High pressure infiltration sintering behavior of WC-Co alloys

In this paper, two average tungsten carbide particle sizes of 2, 0.5?μm are placed respectively, in contact with a WC-16Co substrate, pressed at the pressure of 4.5–5.5?GPa, and heated to temperatures ranging from 1350°C to 1500°C in a large-volume cubic press. During the process Co was forced out of the WC-16Co substrate into the compressed powder. The resulting infiltrated samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Vickers hardness and cutting performance tests. The results of XRD confirmed that the sintered bulks have WC and Co phases. The scanning electron microscopy (SEM) analysis reveals that the WC grains in well-sintered alloys are round in shape and cobalt with lower content is uniformly dispersed in the WC grain boundaries. The sintered sub-micron WC-Co alloy with a cobalt content of 3.8?wt% exhibits a prominent combination of high hardness value of 23.1?GPa and a large fracture toughness value of 8.6?MPa?m^½. The high-speed cutting tests indicating its cutting performance is significantly superior to the commercial YG6X (WC-6?wt%Co with WC grain size of 0.5?μm).     

High pressure sintering behavior of polycrystalline diamond with tungsten

Polycrystalline diamond was investigated under high pressure and high temperature of 5.0 GPa and 1100–1500 °C in the presence of tungsten. In situ resistance measurements indicated that reactions between diamond and tungsten happened at about 960 °C. Phase analysis demonstrated that WC increased and meta-stability of W₂C decreased clearly at the higher temperature. It is clear from the characterization of the sintered body that the electrical resistance decreased and the density of specimens increased as the sintering temperature rose. The specimen sintered at 1500 °C has a homogeneous microstructure and good conductivity.     

Analysing the sintering of heterogeneous powder structures by in situ microtomography

This work analyses the microstructure changes of various copper-based powder systems during sintering from 3D images provided by in situ synchrotron microtomography. The investigated systems include a copper powder with a wide particle size distribution of 0–63 µm poured into a quartz capillary, a pre-sintered copper compact with artificially created large pores and a mixture of copper and alumina particles. The experiments were carried out at the European Synchrotron in Grenoble, France. Powders were sintered up to 1060°C under reducing atmosphere in a furnace located between the X-ray source and the detector. During each experiment, 3D images were taken at various times of the thermal cycle. We have obtained images with a resolution of 1.5 µm and the time of acquisition of every image was ～1 min. Quantitative analysis of these images allowed the changes of various important parameters to be followed. Such parameters characterise the sintering process at the particle length scale: interparticle coordination, pore size distribution and particle centre-to-centre distance. Moreover, by tracking the displacement of each particle centre and comparing it to the displacement predicted by classical mean field assumption, we have been able to assess the magnitude of particle rearrangement occurring during sintering. From these data, the sintering behaviour of heterogeneous powder systems is discussed with particular emphasis of collective particle phenomena.     

The effects of powder properties on in-flight particle velocity and deposition process during low pressure cold spray process

In cold spray process, impacting velocity and critical velocity of particles dominate the deposition process and coating properties for given materials. The impacting velocity and critical velocity of particles depend on the powder properties and cold spray conditions. In the present study, the in-flight particle velocity of copper powder in low pressure cold spraying was measured using an imaging technique. The effects of particle size and particle morphology on in-flight particle velocity and deposition efficiency were investigated. The critical velocity of copper powder was estimated by combining the in-flight particle velocity and deposition efficiency. The effect of annealing of feedstock powder on deposition and critical velocity was also investigated. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 μm. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390 °C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study.     

Facile fabrication of superhydrophobic polytetrafluoroethylene surface by cold pressing and sintering

A series of superhydrophobic polytetrafluoroethylene (PTFE) surfaces were prepared by a facile cold pressing and sintering method, and their microstructures and wetting behaviors could be artificially tailored by altering sintering temperature and using different masks. Specifically, the microstructures mainly depended on the sintering temperature, whereas the wetting behaviors, water contact angle (WCA) and sliding angle (SA), greatly hinged on both the sintering temperature and mask. Then a preferable superhydrophobic surface with WCA of 162 ± 2° and SA of 7° could be obtained when the sintering temperature was 360 °C and the 1000 grit abrasive paper was used as a mask. In addition, it was worth noting that the as-prepared surfaces exhibited excellent stability under UV illumination, which was the most key factor for them toward practical applications.     

Reaction laser sintering of Ni–Al powder alloys

Laser reactive sintering, i.e., laser-induced self-propagating reaction sintering synthesis was carried out on Ni–Al powder alloys. The exothermic behaviors for the alloys with different Al content were characterized by sintering temperature curves produced from reactive heat. The phases transformed from the sintered alloys were identified by X-ray diffraction. Properties and microstructure of the sintered alloys were studied.     

冷却物冷藏间冷风机的空气除霜实验研究

冷却物冷藏间以贮藏果蔬为主,当贮藏品种产于热带地区,贮藏温度高于0℃。由于贮藏温度距水的冰点不远,制冷系统的蒸发温度会低于冰点,在工作过程中蒸发器表面必然结霜。文中对冷间温度高于冰点,蒸发温度低于冰点的冷库制冷系统,进行了空气除霜和"依次除霜法"实验,通过冷风机回风区空气温度、冷风机蒸发管组翅片温度的测量,结合肉眼观察认为,库温为5℃的情况下,依靠制冷压缩机停机阶段,冷风机风扇常开,能够基本除掉上次制冷过程产生的结霜,当制冷系统规律工作8小时,前期制冷过程积累的结冰可以通过"依次除霜法"依次关闭除霜冷风机的供液电磁阀15分钟,在保证库房降温的前提下,能够彻底除掉蒸发器表面的结霜。     

Test of cold denaturation mechanism for proteins as a function of water's structure

At low temperatures and high pressures (ranging from approximately 200 up to 700 MPa), some proteins become thermodynamically unstable by means of a phenomenon know as cold denaturation. A microscopic understanding of the mechanisms leading to cold denaturation is very difficult to develop, due in part to the complexity of the protein–solvent interactions. A possible proposal to explain the cold denaturation of proteins based on a relation between hydrophobicity and the loss of the local low-density water structure at high pressures is explained in detail. In the present paper, this mechanism is tested for the first time by means of full atom numerical simulations. In good agreement with the proposal, cold denaturation resulting in the unfolded state has been found at the high-density liquid (HDL) state of water, at which the amount of open tetragonal hydrogen bonds decreases at cooling.     

Rietveld analysis of high pressure powder diffraction data

Abstract

X-ray and neutron powder diffraction data obtained from samples contained within high pressure cells are generally of lower quality than data collected from samples at ambient conditions. The far smaller sample size as well as possible contamination of the pattern by the pressure cell means that Rietveld refinement techniques must be adapted to extract the maximum useful information from the data. These problems become paramount as larger structures at high pressure are attempted. Techniques such as “leBail extraction”, “soft restraints” and “rigid body refinement” will be discussed with application to analysis of high pressure neutron powder diffraction data.     

单压吸收式制冷气泡泵冷态实验及性能分析

单压吸收式制冷是一种可以利用太阳能等低品位能源驱动的制冷循环,而气泡泵是该制冷系统中的最主要耗能部件,因此对气泡泵的研究尤其重要。冷态模拟实验可以避免热态实验中出现的实验过程不稳定等缺点,产生的气泡更加均匀,获得的实验数据更加准确。实验结果表明输气量和沉浸比对气泡泵性能影响较大。     

Sintering behaviour and microstructures of nanostructured ZnO–ZnS core–shell powder by spark plasma sintering

Densification of nanostructured ZnO–ZnS core–shell powder was carried out by spark plasma sintering to produce a bulk ZnO–ZnS composite. By adjusting the sintering temperature, we could fabricate a bulk ZnO–ZnS composite without destroying the original core–shell structure of the powder. X-ray diffraction and transmission electron microscopy were used to characterize the microstructural properties of the core–shell powder and its sintering behaviour. During spark plasma sintering, phase transition from a sphalerite to a wurtzite structure was observed in the ZnS shell and the crystallographic orientation of the ZnS shell was affected by the ZnO core.     

Effects of high pressure sintering on the microstructure and thermoelectric properties of BiCuSeO

Polycrystalline BiCuSeO oxides are prepared by solid-state reaction followed by pressureless sintering (PLS) and high pressure sintering (HPS) methods. Both the experimental results and the density functional theory calculations indicate that the crystal defect concentrations of BiCuSeO can be reduced under the effect of high pressure. By comparing with the PLS sample, a larger power factor and a lower thermal conductivity can be obtained for the HPS sample. The maximum figure of merit ZT～0.4 @ 800?K was obtained for the HPS sample, which is about 3 times higher than the PLS sample. These results indicate that the effect of high pressure is beneficial to modifying the microstructure and improving the thermoelectric performance of BiCuSeO oxyselenide.     

对冷屏蔽结构设计的分析

空间冷屏是一种包裹在飞行器外的相变制冷装置,旨在提高近地高速飞行器的突防能力,防止其在近地轨道飞行时被红外探测器发现和拦截.为了实现弹道导弹在飞行中段的红外隐身效果,采用深冷介质相变制冷技术,建立了空间冷屏样机.对冷屏系统工作要求主要是:体积、重量轻、换热能力强、外表面维持低温时间长、外表面各点的温差小.分析了为达到以上几点在冷屏蔽系统设计中采取的几个措施.     

Deposition characteristics of Al-12Si alloy coating fabricated by cold spraying with relatively large powder particles

In this paper, the microstructure, microhardness and adhesive strength of Al-12Si coating produced by cold spraying were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was lower than that of small Al-12Si particles. The as-deposited Al-12Si coating had the same crystal structure as Al-12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. Some fine Si particles precipitated in α-Al matrix because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in Al-12Si coating improved the coating microhardness.     

高压下制备高密度的Bi系超导体

 首次直接用高压烧结而不必进行任何退火后处理获得超导Bi_0.8Pb_0.2SrCa₂O_x陶瓷样品。此样品是由名义组分为Bi_0.8Pb_0.2SrCa·Cu₂O_x的超导体在1.0 GPa、500 ℃处理2 h后制得的，其零电阻转变温度为97 K。以前未见报导过如此好的结果。高压下烧结的Bi_0.8Pb_0.2SrCa·Cu₂O_x超导体的密度可达6.23 g/cm³（约为理论密度的95%），这比常规法制得的样品的密度（大约4~5 g/cm³）高得多。     

聚晶金刚石复合片超高压烧结过程的实验研究

 采用X射线衍射（XRD）分析、扫描电子显微镜（SEM）和能谱分析（EDX）等方法,对在压力5.6 GPa、温度1 460 ℃条件下,加热时间分别为2、3、4、6、8、10 min的聚晶金刚石复合片（PDC）样品进行了分析。结果表明：PDC的烧结经历了金刚石的石墨化过程,而且在烧结过程中金刚石层表面的XRD图谱出现了WC衍射峰,强度由弱到强,再由强到弱,直至消失;进一步的研究发现,PDC中助烧结剂Co的晶格常数因C和W原子的固溶会变大,并随PDC加热时间的延长出现一个峰值;从硬质合金基体扩散到金刚石层的Co由于扩散通道各异、所受阻力不同,使得其分布不均匀。     

选择性激光烧结成型件密度的支持向量回归预测

根据不同工艺参数（层厚、扫描间距、激光功率、扫描速度、加工环境温度、层与层之间的加工时间间隔和扫描方式）下的选择性激光烧结成型件密度的实测数据集,应用基于粒子群算法寻优的支持向量回归（SVR）方法,建立了加工工艺参数与成型件密度间的预测模型,并与BP神经网络模型进行了比较.结果表明：基于相同的训练样本和检验样本,成型件密度的SVR模型比其BP神经网络模型具有更强的内部拟合能力和更高的预测精度;增加训练样本数有助于提高SVR预测模型的泛化能力;基于留一交叉验证法的SVR模型的预测误差最小.因此,SVR是一种预测选择性激光烧结成型件密度的有效方法. 关键词： 选择性激光烧结 密度 支持向量机 回归分析     

冷85Rb原子D2线电磁诱导透明的实验研究

对85Rb原子D2线三能级系统的电磁诱导透明进行了详细的实验研究.得到了不同耦合强度下探测光的吸收线型,并对气室中热原子与磁光阱中冷原子的电磁诱导透明光谱进行了比较.     

Synthesis and high-resolution study distinguishing between very similar interstitial iron nitride structures

Angle-dispersive X-ray diffraction and microfluorescence together with precession electron diffraction (PED) and scanning electron microscopy measurements on iron nitride prepared at 15?GPa and 1800?K from iron and sodium azide starting materials reveal synthesis of both hexagonal P6₃22 and trigonal P312 Fe₃N_1+x modifications (a?=?4.745 (1) Å, c?=?4.403 (1) Å, Z?=?2). Nitrogen access to vacant interstitial sites, repulsions between nitrogen ions and metal nitride thermal stability are the factors relating iron nitride-phase relations to those of other early (Hf, Zr, Ti)-N and late (Ni-N) transition metal nitrides subjected to similar pressure and temperature conditions. Here, Fe₃N_1+x can accommodate pressure and x variability by situating nitrogen in a broader range of interstitial crystallographic sites in the intimately related hexagonal and trigonal crystal structures.