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151.
以低材质原木为原料,对制作难度很大的非正方形截面模压矩形材的制作工艺进行了试验研究.结果表明杨木、落叶松、冷杉等实体木材的压缩恢复率均随水蒸气处理(180℃)定型时间的增加而减小;模压矩形材沿其短轴方向由表面至中心密度递减,沿长轴方向表面的中部密度最大;应用改进设计的模具,沿模压的短轴和长轴方向同时施力可克服模压矩形材的环状开裂;压缩矩形材的密度分布有利于提高整体压缩材的力学强度和物理性能.同时,该文对压缩矩形材的生产工艺及影响矩形材变形固定的主要因素进行了考察. 相似文献
152.
153.
Straightness is one of the most important properties for making timber an attractive material for modern mechanized building. Several studies have shown that a lack of straightness is one of the main reasons for choosing materials other than timber in the construction industry. This paper presents a way to model moisture-induced bow from longitudinal shrinkage data predicted from an analysis of images of the surface of Norway spruce studs. For this study, eight studs (45 × 95 × 2500mm and 45 × 120 × 3000mm) of Norway spruce timber were selected. Bow in these studs was measured at two moisture contents below the fiber saturation point. The studs were then split into three slices 11mm thick, and the surfaces of these slices were scanned to obtain color information and images of the tracheid effect. The slices were cut into sticks with dimensions of 10 × 10 × 200mm. The longitudinal shrinkage coefficient of these sticks was measured. A multivariate model was created to model the longitudinal shrinkage coefficient data from the information in the images. The predicted longitudinal shrinkage data was used to model bow. The mean value of the measured longitudinal shrinkage was 0.0121 (SD 0.0123). The root mean square error of prediction (RMSEP) for the multivariate model was 0.0079, which is regarded as good. Thus, it was possible to model moisture-induced bow with good accuracy using the predicted longitudinal shrinkage data. 相似文献
154.
Dorsal Compressive Atlantoaxial Bands and the Craniocervical Junction Syndrome: Association with Clinical Signs and Syringomyelia in Mature Cavalier King Charles Spaniels 下载免费PDF全文
S. Cerda‐Gonzalez N. J. Olby E. H. Griffith 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》2015,29(3):887-892
Background
Dorsal compressive lesions at the atlantoaxial junction (ie, AA bands) occur in dogs with Chiari‐like malformations (CMs), but their clinical relevance is unclear.Objective
Investigate the influence of AA bands on clinical status and syringomyelia (SM) in mature cavalier King Charles spaniels (CKCS).Animals
Thirty‐six CKCS, 5–12 years of age, including 20 dogs with neuropathic pain.Methods
Dogs were examined and assigned a neurologic grade. Magnetic resonance imaging (MRI) of the craniocervical junction was performed with the craniocervical junction extended and flexed (ie, normal standing position). Imaging studies were assessed for the presence of an AA band, CM, SM or some combination of these findings. Band and SM severity were quantified using an objective compression index and ordinal grading scale, respectively.Results
Of 36 CKCS imaged, 34 had CM. Atlantoaxial bands were present in 31 dogs and were more prominent in extended than flexed positions. Syringomyelia was found in 26 dogs, 23 of which also had AA bands. Bands were associated with both the presence (P = .0031) and severity (P = .008) of clinical signs and SM (P = .0147, P = .0311, respectively). Higher compression indices were associated with more severe SM (P = .0137).Conclusions
Prevalence of AA bands in older CKCS is high. Positioning of dogs in extension during MRI enhances the sensitivity of the study for detecting this important abnormality. There were significant associations among AA bands, clinical signs, and SM in dogs with CM; additional work is needed to understand whether or not this relationship is causal. 相似文献155.
Md Qumruzzaman Chowdhury Futoshi Ishiguri Kazuya Iizuka Yuuya Takashima Kahoru Matsumoto Tokiko Hiraiwa Megumi Ishido Hiroyuki Sanpe Shinso Yokota Nobuo Yoshizawa 《Journal of Wood Science》2009,55(2):139-143
Radial variations of wood properties (basic density, fiber length, vessel element length, and compression strength) in plantation-grown
Casuarina equisetifolia in Bangladesh were investigated for effective utilization of the wood. Samples disks at breast height were randomly collected
from trees in a 10-year-old plantation in Cox’s Bazar Forest Division, Bangladesh. The basic density showed a near-constant
value up to 30 mm from the pith and then rapidly increased up to 60 mm from the pith. The fiber length and vessel element
length gradually increased from the pith to bark. When radial variation of wood properties was determined according to relative
distance from the pith, similar radial patterns were observed among the sample trees, indicating that the wood properties
in C. equisetifolia may be related to the growth rate. The compression strength parallel to the grain (CS) increased from the pith to bark. A
significant positive correlation was found between the air-dried density and the CS. The results obtained indicated that wood
around the pith has a relatively low density, and wood outside the pith area has a relatively high density, suggesting that
it could be used as structural lumber.
Part of this report was presented at the 58th Annual Meeting of Japan Wood Research Society, Tsukuba, March 2008 相似文献
156.
通过压缩固结试验,研究应力水平对北方湿地软土压缩性的影响,得出了齐齐哈尔湿地软土的 曲线、压缩系数、压缩模量、主固结系数、次固结系数等主要压缩指标随固结压力的变化规律,以及不同的加载比对 曲线、主固结系数和次固结系数的影响.研究成果可为公路湿地软土地基在路堤填土分级加载情况下固结沉降计算提供相关参数. 相似文献
157.
Stem deformation has often been observed in young black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) plantations. Whenever important stem deformations are observed at the time of harvesting, timber value is negatively affected especially during the wood transformation process. The present work was undertaken to quantify and qualify the importance of stem deformation of black spruce and jack pine in the boreal forest of central Quebec at the stand and tree levels. In 30 black spruce and jack pine plantations, approximately 22% of spruce trees and 27% of pine trees exhibited stem deformation. The proportion of deformed trees was higher in the youngest plantations and decreased with the age of the plantations. Stem deformation caused the formation of compression wood which is another factor that can reduce the value of wood products. Thirty-nine black spruces and 34 jack pines were analysed at the tree level. On average, compression wood represented 14% and 20% of stem volume in 7- and 10-year old black spruce plantations, respectively. These proportions ranged from 18% in the youngest jack pine plantation to 26% in the oldest one. Stems of both species classified as normal contained a lower volume of compression wood than stems classified as deformed or very deformed. Annual percentages of compression wood and annual shoot length increased significantly with tree age (p < 0.0001 for both variables). Statistically significant correlations were also found between the range of displacement of the stem and the percentage of compression wood. The fewer number of trees with deformed stems in older plantations combined with high compression wood formation suggests that, over time, a deformed tree can become normal and straight in appearance. 相似文献
158.
Comparison of internal bond strength and compression shear strength of wood-based materials 总被引:1,自引:1,他引:0
The purpose of this study was to design a compression shear device for easy and fast measurement of the bonded shear strength of wood-based materials to replace the conventional method used to evaluate internal bond strength (IB). To assess the performance of this device, five differently sized specimens, included group I (dimension 5 × 1 cm), group II (5 × 2 cm), group III (5 × 3 cm), group IV (5 × 4 cm), and group V (5 × 5 cm) cut from commercial particleboard and medium-density fiberboard (MDF) (1.8 cm thick) were tested in compression shear. Only group V (5 × 5 cm) was prepared for the IB test. Results indicated that the compression shear strengths (CS) of particleboard and MDF, loaded in the horizontal or the diagonal direction, were greater than the IB, although a significant correlation existed between the two. This finding suggests that the IB of particleboard and MDF could be accurately estimated from the data collected by the CS test.Part of this report was presented at the Third Pacific Rim Bio-based Composites Symposium, Kyoto, December 2–5, 1996 相似文献
159.
By means are measured of the direct observation of the packing behavior of sand in the vacuum suction molding process.The air and powder pressure,density distribution of sand core.The packing mechanisms are studied based on these experimental results.It found that the density distribution of sand core is elucidated by powder pressure theory,and the density distribution is decided by air flowing,sand chocking and shearing. 相似文献
160.
针对铝合金锻件超声信号中含有晶粒散射引起的相干噪声,建立了相应的缺陷回波检测数学模型.提出了基于新的阈值函数Stein无偏估计自适应压缩方法.对离散小波变换各尺度上的小波系数用新的阈值函数进行估值后,进行了小波阈值最小均方误差意义上的迭代,并用小波系数估值进行离散小波反变换.以得到信号的估值,通过反复迭代,得到缺陷回波的最优压缩模型.对含缺陷铝合金锻件的超声信号处理实验的结果表明,与常用的固定阈值方法相比,在相同压缩比下,自适应压缩方法能更好地去除散射噪声及识别缺陷回波信号. 相似文献