC层最大残余应力大幅度减小;在临界点之后,TC层最大残余应力基本不受CMAS弹性模量变化的影响。这些结果对电子束物理气相沉积喷涂的热障涂层失效机理的研究具有重要意义,可以为热障涂层界面的优化提供指导。  相似文献   
80.
本文提出了以物理气相传输法自支撑生长氮化铝单晶的新方法,此方法可以在氮化铝烧结体表面一次性获得大量生长的氮化铝单晶。本研究中,在2373–2523 K的温度条件下经过100 h生长的氮化铝单晶,其最大尺寸为7 × 8 × 12 mm3,典型直径为5–7 mm。这些原生晶体的表面形貌及结晶质量分别通过扫描电子显微镜、拉曼光谱和高分辨X射线衍射进行表征分析。其中,拉曼光谱E2峰位的半高全宽为5.7 cm-1,高分辨X射线衍射得到的对称摇摆曲线的半高全宽为93.6角秒。经过选择性化学腐蚀后的晶体,其表面的平均腐蚀坑密度为7.5 × 104 cm-2。逸出气体分析和辉光放电质谱分析结果表明,碳和氧为晶体内部的主要杂质元素,含量分别为28 ppmw和120 ppmw。此方法为高质量氮化铝单晶的获取提供了一个新的途径,这些单晶可以被切成晶片作为后续氮化铝同质外延生长的优良籽晶。使用这些小的籽晶,我们首次成功制备出了直径高达60 mm的氮化铝单晶体/晶圆,并具有良好的深紫外光透过性。  相似文献   
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