全文获取类型
收费全文 | 291857篇 |
免费 | 16904篇 |
国内免费 | 686篇 |
专业分类
林业 | 24220篇 |
农学 | 14218篇 |
基础科学 | 3009篇 |
45236篇 | |
综合类 | 33595篇 |
农作物 | 17273篇 |
水产渔业 | 17770篇 |
畜牧兽医 | 122818篇 |
园艺 | 6403篇 |
植物保护 | 24905篇 |
出版年
2020年 | 2947篇 |
2019年 | 3721篇 |
2018年 | 4376篇 |
2017年 | 4805篇 |
2016年 | 5061篇 |
2015年 | 4507篇 |
2014年 | 5872篇 |
2013年 | 16426篇 |
2012年 | 7454篇 |
2011年 | 9763篇 |
2010年 | 7957篇 |
2009年 | 8375篇 |
2008年 | 9047篇 |
2007年 | 8082篇 |
2006年 | 8533篇 |
2005年 | 7627篇 |
2004年 | 7454篇 |
2003年 | 7277篇 |
2002年 | 6564篇 |
2001年 | 7933篇 |
2000年 | 7605篇 |
1999年 | 6711篇 |
1998年 | 4334篇 |
1997年 | 4281篇 |
1996年 | 3956篇 |
1995年 | 4696篇 |
1994年 | 4141篇 |
1993年 | 3783篇 |
1992年 | 5454篇 |
1991年 | 5644篇 |
1990年 | 5299篇 |
1989年 | 5292篇 |
1988年 | 4729篇 |
1987年 | 4869篇 |
1986年 | 4804篇 |
1985年 | 4967篇 |
1984年 | 4380篇 |
1983年 | 4040篇 |
1982年 | 3182篇 |
1981年 | 3004篇 |
1980年 | 2972篇 |
1979年 | 3835篇 |
1978年 | 3412篇 |
1977年 | 3043篇 |
1976年 | 2880篇 |
1975年 | 2844篇 |
1974年 | 3159篇 |
1973年 | 3140篇 |
1972年 | 2865篇 |
1971年 | 2844篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
61.
62.
Improvement of both the tensile and impact strength of the same polymeric material has always been a great challenge for the plastic industry. The study focuses on the effect of incorporation of calcium carbonate nanoparticles (0.3 wt% to 15 wt%) into three polypropylene (PP) based matrices viz. PP homopolymer, propylene-ethylene (PP-PE) copolymer and the blend of PP:PP-PE (30:70) to improve their impact behavior without hampering the tensile strength much. A loss in both the tensile and impact properties was observed in PP based nanocomposite. However, PP-PE based nanocomposites showed a significant improvement in impact strength (47 %) at 10 wt% loading with a loss of tensile strength by 22 %. To minimize this loss a blend of PP:PP-PE (30:70) was explored as a matrix. At 10 wt% loading, this matrix showed an improvement of 30 % in impact strength whereas the tensile loss was minimized to 10 %. Further, silane coupling agent which promoted good interfacial adhesion was used for best compositions. The variation of crystalline morphology of the nanocomposites with various formulations was analyzed using differential scanning calorimetry and X-ray diffraction. 相似文献
63.
64.
Alexander Jarryd Ehlers Smith David A. Ehlers Smith Yvette C. Downs Colleen T. 《Landscape Ecology》2019,34(3):537-549
Landscape Ecology - The effects of changing land use, and especially urbanisation, on species and functional diversity are of global concern. Eco-estates are a form of urban housing development... 相似文献
65.
Vimal Nair Min Cheol Kim James A. Golen Arnold L. Rheingold Gabriel A. Castro Paul R. Jensen William Fenical 《Marine drugs》2020,18(11)
A new cytotoxic thiodepsipeptide, verrucosamide (1), was isolated along with the known, related cyclic peptide thiocoraline, from the extract of a marine-derived actinomycete, a Verrucosispora sp., our strain CNX-026. The new peptide, which is composed of two rare seven-membered 1,4-thiazepane rings, was elucidated by a combination of spectral methods and the absolute configuration was determined by a single X-ray diffraction study. Verrucosamide (1) showed moderate cytotoxicity and selectivity in the NCI 60 cell line bioassay. The most susceptible cell lines were MDA-MB-468 breast carcinoma with an LD50 of 1.26 µM, and COLO 205 colon adenocarcinoma with an LD50 of 1.4 µM. Also isolated along with verrucosamide were three small 3-hydroxy(alkoxy)-quinaldic acid derivatives that appear to be products of the same biosynthetic pathway. 相似文献
66.
67.
Epidermal growth factor (EGF)-like activity was measured in mares' colostrum and milk by radioreceptor assay. Milk samples were collected from 22 mares 1 or more times during early lactation. Samples of colostrum were taken after parturition and before the foal first suckled (presuckle), within 6 hours after the foal first suckled (postsuckle), and on days 1, 2, 4, and 8 of lactation. In the 5 mares from which milk samples were obtained at each sampling time, presuckle colostral mean EGF-like activity (17.8 ng/ml) was greatest (P less than 0.05). The mean values for EGF-like activity at all other sampling times were not significantly different from each other (postsuckle colostrum, 9.7 ng/ml; day 1, 9.6 ng/ml; day 2, 8.5 ng/ml; day 4, 8.0 ng/ml; day 8, 7.8 ng/ml). 相似文献
68.
K A Hahn D W Knapp R C Richardson C L Matlock 《Journal of the American Veterinary Medical Association》1992,200(3):355-357
In 11 dogs with nasal adenocarcinoma, IV treatment with 2 to 8 cycles of cisplatin at a dosage of 60 mg/m2 of body surface, given at 3-week intervals, resulted in a survival time of 8 to 128 weeks (mean, 32.7 weeks; median, 20 weeks). Radiography revealed complete remission in 2 dogs and partial remission in 1 dog, for an overall response rate of 27%. Clinical problems such as nasal discharge (n = 11), sneezing (n = 6), and epistaxis (n = 4) resolved in 1 to 2 weeks in all dogs after beginning chemotherapy. 相似文献
69.
Historical and physical signs associated with prostatic disease diagnosed in dogs over a 5.5-year period were defined. One hundred seventy-seven male dogs were determined to have prostatic abnormality. Of the 177 dogs, 87 were determined to have specific prostatic disease. The most common prostatic disease identified in this study was bacterial prostatitis, followed by prostatic cyst, prostatic adenocarcinoma, and benign hyperplasia. The most common prostatic disease identified in neutered dogs was prostatic adenocarcinoma. Mean age at onset of prostatic disease was 8.9 years; statistically significant difference was not observed between age at onset of the various types of prostatic disease identified. Doberman Pinscher was the most common breed with prostate disease. Twenty-nine percent of dogs with a specifically identifiable prostatic disease had signs of systemic illness, 41% had signs of lower urinary tract disease, 28% had signs of gastrointestinal tract abnormalities, and 13% had signs of locomotor difficulty. 相似文献
70.
M O Furr M V Crisman J Robertson O Barta W S Swecker 《Journal of the American Veterinary Medical Association》1992,201(2):307-309
Immune system dysfunction and immunoglobulin deficiency was diagnosed in a 2-year-old horse with disseminated lymphosarcoma. Prolonged (35 days) parenteral nutrition was delivered to support the horse during a period in which immune function studies could be performed. Correction of nutritional compromise by use of parenteral nutrition did not correct the immunoglobulin deficiency, and results of lymphocyte phenotype testing did not indicate abnormal proportions of leukocytes. Lymphoblast transformation studies were suggestive of a circulating immunosuppressive factor in the horse's serum. Normal cell function was detected when the cells were stimulated in precolostral equine serum. 相似文献