سنتز پودر ZrB2بوسیله احیا حرارتی بور

ZrB2 Powders Synthesis by Borothermal Reduction

سنتز پودر ZrB₂بوسیله احیا حرارتی بور

ABSTRACT

IHigh-purity zirconium diboride (ZrB2) powders with submicrometer particle size were synthesized by borothermal reduction of nanometric ZrO2 powders in vacuum. The reaction process was experimentally and thermodynamically assessed. B2O3 was identified as a possible intermediate reaction product. ZrO2 completely converted to ZrB2 when thermally treated at 10001C for 2 h in a vacuum, but the removal of residual boron-related species required a temperature above ۱۵۰۰۱C. ZrB2 powders obtained at 10001–۱۲۰۰۱C showed a faceted morphology, whereas those prepared above 15001C had a nearly spherical morphology. The particle size that was calculated from the measured surface area increased with the increasing synthesis temperature from 0.15 lm at 10001C to ۰٫۶۶ lm at 16501C. The oxygen content of the ZrB2 powders synthesized at 16501C was as low as 0.43 wt%.

 

سرامیک­ های فوق ­دمابالا جهت کاربرد در محیط­ های سرسخت

Ultra-High Temperature Ceramic Materials for
Extreme Environment Applications

سرامیک­ های فوق ­دمابالا جهت کاربرد در محیط­ های سرسخت

ABSTRACT

For the purposes of this paper, we will simply define UHTC materials by their usefulness in a real structural (load-bearing) application where the very high temperatures are generated rapidly by burning fuels or friction with the atmosphere (not steady state). This will quickly eliminate most of the materials mentioned above. While oxides are reasonable to consider for use in oxidizing environments, poor thermal shock resistance due to high thermal expansion and low thermal conductivity eliminates them from further discussion. The silicon based refractory compounds (SiC, Si3N4, MoSi2, etc.) possess excellent oxidation resistance up to 1700°C due to the formation of a layer of SiO2 glass that inhibits oxygen diffusion to the parent material.4 This is the primary reason for the popularity of these materials for a wide variety of applications. However, active oxidation (the direct formation ofSiO(g) instead of a protective SiO2 layer) can occur at very high temperatures (> 1350°C, depending on PO2) and reduced system pressures. In addition, decomposition of already-formed SiO2, or the interface reaction between SiC and SiO2 results in SiO(g) formation at high temperatures and reduced pressure environments. Other materials, such as TiB2, TiC, NbB2, NbC, while having high melting temperatures, form oxides with low melting points (TiO2 – Tm = 1840°C and Nb2O5 – Tm = 1485°C). Graphite has the highest melting temperature of any material known, but starts to burn thet 800°C. While it is a most widely used material in high-temperature applications, it must be protected by coatings for long-term use.

مینرال­ های گروه سیلیمانیت؛ سیلیمانیت، کیانیت و آندالوزیت

The Sillimanite Minerals: Andalusite, Kyanite, and Sillimanite

 مینرال­ های گروه سیلیمانیت؛

سیلیمانیت، کیانیت و آندالوزیت

ABSTRACT

The chemistry and the mineralogy of the three Al2O3.SiO2 sillimanite minerals (anadlusite, kyanite, and sillimanite) are described. Their P–T diagram is discussed. The structural differences among the three are reviewed, emphasizing the coordination of the Al3+ cations that link the double octahedral chains within the structures. Their decompositions to produce mullite and silica are described and contrasted. The effect of nanomilling on those decompositions is discussed. Finally, the locations of commercial deposits and the industrial applications are addressed.The sillimanite minerals are the three anhydrous aluminosilicates: andalusite, kyanite, and sillimanite [1,2]. Kyanite is also referred to as cyanita, cyanite, and disthene. Because all three have the same 1:1 molar ratio of alumina (Al2O3) to silica (SiO2), they are often written simply as Al2O3·SiO2 or Al2SiO5. Their ideal composition is 62.92 wt% alumina and 37.08 wt% silica. However, in natural states involving significant impurities, the alumina content is usually less than 60 wt%. There are reports of higher alumina content deposits associated with the presence of higher alumina content minerals. That such a mineral group exists is not surprising, for the three most common elements in the Earth’s crust are O, Si, and Al.

 

زیرکنیا

Zirconia

زیرکنیا

ABSTRACT

Zirconia is a very important industrial ceramic for structural applications because of its high toughness, which has proven to be superior to other ceramics. In addition, it has applications making use of its high ionic conductivity. The thermodynamically stable, room temperature form of zirconia is baddeleyite. However, this mineral is not used for the great majority of industrial applications of zirconia. The intermediate-temperature phase of zirconia, which has a tetragonal structure, can be stabilized at room temperature by the addition of modest amounts (below ∼۸ mol%) of dopants such as Y3+ and Ca2+. This doped zirconia has mechanical toughness values as high as 17 MPa • m1/2. On the other hand, the high-temperature phase of zirconia, which has a cubic structure, can be stabilized at room temperature by the addition of significant amounts (above ∼۸ mol%) of dopants. This form of zirconia has one of the highest ionic conductivity values associated with ceramics, allowing the use of the material in oxygen sensors and solid-oxide fuel cells. Research on this material actively continues and many improvements can be expected in the years to come.

سیلیس و کوارتز

Quartz and Silicas

سیلیس و کوارتز

ABSTRACT

Silica is the most ubiquitous mineral in the earth’s crust, existing in a wide variety of crystalline and noncrystalline forms due to the flexibility of the linkage among SiO4 tetrahedra. The thermodynamically stable, room temperature form of silica is quartz, which is itself a widely available mineral and ingredient in many commercial ceramics and glasses. In addition to historically abundant raw material sources, crystalline and noncrystalline silicas can be produced by a wide range of synthetic routes. For example, synthetic quartz can be produced by hydrothermal growth in an autoclave, and synthetic vitreous silica can be produced from silicon tetrachloride by oxidation or hydrolysis in a methane–oxygen flame. Pure silicas serve as model systems in the study of ceramics and glasses, but at the same time, are used in a wide and steadily increasing variety of sophisticated technological applications, from piezoelectric crystals to optical fibers to waveguides in femtosecond lasers. Increased understanding of these ubiquitous materials is aided by improved experimental tools such as new neutron scattering facilities and increasingly sophisticated computer simulation methods.

اکسیدهای دیرگداز

Refractory Oxides

اکسیدهای دیرگداز

ABSTRACT

Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure–property relations, describes typical processing routes, andsummarizes the properties of these materials.The term refractory refers to materials that are resistant to the effects of heat. Refractory oxides, therefore, are ceramic materials that can be used at elevated temperatures. These nondescript restrictions allow nearly any oxide to be classified as refractory. For this article, refractory oxides will refer, somewhat arbitrarily, to common crystalline compounds with melting temperatures of at least 1,800°C. These compounds can contain one or more metal or metalloid cations bonded to oxygen. As an introduction to the topic, this section provides a brief historic overview of materials commonly used in the refractories industry, including some lower melting temperature materials. The section also reviews some current trends in the industries that produce and use refractory oxides. The other sections of this chapter focus on phase-pure oxide ceramics that can be used at elevated temperatures.

مولایت

Mullite

مولایت

ABSTRACT

Mullite is the only stable intermediate phase in the alumina–silica system at atmospheric pressure. Although this solid solution phase is commonly found in human-made ceramics, only rarely does it occur as a natural mineral. Yet mullite is a major component of aluminosilicate ceramics and has been found in refractories and pottery dating back millennia. As the understanding of mullite matures, new uses are being found for this ancient material in the areas of electronics and optics, as well as in high temperature structural products. Many of its high temperature properties are superior to those of most other metal oxide compounds, including alumina. The chemical formula for mullite is deceptively simple: 3Al2O3 .۲SiO2. However, the phase stability, crystallography, and stoichiometry of this material remain controversial. For this reason, research and development of mullite is presented in an historical perspective that may prove useful to engineers and scientists who encounter this material under nonequilibrium conditions in their work. Emphasis is placed on reviewing studies where the primary goal was to create single-phase mullite monoliths with near theoretical density.

ترکیبات حاوی سرب Lead Compounds

Lead Compounds

ترکیبات حاوی سرب

ABSTRACT

Lead compounds include over forty naturally occurring minerals from which five lead oxides can be derived. The lead oxides, as well as some lead silicates, are used as raw materials in lead-containing glasses and crystalline electronic ceramics. The presence of lead in glass increases the refractive index, decreases the viscosity, increases the electrical resistivity, and increases the X-ray absorption capability of the glass. The lead in electronic ceramics increases the Curie temperature and modifies various electrical and optical properties. The refinement of metallic lead from minerals and recycled goods such as lead acid batteries and cathode ray tubes is a multistep process, supplemented by oxidation steps to produce lead oxides. Lead compounds are known to be toxic and are therefore highly regulated.

 

تهیه پودر ZrB2با اندازه نانو بوسیله سنتز خود انتشار دما بالا

Preparation of nano-size ZrB2 powder by self-propagating

high-temperature synthesis

تهیه پودر ZrB₂با اندازه نانو بوسیله سنتز خود انتشار دما بالا

ABSTRACT

Preparation of nano-size ZrB2 powder via SHS was demonstrated by adding 10–۵۰ wt.% NaCl into Zr–B elemental starting powder. Reactions took place completely even with high NaCl content. Adiabatic temperature of reactions, reaction wave velocity, average crystallite size and particle size of the formed ZrB2 decreased significantly with increasing NaCl content. ۳۰ wt.% NaCl addition was found to be the optimum and obtained ZrB2 particles were mostly finer than 200 nm. Hindrance of mass transport among ZrB2 crystals is believed to be  the basis of grain refinement effect of NaCl. Obtaining nano-sized powder was considered very difficult in SHS due to inevitable high temperatures. Owing to the introduction of NaCl into SHS, process control and preparation of ceramic powder having nano-sized particles were rendered possible. The development has the potential to enrich the spectrum and the properties of materials that could be produced via SHS.

ابزار جوشکاری اصطکاکی اغتشاشی friction stir welding tools

friction stir welding tools

ابزار جوشکاری اصطکاکی اغتشاشی

ABSTRACT

Friction stir welding (FSW) is a widely used solid state joining process for soft materials such as aluminium alloys because it avoids many of the common problems of fusion welding. Commercial feasibility of the FSW process for harder alloys such as steels and titanium alloys awaits the development of cost effective and durable tools which lead to structurally sound welds consistently. Material selection and design profoundly affect the performance of tools, weld quality and cost. Here we review and critically examine several important aspects of FSW tools such as tool material selection, geometry and load bearing ability, mechanisms of tool degradation and process economics.