全文获取类型
收费全文 | 8282篇 |
免费 | 559篇 |
国内免费 | 267篇 |
专业分类
电工技术 | 385篇 |
综合类 | 359篇 |
化学工业 | 1706篇 |
金属工艺 | 829篇 |
机械仪表 | 454篇 |
建筑科学 | 217篇 |
矿业工程 | 164篇 |
能源动力 | 189篇 |
轻工业 | 1000篇 |
水利工程 | 37篇 |
石油天然气 | 278篇 |
武器工业 | 98篇 |
无线电 | 740篇 |
一般工业技术 | 799篇 |
冶金工业 | 343篇 |
原子能技术 | 183篇 |
自动化技术 | 1327篇 |
出版年
2025年 | 65篇 |
2024年 | 203篇 |
2023年 | 187篇 |
2022年 | 360篇 |
2021年 | 311篇 |
2020年 | 224篇 |
2019年 | 214篇 |
2018年 | 184篇 |
2017年 | 237篇 |
2016年 | 248篇 |
2015年 | 231篇 |
2014年 | 399篇 |
2013年 | 422篇 |
2012年 | 561篇 |
2011年 | 654篇 |
2010年 | 537篇 |
2009年 | 602篇 |
2008年 | 531篇 |
2007年 | 525篇 |
2006年 | 447篇 |
2005年 | 321篇 |
2004年 | 278篇 |
2003年 | 240篇 |
2002年 | 203篇 |
2001年 | 115篇 |
2000年 | 111篇 |
1999年 | 84篇 |
1998年 | 81篇 |
1997年 | 51篇 |
1996年 | 65篇 |
1995年 | 73篇 |
1994年 | 67篇 |
1993年 | 47篇 |
1992年 | 36篇 |
1991年 | 43篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 27篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1951年 | 1篇 |
排序方式: 共有9108条查询结果,搜索用时 0 毫秒
91.
Short syntheses of (Z)-7-dodecen-1-yl acetate, (Z)-7-tetradecen-1-yl acetate, (Z)-9-dodecen-1-yl acetate, and (Z)-9-tetradecen-1-yl acetate from 7-hydroxyheptanal and 9-oxononanoic acid precursors obtained by oxidative cleavage of easily available aleuritic acid are reported. The key step in these syntheses is a stereoselective Wittig reaction between aldehyde and alkyl-phosphonium salt. Wittig-Horner type reaction of 7-hydroxyheptanal and diethyl cyanomethylphosphonate gave the ,-unsaturated nitrile derivative which after protection of the hydroxyl group was reduced to the corresponding aldehyde. Wittig reaction of the latter, followed by acetylation, completed the synthesis of (E,Z)-7,9-dodecadien-1-yl acetate, the sex pheromone of the European grapevine mothLobesia botrana Schiff. 相似文献
92.
The effect of temperature on product formation during the sulphonation of indigo with concentrated sulphuric acid and 30 % fuming sulphuric acid was studied with the aid of high performance liquid chromatographic (HPLC) analysis. Trisulphoindigo was prepared by the sulphonation of indigo with concentrated sulphuric acid at 160°C. The di- and tetra-sulphoindigo side products formed during the reaction were removed by preparative HPLC to yield a product of high chromatographic purity. The trisulphoindigo content of the food dye FD & C Blue No. 2 and the purity of commercially available trisulphoindigo were determined by HPLC analysis. 相似文献
93.
2-乙酰基-7-甲氨基酮1与溴反应得2-乙酰基-7-甲氨基-4-溴酮2。2与氢氧化钠反应得3-乙酰基-5-溴酚酮3。化合物2和3是两种新化合物,其结构已经红外光谱、核磁共振谱及元素分析证实。 相似文献
94.
M. Feuerstein G. Engelhardt P. L. McDaniel J. E. MacDougall T. R. Gaffney 《Microporous and mesoporous materials》1998,26(1-3)
The location of Li+ and Na+ cations in a series of dehydrated low-silica LiNaX zeolites (LiNaLSX, framework Si/Al ratio=1.0) were characterized by 7Li and 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Depending on the Li+ content, up to three lines were observed in the 7Li MAS NMR spectra attributed to Li+ cations on SI′, SII and SIII sites. The 23Na MAS NMR spectra of the pure sodium form NaLSX and of LiNaLSX samples with low Li+ contents contain up to five lines belong to Na+ cations located on SI, SI′, SII, and two different SIII′ sites. LiNaLSX zeolites containing more than 40% of Li+ show only a single narrow line in the 23Na MAS NMR spectra attributed to mobile sodium cations. The populations of the different cation sites were determined from the relative line intensities of the MAS NMR spectra. Below about 70% Li+ exchange, lithium cations are located only on sites SI′ and SII. Between 70% and 100% Li+ content these sites are fully occupied by Li+, and the population of site SIII by Li+ increases. It was found that the nitrogen-adsorption capacity correlates well with the occupation of Li+ at site SIII. 相似文献
95.
Jeong-Shik Park Young-Soon Kim Suk-Woo Yang Ik-Joon Cho Chun-Yeong Kim Hyung-Shik Shin 《Korean Journal of Chemical Engineering》1998,15(3):304-309
The superconducting Y1Ba2Cu3O7-x
(123) and the Y2Ba1Cu1O5(211) powders in this study were prepared by pyrophoric synthesis method with Y2O3 (99.9 %), BaCO3 (99.9 %), and CuO (99.9 %) powders. Samples of 123 and 211 pellets were first prepared and then piled to have a 123/211/123
arrangement before a partial melting process was applied for phase change of the center piece (211) to 123 phase through a
peritectic reaction. The process parameters were a melting temperature of 1,040–1,070 °C and the mass of the 123 piece ranging
from 0.4 to 1.2 g. The superconductivity, such as critical temperature (Tc), and mass susceptibility(χ) of bulk 123 and 211 samples were measured by AC four point probe method and AC susceptometer, respectively. The experimental
results can be summarized as follows: the best preparation condition in the range examined was a melting temperature of 1,060
°C and mass ratio [123(A)/211(B)] of 2:1 with melting time of 30 minutes to yield the Tc of 88.5 K. 相似文献
96.
F. Campos J. Atkinson J. T. Arnason B. J. R. Philogène P. Morand N. H. Werstiuk G. Timmins 《Journal of chemical ecology》1989,15(7):1989-2001
2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), the major hydroxamic acid present in corn, and its tritiated derivative, were prepared synthetically for use in the determination of the toxicokinetics of this insect deterrent in the European corn borer (ECB),Ostrinia nubilalis. In growth studies with DIMBOA (0, 0.05, 0.2, and 0.5 mg/g diet), the mean time to pupation and adult emergence were significantly lengthened by an increase in concentration. Pupal and adult weights, for both female and male, decreased with an increase in concentration. Increased larval and pupal mortality occurred at the highest concentration of DIMBOA. DIMBOA, at concentrations of 0.2 and 0.5 mg/g diet, resulted in a decrease in the number of egg masses produced per female, and at 0.5 mg/g diet, in a decrease in the number of eggs per egg mass. Larvae fed from the neonate stage on a diet containing 0.2 mg [3H]- + [1H]DIMBOA/g diet showed an increase in the content of label from fourth to fifth instar, but levels declined at pupation and emergence. A large amount of the labeled compounds was excreted by the insect in the pupal case. In dose-related studies, both uptake and excretion increased with an increase in concentration of DIMBOA (0.05, 0.2, 0.4 mg/g diet), while a body burden (concentration in the tissues/concentration in the frass) of approximately 0.25 was maintained for all concentrations. At the highest dose of DIMBOA (0.4 mg/g), the ECB increased consumption, possibly to compensate for the toxic effects of the compound. In topical application studies, elimination of the labeled compound in the frass was rapid, reaching 65% by 4 hr and 88% by 48 hr. Accumulation of label in tissues other than hemolymph was low. The results show that the ECB does possess adaptive mechanisms to deal with the effects of this host-derived compound. 相似文献
97.
Sheng Yan Lok Yan Yim Rachel Chun Yee Tam Albert Chan Liwei Lu Chak Sing Lau Vera Sau-Fong Chan 《International journal of molecular sciences》2016,17(8)
Systemic lupus erythematosus (SLE) is a chronic multi-organ autoimmune disease characterized by hyperactivated immune responses to self-antigens and persistent systemic inflammation. Previously, we reported abnormalities in circulating and bone marrow (BM)-derived plasmacytoid dendritic cells (pDCs) from SLE patients. Here, we aim to seek for potential regulators that mediate functional aberrations of pDCs in SLE. BM-derived pDCs from NZB/W F1 mice before and after the disease onset were compared for toll-like receptor (TLR) induced responses and microRNA profile changes. While pDCs derived from symptomatic mice were phenotypically comparable to pre-symptomatic ones, functionally they exhibited hypersensitivity to TLR7 but not TLR9 stimulation, as represented by the elevated upregulation of CD40, CD86 and MHC class II molecules upon R837 stimulation. Upregulated induction of miR-155 in symptomatic pDCs following TLR7 stimulation was observed. Transfection of miR-155 mimics in pre-symptomatic pDCs induced an augmented expression of Cd40, which is consistent with the increased CD40 expression in symptomatic pDCs. Overall, our results provide evidence for miR-155-mediated regulation in pDC functional abnormalities in SLE. Findings from this study contribute to a better understanding of SLE pathogenesis and ignite future interests in evaluating the molecular regulation in autoimmunity. 相似文献
98.
Emmanuel Vázquez-Mayorga ángel G. Díaz-Sánchez Ruben K. Dagda Carlos A. Domínguez-Solís Raul Y. Dagda Cynthia K. Coronado-Ramírez Alejandro Martínez-Martínez 《International journal of molecular sciences》2016,17(8)
Mutations the in human DJ-1 (hDJ-1) gene are associated with early-onset autosomal recessive forms of Parkinson’s disease (PD). hDJ-1/parkinsonism associated deglycase (PARK7) is a cytoprotective multi-functional protein that contains a conserved cysteine-protease domain. Given that cysteine-proteases can act on both amide and ester substrates, we surmised that hDJ-1 possessed cysteine-mediated esterase activity. To test this hypothesis, hDJ-1 was overexpressed, purified and tested for activity towards 4-nitrophenyl acetate (pNPA) as µmol of pNPA hydrolyzed/min/mg·protein (U/mg protein). hDJ-1 showed maximum reaction velocity esterase activity (Vmax = 235.10 ± 12.00 U/mg protein), with a sigmoidal fit (S0.5 = 0.55 ± 0.040 mM) and apparent positive cooperativity (Hill coefficient of 2.05 ± 0.28). A PD-associated mutant of DJ-1 (M26I) lacked activity. Unlike its protease activity which is inactivated by reactive oxygen species (ROS), esterase activity of hDJ-1 is enhanced upon exposure to low concentrations of hydrogen peroxide (<10 µM) and plateaus at elevated concentrations (>100 µM) suggesting that its activity is resistant to oxidative stress. Esterase activity of DJ-1 requires oxidation of catalytic cysteines, as chemically protecting cysteines blocked its activity whereas an oxido-mimetic mutant of DJ-1 (C106D) exhibited robust esterase activity. Molecular docking studies suggest that C106 and L126 within its catalytic site interact with esterase substrates. Overall, our data show that hDJ-1 contains intrinsic redox-sensitive esterase activity that is abolished in a PD-associated mutant form of the hDJ-1 protein. 相似文献
99.
MicroRNA-331-3p Suppresses Cervical Cancer Cell Proliferation and E6/E7 Expression by Targeting NRP2
Tomomi Fujii Keiji Shimada Aya Asano Yoshihiro Tatsumi Naoko Yamaguchi Masaharu Yamazaki Noboru Konishi 《International journal of molecular sciences》2016,17(8)
Aberrant expression of microRNAs (miRNAs) is involved in the development and progression of various types of cancers. In this study, we investigated the role of miR-331-3p in cell proliferation and the expression of keratinocyte differentiation markers of uterine cervical cancer cells. Moreover, we evaluated whether neuropilin 2 (NRP2) are putative target molecules that regulate the human papillomavirus (HPV) related oncoproteins E6 and E7. Cell proliferation in the human cervical cancer cell lines SKG-II, HCS-2, and HeLa was assessed using the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (MTS) assay. Cellular apoptosis was measured using the TdT-mediated dUTP nick end labeling (TUNEL) and Annexin V assays. Quantitative RT-PCR was used to measure the messenger RNA (mRNA) expression of the NRP2, E6, E7, p63, and involucrin (IVL) genes. A functional assay for cell growth was performed using cell cycle analyses. Overexpression of miR-331-3p inhibited cell proliferation, and induced G2/M phase arrest and apoptosis in SKG-II, HCS-2 and HeLa cells. The luciferase reporter assay of the NRP2 3′-untranslated region revealed the direct regulation of NRP2 by miR-331-3p. Gene expression analyses using quantitative RT-PCR in SKG-II, HCS-2, and HeLa cells overexpressing miR-331-3p or suppressing NRP2 revealed down-regulation of E6, E7, and p63 mRNA and up-regulation of IVL mRNA. Moreover, miR-331-3p overexpression was suppressed NRP2 expression in protein level. We showed that miR-331-3p and NRP2 were key effectors of cell proliferation by regulating the cell cycle, apoptosis. NRP-2 also regulates the expression of E6/E7 and keratinocyte differentiation markers. Our findings suggest that miR-331-3p has an important role in regulating cervical cancer cell proliferation, and that miR-331-3p may contribute to keratinocyte differentiation through NRP2 suppression. miR-331-3p and NRP2 may contribute to anti-cancer effects. 相似文献
100.
Yaping Liu Xingyu Su Jie Hao Maoxin Chen Weijia Liu Xiaogang Liao Gang Li 《International journal of molecular sciences》2016,17(1)
Prolactin family 7, subfamily d, member 1 (PRL7D1) is found in mouse placenta. Our recent work showed that PRL7D1 is also present in mouse testis Leydig cells, and the expression of PRL7D1 in the testis exhibits an age-related increase. In the present study, we generated transgenic mice with Leydig cell-specific PRL7D1 overexpression to explore its function during male reproduction. Prl7d1 male mice exhibited subfertility as reflected by reduced sperm counts and litter sizes. The testes from Prl7d1 transgenic mice appeared histologically normal, but the frequency of apoptotic germ cells was increased. Prl7d1 transgenic mice also had lower testosterone concentrations than wild-type mice. Mechanistic studies revealed that Prl7d1 transgenic mice have defects in the testicular expression of steroidogenic acute regulatory protein (STAR) and hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase cluster (HSD3B). Further studies revealed that PRL7D1 overexpression affected the expression of transferrin (TF) in Sertoli cells. These results suggest that PRL7D1 overexpression could lead to increased germ cell apoptosis and exert an inhibitory effect on testosterone production in Leydig cells by reducing the expression of certain steroidogenic-related genes. In addition, PRL7D1 appears to have important roles in the function of Sertoli cells, which, in turn, affects male fertility. We conclude that the expression level of PRL7D1 is associated with the reproductive function of male mice. 相似文献