全文获取类型
收费全文 | 298610篇 |
免费 | 7068篇 |
国内免费 | 1833篇 |
专业分类
电工技术 | 5693篇 |
技术理论 | 5篇 |
综合类 | 1166篇 |
化学工业 | 49903篇 |
金属工艺 | 11725篇 |
机械仪表 | 8513篇 |
建筑科学 | 7717篇 |
矿业工程 | 1740篇 |
能源动力 | 6473篇 |
轻工业 | 32280篇 |
水利工程 | 3256篇 |
石油天然气 | 6105篇 |
武器工业 | 250篇 |
无线电 | 29332篇 |
一般工业技术 | 55612篇 |
冶金工业 | 55502篇 |
原子能技术 | 5771篇 |
自动化技术 | 26468篇 |
出版年
2021年 | 3492篇 |
2020年 | 2501篇 |
2019年 | 3113篇 |
2018年 | 4908篇 |
2017年 | 4781篇 |
2016年 | 5236篇 |
2015年 | 3998篇 |
2014年 | 6058篇 |
2013年 | 14413篇 |
2012年 | 9958篇 |
2011年 | 12366篇 |
2010年 | 9917篇 |
2009年 | 10290篇 |
2008年 | 11037篇 |
2007年 | 10944篇 |
2006年 | 9441篇 |
2005年 | 8244篇 |
2004年 | 7471篇 |
2003年 | 6922篇 |
2002年 | 6862篇 |
2001年 | 6786篇 |
2000年 | 6367篇 |
1999年 | 6387篇 |
1998年 | 15338篇 |
1997年 | 11229篇 |
1996年 | 8379篇 |
1995年 | 6195篇 |
1994年 | 5582篇 |
1993年 | 5548篇 |
1992年 | 4166篇 |
1991年 | 3965篇 |
1990年 | 3991篇 |
1989年 | 3948篇 |
1988年 | 3700篇 |
1987年 | 3160篇 |
1986年 | 3185篇 |
1985年 | 3564篇 |
1984年 | 3446篇 |
1983年 | 3167篇 |
1982年 | 2836篇 |
1981年 | 3073篇 |
1980年 | 2820篇 |
1979年 | 2925篇 |
1978年 | 2823篇 |
1977年 | 3139篇 |
1976年 | 4305篇 |
1975年 | 2508篇 |
1974年 | 2340篇 |
1973年 | 2384篇 |
1972年 | 2006篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
941.
B Acea Nebril J Aguirrezabalaga González F Fraguela Mari?a L Taboada Filgueira D Freire Rodríguez C Gómez Freijoso 《Canadian Metallurgical Quarterly》1997,21(7):705-707
Gut involvement in bladder tumours is low, even exceptional in the presence of surface, low-grade neoplasia. The authors explain their experience in the diagnosis and management of a patient treated endoscopically for a vesical surface tumour which subsequently exhibited peritoneal and gut metastatic seeding. The various mechanisms for gut dissemination of vesical neoplasias and the repercussion of their endoscopic management are discussed. 相似文献
942.
M Ramirez-Salomon R Soler-Bientz R de la Garza-González CM Palacios-Garza 《Canadian Metallurgical Quarterly》1997,23(9):586-587
It is important for endodontic instruments to have a low fracture rate. If a fracture does occur, it would be desirable to have the ability to bypass the broken segment and complete the root canal treatment. One hundred sixty-two root canals in 52 maxillary and mandibular first molars were cleaned and shaped with Lightspeed instruments by three endodontists in their private practices. The canals were instrumented using the technique recommended by the manufacturer. All canals were instrumented to at least a size 45 at the working length. Six instruments separated during treatment. All six had been used more times than recommended by the manufacturer. Five of the six were easily bypassed and treatment completed. 相似文献
943.
There has been an increasing interest in the use of code-division multiple access (CDMA) in cellular mobile and wireless personal communications. The choice of such multiaccess technique is attractive because of its potential capacity increases and other technical factors such as privacy and multipath rejection capabilities. However, it is well known that the performance of CDMA can be significantly degraded due to cochannel interference (CI) and the near-far effects. We consider the performance of direct-sequence (DS)-based CDMA over fading channels that are modeled as slowly varying Rayleigh-fading discrete multipath channels. Specifically, we propose and analyze an adaptive multistage interference cancellation strategy for the demodulation of asynchronous DS spread-spectrum multiple-access signals. Numerical results show that the proposed multistage detector, which alleviates the detrimental effects of the near-far problem, can significantly improve the system performance 相似文献
944.
P Nádvornik P Krupa J Chrastina V Smrcka Z Novák E Zborilová 《Canadian Metallurgical Quarterly》1997,139(4):359-360
The authors propose a new method for performing stereotactic callosotomy, which they have named circular callosotomy. The operating device is the original Riechert-Mundinger's string electrode, which can be protruded through a side window and by rotating the probe it is possible to cut the commisural pathways to the extent required. The anatomical results of the operation can be checked using MRI scanning. 相似文献
945.
L Palazzo B Landi C Cellier G Roseau S Chaussade D Couturier J Barbier 《Canadian Metallurgical Quarterly》1997,29(9):850-853
BACKGROUND AND STUDY AIMS: Granular cell tumors of the esophagus are rare tumors. A definite diagnosis is achieved by endoscopic biopsies in only 50% of cases. Endoscopic ultrasonography (EUS) is the best procedure in the evaluation of upper gastrointestinal tract submucosal tumors. The aim of this study was to describe the endosonographic findings of esophageal granular cell tumors. METHODS: From January 1989 to March 1994, 15 patients with 21 granular cell tumors which had negative biopsies were examined by EUS (Olympus GF UM3 or GF UM20,7,5 and 12 MHz). In five cases, the tumor was also studied with a 20 MHz Olympus miniprobe. The final histological diagnoses were obtained by subsequent endoscopic snare resection in 20 cases and surgically in one case. RESULTS: The endosonographic features (with the GF UM3 or GF UM20) of esophageal granular cell tumors were: a) a tumor size of less than 2 cm in 95% of cases; b) an hypoechoic solid pattern in 100% of cases; c) a tumor arising in the inner layers in 95% (second echo-poor layer n=15; third echo-rich layer n=5). In one case, the endosonographic finding was transmural malignant infiltration of the esophageal wall (histologically confirmed). CONCLUSION: When a granular cell tumor of the esophagus is suspected, EUS can show the inner layer location of the tumor and thus contribute to planning the endoscopic resection or follow up. When the tumor also invades the outer layers, EUS can contribute to planning the surgical resection. 相似文献
946.
We propose that chronically denervated Schwann cells may be less able to respond to axonal signals than their acutely denervated counterparts, and that this lack of sensitivity may be one reason why axons fail to regenerate into chronically denervated nerve stumps. To test this proposal we have used in situ hybridization, and quantitative and qualitative immunohistochemistry to compare the expression of c-erbB2 and c-erbB4 receptors in Schwann cells denervated for up to 6 months in vivo, with that seen in Schwann cells denervated for similar periods of time but then exposed to regenerating axons. The results were correlated with the extent of axonal regeneration in each experimental group as assessed from transverse sections which had been double-immunolabelled using anti S-100 and anti-beta tubulin III antibodies. Since c-erbBs are receptors for neuronally derived neuregulins we probed the appropriate axotomised DRG neurons for expression of GGF2 mRNA. When the denervated distal stumps were anastomosed to acutely transected proximal stumps, GGF expression in DRGs increased transiently during the first week: we assume that secreted GGF2 derived from regrowing axon sprouts would have been available to Schwann cells in all distal stumps. Endoneurial cell proliferation (predominantly Schwann cell proliferation); levels of expression of c-erbB receptors by Schwann cells, and the degree to which axons regenerated into the distal stumps all decreased as the period of prior denervation increased: the longer the time of denervation, the lower the expression of c-erbBs in Schwann cells, and the smaller the percentage of bands of Bungner which were re-innervated. 相似文献
947.
PURPOSE: Ultrasound estimated bladder weight was compared before and after surgery for benign prostatic hyperplasia (BPH) to reveal a possible reversible change in bladder hypertrophy. MATERIALS AND METHODS: Ultrasound estimated bladder weight was measured before and after subcapsular (17) or transurethral (16) prostatectomy in 33 male patients with BPH. Sequential changes in the American Urological Association symptom score and urinary flow rate were also examined. RESULTS: Along with a significant improvement in the American Urological Association symptom scores and maximum flow rate, ultrasound estimated bladder weight decreased from 52.9 +/- 22.6 to 31.6 +/- 15.8 gm. in 12 weeks after treatment. In all but 4 patients (29 of 33, or 87.9%) ultrasound estimated bladder weight decreased to less than 35.0 gm. in 12 weeks after treatment. Interestingly, in all patients with an initial ultrasound estimated bladder weight of greater than 80 gm. the bladder weight still remained at an abnormally high level 12 weeks after treatment. CONCLUSIONS: Bladder hypertrophy was completely reversible after the surgical treatment of the obstruction in the majority of patients with BPH. The measurement of ultrasound estimated bladder weight was of value in monitoring therapeutic effects in BPH patients. An extraordinarily high ultrasound estimated bladder weight of 80 gm. or more might suggest degenerative and irreversible pathological changes in the bladder detrusor. 相似文献
948.
PI Mitchell L León Vintró H Dahlgaard C Gascó JA Sánchez-Cabeza 《Canadian Metallurgical Quarterly》1997,202(1-3):147-153
Diabetes complicates 2-3% of all pregnancies and is associated with an increase in both perinatal morbidity and mortality, though reasons for these adverse outcomes are unknown. Estrogen biosynthesis is a critical factor during pregnancy and is carried out in the placenta via aromatase (cytochrome P450 19A1), which catalyzes the conversion of C-19 androgens to C-18 estrogens. Previous studies have shown that hormones such as insulin-like growth factors and insulin regulate aromatase activity when studied in vitro. Interestingly, levels of these hormones are altered in patients with diabetes. Thus, we hypothesized that the presence of maternal diabetes may alter placental aromatase activity and thus estrogen biosynthesis, possibly serving as one factor in the adverse outcomes of babies born to mothers with diabetes. To this end, we measured the production of 19-hydroxyandrostenedione, 19-oxoadrostenedione and estrone in 30 placental tissues from diabetic patients, using [7-3H]androst-4-ene-3,17-dione as a model substrate for aromatase (P450 19A1). A statistical difference was detected in the percentage of 19-oxoandrostenedione formed between the overt and control groups (P < 0.05). Additionally, NADPH P450-reductase levels were measured in these same tissues to determine whether alterations in this enzyme necessary for aromatase activity could be affected by diabetes. No differences in reductase levels were detected among the patient groups. However, a statistical correlation was found between NADPH P450-reductase activity and the formation velocities of all three estrogen products (P < 0.05). Thus, it appears that the presence of diabetes does not affect placental aromatase activity. 相似文献
949.
O Zanetti G Binetti E Magni L Rozzini A Bianchetti M Trabucchi 《Canadian Metallurgical Quarterly》1997,95(3):152-157
The operative results of 23 patients with a specific or unspecific spondylodiscitis were documented over 2 years after the focus of the inflammation had been eradicated, bone chip had been interposed and a CDH instrumentation had been performed by an anterior approach only. These outcomes were compared with the results of 32 patients in whom the focus had been removed and the defect had been filled with bone graft from an anterior approach, followed by stabilisation with CD instrumentation through an additional dorsal approach. In the cases where CDH instrumentation was applied, the range of fusion averaged 1.3 segments. This was clearly less extensive than in dorsoventral stabilisation, in which on average 3.5 segments were fused. In 47 of 55 cases mobilisation was achieved without orthesis. Eight months after the operations bony fusion could be observed radiologically in all patients. The mean preoperative kyphotic angle of the affected segments was 14.4 degrees, compared to 4 degrees after the operation. The mean loss of reposition was measured to be about 2.7 degrees in both groups. Average operation time and blood loss were about 50% higher in the patients treated dorsoventrally. We conclude that even in the case of florid spondylodiscitis, a short-range anterior fusion of the affected spinal segment may be performed by use of a stable-angle implant without an increased risk of infection-related loosening. 相似文献
950.
J Tyson L Tranebjaerg S Bellman C Wren JF Taylor J Bathen B Aslaksen SJ S?rland O Lund S Malcolm M Pembrey S Bhattacharya M Bitner-Glindzicz 《Canadian Metallurgical Quarterly》1997,6(12):2179-2185
The Jervell and Lange-Nielsen syndrome (JLNS) comprises profound congenital sensorineural deafness associated with syncopal episodes. These are caused by ventricular arrhythmias secondary to abnormal repolarisation, manifested by a prolonged QT interval on the electrocardiogram. Recently, in families with JLNS, Neyroud et al. reported homozygosity for a single mutation in KVLQT1 , a gene which has previously been shown to be mutated in families with dominantly inherited isolated long QT syndrome [Neyroud et al . (1997) Nature Genet ., 15, 186-189]. We have analysed a group of families with JLNS and shown that the majority are consistent with mutation at this locus: five families of differing ethnic backgrounds were homozygous by descent for markers close to the KVLQT1 gene and a further three families from the same geographical region were shown to be homozygous for a common haplotype and to have the same homozygous mutation of the KVLQT1 gene. However, analysis of a single small consanguineous family excluded linkage to the KVLQT1 gene, establishing genetic heterogeneity in JLNS. The affected children in this family were homozygous by descent for markers on chromosome 21, in a region containing the gene IsK . This codes for a transmembrane protein known to associate with KVLQT1 to form the slow component of the delayed rectifier potassium channel. Sequencing of the affected boys showed a homozygous mutation, demonstrating that mutation in the IsK gene may be a rare cause of JLNS and that an indistinguishable phenotype can arise from mutations in either of the two interacting molecules. 相似文献