采用 SVM 方法的文本情感极性分类研究

文本情感极性分类是文本情感分析首先要解决的关键问题。在分析影响文本情感分类的各类因素的基础上,首先构建了情感词典,并进行情感特征选取以及情感特征加权,然后使用SVM分类的方法对文本进行情感识别及分类,最后在语料数据集的基础上,在单机平台上和Spark分布式计算平台上执行分类模型,对比分析其分类精度和时间代价。实验结果验证了本文构建的情感极性分类模型在单机和分布式云平台上中的有效性。     

Fatigue crack propagation in some PM steels

Abstract

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0·90 and 1·0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when ρ_r is ～0·9 but when ρ_r is ～1·0 the exponents are between 2·6 and 4·0, i.e in the range typical of wrought steels (2–4). At both densities, threshold stress intensities are between 5·5 and 10·8 MPa m_1/2 when R = 0·1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0·8, reductions in threshold to between 2·7 and 5 MPa m_1/2 are attributed to crack closure effects. At ρ_r = 0·90, Fe–0·5C fails by progressive rupture of sinter necks. Astaloy A, with 0·2%C and 0·6%C, and Distaloy AB–0·6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When ρ_r is ～1·0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when K_maxis high.     

Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m^?2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.     

Microstructure and properties of Al2O3 dispersion-strengthened copper fabricated by reactive synthesis process

Al2O3 dispersion-strengthened copper alloy was prepared by reactive synthesis and spark plasma sintering(SPS) process. Studies show that nano-sized c-Al2O3 particles with 27.4 nm mean size and 50-nm interval are homogeneously distributed in copper matrix. The density of SPS alloy is about 99 %, meanwhile, the electrical conductivity of sintered alloy is 72 % IACS and the Rockwell hardness can reach to HRB 91.     

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

Availability analysis of a coke oven gas fueled spark ignition engine

The current work investigates a coke oven gas fueled spark ignition (SI) engine from the perspective of the first and second laws in order to understand the energy conversion performance of fuels and achieve highly efficient utilization. A detailed energy and exergy analysis is applied to a quasi-dimensional two-zone spark ignition engine model which combines turbulence flame propagation speed model at 1500 rpm by changing gas fuel types, compression ratio, load and ignition timing. It was found that the irreversibility of methane is the maximum and that of syngas is the minimum among the three different fuels. The irreversibility in the combustion process of a coke oven gas fueled SI engine is reduced when the compression ratio or the throttle valve opening angle is increased and the ignition timing is delayed. Increasing the compression ratio and delaying the ignition timing can improve the first and second law efficiency and reduce the brake specific fuel consumption (BSFC). The power performance and fuel economy are good and the energy is also used effectively when the compression ratio is 11, the throttle angle is 90% and the ignition time is ?10° CA ATDC respectively.     

Microstructural behaviour of spark plasma sintered composites containing bimodal micro- and nano-sized Al2O3 particles

Spark plasma sintering (SPS) has been recognised, in the recent past, as a very useful method to produce metal matrix composites with enhanced properties if compared with traditional powder metallurgy techniques. Obviously, the materials final properties are strongly related to the reinforcement types and percentages as well as to the processing parameters employed during synthesis. The present paper analyses the effect of microscopic and nanometric alumina particles, blended to pure aluminium in different combinations, on the final properties of metal matrix composites produced via SPS. A strong variation in the microstructural behaviour has been observed by varying the nano to micro alumina particles percentage blended with aluminium ones.     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Improvement in microstructure and mechanical properties of Ti(C,N) cermet prepared by two-step spark plasma sintering

A fine grained Ti(C, N) cermet tool material was prepared by two-step spark plasma sintering. Microstructure evolution and densification mechanisms of Ti(C, N) during spark plasma sintering were studied. Effect of two-step sintering process and Ni content on microstructure and mechanical properties were also investigated. The critical activated densification temperature of Ti(C, N) is about 1300?℃, and the rapidest densification rate takes place at 1300?℃~1400?℃. Grains are in the size of 1?µm when the Ti(C, N) cermet was prepared by two-step spark plasma sintering. The optimal flexural strength, fracture toughness and Vickers hardness are 1094?±?42?MPa, 7.2?±?0.5?MPa?m^1/2 and 18.3?±?0.4?GPa, respectively. The Ti(C, N) cermets containing more content of Ni have higher toughness, which is due to the remarkable toughening effect of crack bridging by large grains.     

Effect of tantalum addition on microstructure and oxidation of spark plasma sintered ZrB2-20vol% SiC composites

Almost full density (>99% theoretical density (ρ_th)) was achieved for ZrB₂-20vol% SiC-Xwt.% Ta (X = 2,5, 5 and 10) composites after Spark Plasma Sintering (SPS) (Temperature: 1900 °C, Pressure: 50 MPa; Time: 3 min). The microstructure of ZrB₂-based composites exhibited core-rim structure and it consists of major crystalline phases (ZrB₂ core, (Zr, Ta)B₂ rim, SiC), minor amounts of ZrO₂ and (Zr, Ta)C solid solution phases. Both the specific weight (from 22.91 to 18.77 mg/cm²) and oxide layer thickness (401–195 μm) of ZrB₂-20vol% SiC composites decreased with increasing addition of Ta after the isothermal oxidation at 1500 °C for 10 h in air. The cross-sectional microstructure of oxidized samples displayed presence of a stack of three distinctive layers, which includes thick dense SiO₂ top layer, SiC depleted intermediate layer and unreacted bulk. The present work clearly demonstrated the advantage of tantalum addition in improving the oxidation resistance of ZrB₂-20vol% SiC.