全文获取类型
收费全文 | 4881篇 |
免费 | 321篇 |
国内免费 | 13篇 |
专业分类
电工技术 | 44篇 |
综合类 | 1篇 |
化学工业 | 948篇 |
金属工艺 | 104篇 |
机械仪表 | 119篇 |
建筑科学 | 88篇 |
矿业工程 | 7篇 |
能源动力 | 119篇 |
轻工业 | 868篇 |
水利工程 | 56篇 |
石油天然气 | 17篇 |
无线电 | 217篇 |
一般工业技术 | 651篇 |
冶金工业 | 1432篇 |
原子能技术 | 26篇 |
自动化技术 | 518篇 |
出版年
2024年 | 18篇 |
2023年 | 41篇 |
2022年 | 63篇 |
2021年 | 222篇 |
2020年 | 146篇 |
2019年 | 195篇 |
2018年 | 198篇 |
2017年 | 207篇 |
2016年 | 175篇 |
2015年 | 133篇 |
2014年 | 173篇 |
2013年 | 265篇 |
2012年 | 262篇 |
2011年 | 269篇 |
2010年 | 184篇 |
2009年 | 216篇 |
2008年 | 173篇 |
2007年 | 148篇 |
2006年 | 119篇 |
2005年 | 103篇 |
2004年 | 71篇 |
2003年 | 71篇 |
2002年 | 56篇 |
2001年 | 50篇 |
2000年 | 43篇 |
1999年 | 62篇 |
1998年 | 436篇 |
1997年 | 278篇 |
1996年 | 181篇 |
1995年 | 112篇 |
1994年 | 102篇 |
1993年 | 83篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 17篇 |
1989年 | 31篇 |
1988年 | 29篇 |
1987年 | 22篇 |
1986年 | 21篇 |
1985年 | 16篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1978年 | 7篇 |
1977年 | 32篇 |
1976年 | 70篇 |
1975年 | 3篇 |
1974年 | 3篇 |
排序方式: 共有5215条查询结果,搜索用时 250 毫秒
41.
42.
43.
Daniela F. S. Fonseca ?ngelo C. Salvador Sónia A. O. Santos Carla Vilela Carmen S. R. Freire Armando J. D. Silvestre Sílvia M. Rocha 《International journal of molecular sciences》2015,16(6):14194-14209
The lipophilic composition of wild Arbutus unedo L. berries, collected from six locations in Penacova (center of Portugal), as well as some general chemical parameters, namely total soluble solids, pH, titratable acidity, total phenolic content and antioxidant activity was studied in detail to better understand its potential as a source of bioactive compounds. The chemical composition of the lipophilic extracts, focused on the fatty acids, triterpenoids, sterols, long chain aliphatic alcohols and tocopherols, was investigated by gas chromatography–mass spectrometry (GC–MS) analysis of the dichloromethane extracts. The lipophilic extractives of the ripe A. unedo berries ranged from 0.72% to 1.66% (w/w of dry weight), and consisted mainly of triterpenoids, fatty acids and sterols. Minor amounts of long chain aliphatic alcohols and tocopherols were also identified. Forty-one compounds were identified and among these, ursolic acid, lupeol, α-amyrin, linoleic and α-linolenic acids, and β-sitosterol were highlighted as the major components. To the best of our knowledge the current research study provides the most detailed phytochemical repository for the lipophilic composition of A. unedo, and offers valuable information for future valuation and exploitation of these berries. 相似文献
44.
Marianna Erbano Guilherme Schnell e Schühli élide Pereira dos Santos 《International journal of molecular sciences》2015,16(4):7839-7850
The genetic diversity and population structure of Salvia lachnostachys Benth were assessed. Inter Simple Sequence Repeat (ISSR) molecular markers were used to investigate the restricted distribution of S. lachnostachys in Parana State, Brazil. Leaves of 73 individuals representing three populations were collected. DNA was extracted and submitted to PCR-ISSR amplification with nine tested primers. Genetic diversity parameters were evaluated. Our analysis indicated 95.6% polymorphic loci (stress value 0.02) with a 0.79 average Simpson’s index. The Nei-Li distance dendrogram and principal component analysis largely recovered the geographical origin of each sample. Four major clusters were recognized representing each collected population. Nei’s gene diversity and Shannon’s information index were 0.25 and 0.40 respectively. As is typical for outcrossing herbs, the majority of genetic variation occurred at the population level (81.76%). A high gene flow (Nm = 2.48) was observed with a correspondingly low fixation index. These values were generally similar to previous studies on congeneric species. The results of principal coordinate analysis (PCA) and of arithmetic average (UPGMA) were consistent and all three populations appear distinct as in STRUCTURE analysis. In addition, this analysis indicated a majority intrapopulation genetic variation. Despite the human pressure on natural populations our study found high levels of genetic diversity for S. lachnostachys. This was the first molecular assessment for this endemic species with medicinal proprieties and the results can guide for subsequent bioprospection, breeding programs or conservation actions. 相似文献
45.
46.
Choi Paul Santos Christian Ariane Kim Min-Kun Jung Hyunsook Hong Do-Young Koo Junemo 《Journal of Mechanical Science and Technology》2021,35(11):5007-5022
Journal of Mechanical Science and Technology - This study delivers equations useful for low-height pleated fibrous filter design: two pressure drop equations and one set of optimum design equations... 相似文献
47.
48.
Anauara L. e Silva Lucas E. de Lima Terceiro Matheus F. de Lima Rafael Costa‐Silva Ednalva A. V. dos Santos Maria de Ftima Agra 《Microscopy research and technique》2020,83(3):287-296
Micromorphological studies were carried out using multiple microscopic techniques on the leaves and stem bark of Byrsonima sericea DC. (Malpighiaceae), a species popularly known as “murici” and used medicinally, in order to identify both qualitative and quantitative features of leaf and stem anatomy and histochemistry as differential parameters to support both the quality control of its ethnodrugs and the taxonomy of the genus. The study was conducted using traditional techniques of plant anatomy, histochemical tests, and the stomatal index (SI). Byrsonima sericea has hypostomatic leaves, anomocytic stomata, and its epidermal walls are anticlinal and straight on the adaxial and curved on the abaxial faces. T‐shaped trichomes were observed mainly on the abaxial surface. The leaf epidermis showed waxes syntopism on both surfaces, with the occurrence of different crystalloid forms on a single phylloplane. The mesophyll is dorsiventral, with 3‐4 collateral vascular bundles. Phenolic compounds, starch, and proteins were identified in the petiole and stem. The SI was 14.5 ± 0.53% (p < .05), but did not showed significant variations. A set of characters were found to be distinctive for the studied species, however, constituting parameters that could be used to separate B. sericea from other species of the genus. 相似文献
49.
50.
Cristiano Silva do Nascimento Brenda N. Santos Sueli Rodrigues 《International Journal of Food Science & Technology》2022,57(8):5186-5194
In the present study, a prebiotic acerola juice containing gluco-oligosaccharides and dextran was produced and processed by high-intensity ultrasound (HIUS). After a simulated in vitro digestion, the gluco-oligosaccharides and dextran maintained 90% and 80% of their initial concentration in all prebiotic’s juices. At the same time, Vitamin C and phenolic compounds concentration increased significantly by 19% and 7% (P < 0.05). After the in vitro digestion, the prebiotic juice HIUS processed by 10 min showed the highest increase in gluco-oligosaccharides and bioactive compound concentrations. The HIUS processing imparted some dextran hydrolysis and improved its fermentability by Lacticaseibacillus casei. Gluco-oligosaccharides were extensively consumed as substrate in simulated intestinal conditions, promoting the L. casei NRRL B-442 growth and production of organic acids and short-chain organic acids. The prebiotic juice HIUS processed for 6 min showed the best responses regarding the metabolism of L. casei NRRL B-442. The results showed high-intensity ultrasound processed acerola juices containing gluco-oligosaccharides and dextran as a prebiotic food. 相似文献