全文获取类型
收费全文 | 44213篇 |
免费 | 2585篇 |
国内免费 | 69篇 |
专业分类
电工技术 | 304篇 |
综合类 | 130篇 |
化学工业 | 10472篇 |
金属工艺 | 782篇 |
机械仪表 | 976篇 |
建筑科学 | 1475篇 |
矿业工程 | 100篇 |
能源动力 | 1129篇 |
轻工业 | 8692篇 |
水利工程 | 439篇 |
石油天然气 | 248篇 |
武器工业 | 2篇 |
无线电 | 2181篇 |
一般工业技术 | 7081篇 |
冶金工业 | 6456篇 |
原子能技术 | 174篇 |
自动化技术 | 6226篇 |
出版年
2024年 | 97篇 |
2023年 | 471篇 |
2022年 | 941篇 |
2021年 | 1513篇 |
2020年 | 1069篇 |
2019年 | 1156篇 |
2018年 | 1742篇 |
2017年 | 1739篇 |
2016年 | 1879篇 |
2015年 | 1391篇 |
2014年 | 1850篇 |
2013年 | 3622篇 |
2012年 | 3074篇 |
2011年 | 2980篇 |
2010年 | 2255篇 |
2009年 | 2067篇 |
2008年 | 2004篇 |
2007年 | 1779篇 |
2006年 | 1365篇 |
2005年 | 1107篇 |
2004年 | 989篇 |
2003年 | 955篇 |
2002年 | 786篇 |
2001年 | 603篇 |
2000年 | 521篇 |
1999年 | 528篇 |
1998年 | 2222篇 |
1997年 | 1475篇 |
1996年 | 952篇 |
1995年 | 556篇 |
1994年 | 461篇 |
1993年 | 434篇 |
1992年 | 172篇 |
1991年 | 137篇 |
1990年 | 102篇 |
1989年 | 115篇 |
1988年 | 134篇 |
1987年 | 116篇 |
1986年 | 94篇 |
1985年 | 119篇 |
1984年 | 112篇 |
1983年 | 73篇 |
1982年 | 73篇 |
1981年 | 101篇 |
1980年 | 97篇 |
1979年 | 66篇 |
1978年 | 53篇 |
1977年 | 130篇 |
1976年 | 243篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
91.
Isidoro Izquierdo Cubero Maria T. Plaza López-Espinosa Rafael Acuña Castillo 《Journal of chemical ecology》1991,17(8):1529-1541
The synthesis of the title compound13 has been carried out through the preparation of its precursor, (3R,4R,5S,6R)-3,4,5-trihydroxy-1,7-dioxaspiro[5.5]undecane (6), obtained fromd-fructose using Wittig's methodology, reduction, and spiroketalation. Compound6 was transformed into13 by a Barton deoxygenation at C-5 followed by a Corey dideoxygenation at C-3,4 of the appropriately protected derivatives.Enantiospecific synthesis of spiroacetals. Part II. For Part I, see Izquierdo and Plaza (1990). 相似文献
92.
L B Zago S M Friedman N H Slobodianik M L de Portela M E Río J C Sanahuja 《Archivos latinoamericanos de nutrición》1983,33(4):802-813
In order to analyze the interrelationships between free erythrocyte protoporphyrins and the usual iron intake in adult students, biochemical, and hematological values, and dietary daily intake, obtained using the recall method during seven days, were studied. Hematocrit (Hto.), hemoglobin (Hb) and free erythrocyte protoporphyrins (FEP) were determined in a group of 145 female university students, healthy according to the standard parameters of the Buenos Aires University Health Department. Mean iron intake was 23.0 +/- 1.5 mg per day, about 44% being provided by animal sources; 74.5% of the population was within the recommended daily intake according to FAO/WHO; only 0.7% of the population did not cover protein requirements while 35% did not cover energy needs. Hto. and Hb were below normal levels in 7.8% of the population when compared with standards according to ICNND. To obtain information about normal values to FEP, expressed as microgram/100 ml red cells (FEP% r.c.) and FEP/Hb ratio, the group of students with adequate intake of energy and proteins who had normal values for Hb and Hto. was selected. This group, including 94 women, had a mean FEP% r.c. of 15.71 +/- 7.26 and a mean FEP/Hb ratio of 0.44 +/- 0.21. There was observed an inverse correlation between FEP% r.c. and FEP/Hb with total iron intake (r = 0.80 and r = 0.78, respectively) and between FEP% r.c. and Hb concentration (r = 0.81). These results confirm the usefulness of the free erythrocyte protoporphyrins determination as a good index of iron stores and usual intake of this population. 相似文献
93.
Summary The Stefan-Winkelmann diffusion technique has been used to determine vapor pressures of high boiling point compound-solvents mixtures. Data are reported at a temperature of 67°C for the mixtures polyphenyl ether (6 rings) — benzene and carbowax 1500 — benzene and at a temperature of 100°C for the mixtures polyphenyl ether (6 rings) — toluene and tricresylphosphate-toluene. The range of concentration is only limited by the appearance of a solid phase, and results are in good agreement with those determined by vapor pressure osmometry.Nomenclature A
Vapour source
- B
Flowing gas phase
- c
Concentration, g-mol/cm3
- DAB
Molecular diffusion coefficient, cm2/s
- hi
Total interface depth, cm
- K
Parameter defined by equation (6), dimensionless
- I
Polymer or high boiling point compound
- MA
Molecular weight
- NAZ
Molar flux, g-mol/(cm2) (s)
- PA
Partial pressure at the interface, atm
- P
Total pressure, atm
- R
Gas constant, (atm) (cm3) / (°K) (g-mol)
- T
Temperature, °K
- t
Time, s
- YA
Molar fraction at the interface, dimensionless
- zo
Initial interface depth, cm
- zi
Drop of the interface depth, cm
- z
Interface position
Greek Letters A
Density of the liquid phase, g/cm3 相似文献
94.
J. Hrivňák L. Soják J. Krupčík Y. P. Duchesne 《Journal of the American Oil Chemists' Society》1973,50(3):68-71
The influence of temperature on the gas chromatographic separation ofcis-trans isomers of the methyl esters of some monounsaturated fatty acids was studied on capillary columns coated with Apiezon L,
BDS and DEGS. As far as methyl oleate and methyl elaidate are concerned, the separation is better at lower temperatures on
Apiezon L (180–210 C) and at higher temperatures on polyester phases (BDS, DEGS; 150–180 C). The influence of temperature
on the separation ofcis-trans isomers on the three stationary phases under study is explained by the higher values of δECL/δt forcis isomers. The variation of the equivalent carbon chain length with temperature can be used for the identification ofcis-trans isomers in natural mixtures. 相似文献
95.
An optimization strategy has been applied to describe the chemical composition at the furnace bottom in the Kraft recovery boiler of a pulp production process. The concentrations of each involved chemical species were calculated through an optimization approach, minimizing the Gibbs free energy of the system. Various systems were proposed and tested, assuming different chemical species and phases number. Because serious initialization problems were found at this stage for some of the proposed systems, an optimization heuristic method (PSO) was used for the first approach to the problem. Once the appropriate phases number and chemical species in the system were determined, the initialization problems disappeared and the use of a deterministic optimization method (SQP) became viable. The proposed approach has shown to be satisfactory to reproduce industrial data and also data reported in the open scientific literature. 相似文献
96.
Summary The synthesis is reported of copolymers of styrene with methacrylic acid and of methyl methacrylate with methacrylic acid by radical copolymerization, of copolymers of methyl methacrylate with methacrylic acid by partial alkaline hydrolysis of poly(methyl methacrylate), and of block copolymers of styrene with methacrylic acid. Modified titration curves of all these copolymers were recorded in water and water-ethanol solutions. In a solution containing 50 mass.% ethanol, only small differences could be observed between the potentiometric behaviour of the individual copolymers and polymethacrylic acid. Also, there were no essential differences in any of the solvents used between the potentiometric behaviour of block copolymers of styrene with methacrylic acid, on the one hand, and polymethacrylic acid, on the other. On the contrary, maxima and minima were always observed on the modified titration curves of statistical copolymers with a higher content of the hydrophobic comonomer in solutions with a high water content. Thus, using the modified titration curves, it is possible to decide whether a given copolymer is of the block or statistical type. 相似文献
97.
The effects of the nature of the polyols used in polyurethane (PUR) synthesis on the structural transformations after ageing by UV irradiation has been studied. The investigations were performed on PUR prepared from polyester and polyether diol oligomers. The characterization of the samples before and after ageing in view of the structural changes, which influence the course of the photooxidative degradation and photocrosslinking as well as in view of changes of mechanical properties has been done. Comparative investigations were performed by spectroscopic and viscometric measurements. The results show pronounced heterogeneity of the degradative reactions, including the existence of crosslinking processes and producing thus very inhomogeneous polymeric material. The course and the intensity of photooxidative degradation of PUR differ together with changes of mechanical properties depending whether polyester or polyether polyol have been used. The processes of photooxidative degradation is less expressed in polyesterurethane then in polyurethane based on polyether, under the same conditions of experiments. Different structures of polyester diols caused the various ageing behaviour of PUR too. 相似文献
98.
We propose a new entropy coding scheme, denoted in this paper as the UVLC (universal variable length coding). It is universal in the sense that its efficiency is close to one for a large class of images. The UVLC, when it is applied to block orthogonal transforms, processes groups of blocks at the bit level, using universal codes designed for binary memoryless sources. It can be used for every video coding application, from high definition TV (HDTV) to high quality videotelephony (above 2 Mbit/s) for transform or subband coding. Last but not least, its implementation is very regular and can be realized in a single chip for the encoding oftv at theccir 601 format. 相似文献
99.
Inspired by the relational algebra of data processing, this paper addresses the foundations of data analytical processing from a linear algebra perspective. The paper investigates, in particular, how aggregation operations such as cross tabulations and data cubes essential to quantitative analysis of data can be expressed solely in terms of matrix multiplication, transposition and the Khatri–Rao variant of the Kronecker product. The approach offers a basis for deriving an algebraic theory of data consolidation, handling the quantitative as well as qualitative sides of data science in a natural, elegant and typed way. It also shows potential for parallel analytical processing, as the parallelization theory of such matrix operations is well acknowledged. 相似文献
100.