تفجوشی، شکل دهی، واکنش و رشد بلور توسط سیستم پلاسمای جرقه ای (SPS)

Sintering, consolidation, reaction and crystal growth by

(the spark plasma system (SPS

تفجوشی، شکل دهی، واکنش و رشد بلور توسط سیستم پلاسمای جرقه ای (SPS)

ABSTRACT

The graphite die set in spark plasma system (SPS) is heated by a pulse direct current. Weak plasma, discharge impact, electric field and electric current, which are based on this current, induce good effects on materials in the die. The surface films of aluminum and pure WC powders are ruptured by the spark plasma. Pure AlN powder is sintered without sintering additives in the electric field. The spark plasma leaves discharge patterns on insulators. Organic fibers are etched by the spark plasma. Thermosetting polyimide is consolidated by the spark plasma. Insoluble polymonomethylsilane is rearranged into the soluble one by the spark plasma. A single crystal of CoSb3 is grown from the compound powders in the electric field by slow heating. Coupled crystals of eutectic powder are connected with each other in the electric field.

 

اثر زیرلایه روی جوانه زنی و رشد نانو ورق های عمودی گرافن

Influence of substrate on nucleation and growth of

verticalgraphene nanosheets

اثر زیرلایه روی جوانه زنی و رشد نانو ورق های عمودی گرافن

ABSTRACT

The present study reports the role of substrate on nucleation and growth of vertical graphene nanosheets (VGNs) under electron cyclotron resonance chemical vapor deposition (ECR-CVD). The VGNs are grown on Pt, Ni, Au, Cu, Si(100), Si(111), SiO2 and quartz substrates simultaneously. The morphology of VGNs is found to vary significantly with substrate. VGNs on Pt have the highest aerial density of vertical sheets while quartz have the lowest. The structural defects in VGNs vary with substrate as evidenced from Raman spectroscopy. The observation of defect related Raman bands such as D'' and D* at 1150 and 1500 cm-1, respectively revealed the existence of pentagon-heptagon rings or carbon onions in VGNs. Formation of such defects at early stage of nucleation dictates the growth mechanism and hence the morphology. A phenomenological four stage model is discussed, to substantiate the nucleation and growth mechanism of VGNs on different substrates, by evoking substrate - plasma interaction during growth.

 

تحولات مارتنزیتی در آلیاژهای غیرآهنی آلیاژهای حافظه‎دار

Martensitic transformations in nonferrous shape memory alloys

تحولات مارتنزیتی در آلیاژهای غیرآهنی آلیاژهای حافظه‎دار

ABSTRACT

In the present paper, the important developments on martensitic transformations in non-ferrous shape memory alloys within nearly 10 years are critically reviewed. Since the alloys include not only noble-metal alloys but also Ti–Ni based alloys, the field is very wide both in contents and in the kind of alloys. We tried to describe items with uniformity, which are common to all alloys, but specific items were also discussed when they are important. Special attention was paid to similarity and dissimilarity among alloys to highlight key points on the issues concerned.

 

رشد نانولوله‌های کربنی روی فیبرهای کربنی توسط CVD حرارتی

Growth of Carbon Nanotubes on Carbon Fiber by

Thermal CVD Using Ni Nanoparticles as Catalysts

رشد نانولوله‌های کربنی روی فیبرهای کربنی توسط CVD حرارتی

با استفاده از نانوذرات نیکل به عنوان کاتالیزور

ABSTRACT

Nickel nanoparticles and thin film on carbon fiber have been prepared through electroless deposition. Moreover, carbon nanotubes were grown on carbon fiber covered by nickel nanoparticles using thermal chemical vapor deposition. The effects of changes in the thickness of the nickel catalyst layer and the growth temperature of carbon nanotubes were studied systemically, and the results are discussed in the present work.

 

اثر نرخ بارگذاری روی چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی

Fracture Toughness of a Hybrid Rubber Modified

Epoxy. II. Effect of Loading Rate

چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی.

II. اثر نرخ بارگذاری

ABSTRACT

Effect of loading rate on toughness characteristics of hybrid rubber-modified epoxy was investigated. Epoxy was modified by amine-terminated butadiene acrylonitrile (ATBN) and recycled tire. Samples were tested at various loading rates of 1–1000 mm/min. Fracture toughness measurements revealed synergistic toughening in hybrid system at low loading rates (1–10 mm/min); hybrid system exhibited higher fracture toughness value in comparison with the ATBN-modified resin with same modifier content. However, synergistic toughening was eliminated by increasing the loading rate. At higher loading rates (10–1000), the fracture toughness of hybrid system decreased gradually to the level lower than that of ATBN-modified epoxy. Fractography of the damage zones showed the toughening mechanisms of ATBN-modified system was less affected by increasing the loading rate compared to that of hybrid system.

 

چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی

Fracture Toughness of a Hybrid-Rubber-Modified

Epoxy. I. Synergistic Toughening

چقرمگی شکست اپوکسی اصلاح شده با لاستیک هیبریدی

 I . اثر تشدید کننده چقرمه شدن

ABSTRACT

The fracture behavior of a hybrid-rubber-modified epoxy system was investigated. The modified epoxy included amine-terminated butadiene acrylonitrile (ATBN) rubber and recycled tire particles as fine and coarse modifiers, respectively. The results of the fracture toughness (KIC) measurement of the blends revealed synergistic toughening in the hybrid system when 7.5-phr small particles (ATBN) and 2.5-phr large particles (recycled tire) were incorporated. Transmission optical micrographs showed different toughening mechanisms for the blends; fine ATBN particles increased the toughness by increasing the size of the damage zone and respective plastic deformation in the vicinity of the crack tip. However, in the case of hybrid resin, coarse recycled rubber particles acted as large stress concentrators and resulted in the branching of the original crack tip. Mode mixity at the branch tips led to synergistic KIC in the hybrid system. It seemed that the ductility of the matrix played an effective role in the nature of the crack-tip damage zone in the hybrid epoxies

 

خصوصیات ترموالکتریکی بهبود یافته در ساماریم بورید

Enhanced thermoelectric properties of samarium boride

خصوصیات ترموالکتریکی بهبود یافته در ساماریم بورید

ABSTRACT

SmB62 single crystals were successfully grown by the floating zone (FZ) method. The high-temperature thermoelectric properties were investigated, together with magnetic properties and specific heat at low-temperature. The electrical resistivity, ρ, shows variable-range-hopping (VRH) behavior with significantly lower values than other rare-earth RB62 (RB66) compounds. An effective magnetic moment, μeff, of 0.42 μB/Sm was estimated, which if straightforwardly taken indicates a mixed valency for SmB62 with Sm2+:Sm3+ = 1:1, which is the first ever indicated for RB66-type compounds. Localization length of the VRH at the Fermi level, ξ, was estimated to be 3.33 Å indicating that carriers in SmB62 are much less localized than in YB66 which has 0.56 Å. The thermoelectric behavior of SmB62 is striking, with ρ reduced by two orders of magnitude while maintaining large Seebeck coefficients, and as a result the power factor is ∼30 times higher than other rare-earth phases. Overall the figure of merit ZT amounts to ∼0.13 at 1050 K, with an extrapolated value of ∼0.4 at 1500 K, an expected working temperature for topping cycles in thermal power plants; that gives a ∼40 times enhancement for Sm. Since there are few thermoelectric materials applicable for very-high temperature applications, this discovery gives new interest in the samarium higher borides.

 

اثر افزودن کاربید تیتانیوم روی خواص ترموالکتریک سرامیک ها

Effect of titanium carbide addition on the thermoelectric properties  of B4C ceramics

اثر افزودن کاربید تیتانیوم روی خواص ترموالکتریک سرامیک های B4C

ABSTRACT

TiB2/B4C composite ceramics are prepared via hot pressing, in which the TiB2 particles are formed by introducing TiC0.78 and through the reaction between B4C and TiC0.78. The electrical and thermal conductivities and Seebeck coefficient of samples containing 0, 12.5 and 25.4 vol% TiB2 are measured from room temperature up to 1200 K. The results show that the transport properties of the samples vary with the TiB2 content. The transport properties of a 12.5 vol% TiB2/B4C sample are dominated by the B4C matrix as in undoped B4C ceramics, whilst the transport properties of a 25.4 vol% TiB2/B4C sample are dominated by TiB2 particles. The figure of merit of the 25.4 vol% TiB2/B4C sample is higher than that of the undoped B4C ceramic sample between room temperature and about 700 K, which indicates that it is possible to improve the thermoelectric properties by selecting an optimum combination of different materials.

 

رشد سلولی و دندریتی Cellular and dendritic growth

Cellular and dendritic growth

رشد سلولی و دندریتی 

ABSTRACT

Many of the alloys used in practice, such as steel, aluminum-copper alloys, nickel-base and copper-base alloys, are single phase alloys, which means that the final product of solidification is a solid solution. Depending on the thermal and compositional field, cellular or, in most practical cases, dendritic morphology will occur. In other cases, even when the room temperature microstructure is mostly eutectic some primary phases solidify before the eutectic. They can be solid solutions, carbides, intermetallic phases, inclusions, etc. Their morphology affects mechanical properties, and thus, understanding how this morphology can be controlled is a mater of significant practical importance. A detailed discussion of primary phase growth, and in particular of dendrite growth, will be provided in the following sections.

 

خوردگی حفره‌ای در جوش فلزات فولاد ضد زنگ سوپر دوبلکس

Effects of Heat Input on Pitting Corrosion in

Super Duplex Stainless Steel Weld Metals

اثر حرارت ورودی بر خوردگی حفره‌ای در جوش فلزات فولاد ضد زنگ سوپر دوبلکس

ABSTRACT

Due to the difference in reheating effects depending on the heat input of subsequent weld passes, the microstructure of the weld metal varies between acicular type austenite and a mixture of polygonal type and grain boundary mixed austenite. These microstructural changes may affect the corrosion properties of duplex stainless steel welds. This result indicates that the pitting resistance of the weld can be strongly influenced by the morphology of the secondary austenite phase. In particular, the ferrite phase adjacent to the acicular type austenite phase shows a lower Pitting Resistance Equivalent (PRE) value of 25.3, due to its lower chromium and molybdenum contents, whereas the secondary austenite phase maintains a higher PRE value of more than 38. Therefore, it can be inferred that the pitting corrosion is mainly due to the formation of ferrite phase with a much lower PRE value.