Variety Of High-temperature Applications Research Articles

A review on additive manufacturing of refractory tungsten and tungsten alloys

We review the progress of additive manufacturing effort on refractory metal tungsten and tungsten alloys. These materials are excellent candidates for a variety of high temperature applications but extremely challenging to fabricate via additive manufacturing due to a series of existing issues during the manufacturing. We outline these issues and discuss the current understanding and progress to tackle them. Laser powder-bed-fusion, laser directed-energy-deposition, and electron beam powder-bed-fusion are three common techniques that have been applied to additively manufacture pure tungsten. This overview discusses current observations and understanding on the issues associated with each of these techniques. We identify future research opportunities in additive manufacturing of refractory metals.

Effect of powder size and geometry on consolidation of Mo30%W alloy by spark plasma sintering

Molybdenum alloys are important structural components for a variety of high temperature applications in the aerospace and nuclear industries. Spark plasma sintering has been successful in producing highly dense refractory metals with desirable microstructural characteristics, where the final microstructure and sintering mechanisms depend heavily on the starting powder size and morphology. To understand how powder size and morphology affect consolidation of a molybdenum 30 wt% tungsten alloy (Mo30W), three different powder feedstocks were sintered and the solid compacts characterized. These powders were prepared utilizing chemical reduction, ball-milling, and gas atomization processes. Additionally, to determine the effect of particle size on sintering, two different size distributions of the gas atomized material were also tested. In-situ sintering dilatometry data was analyzed to determine the apparent activation energy for the initial stage sintering of each powder. Powder geometry was found to have the greatest impact on the sintering of Mo30W powders, with the rougher, more angular powders densifying at lower temperatures compared with the smoother spherical powders densifying at higher temperatures. Powder size contributed much less to the densification of the powders using SPS. From these experiments, optimum sintering temperatures can be determined, with 1600 °C being sufficiently high to fully densify the chemical reduction and ball-milled materials and a maximum hold temperature of >1800 °C being required for the spherical atomized powders.

Optical properties of high temperature molten salt mixtures for volumetrically absorbing solar thermal receiver applications

Molten salts are promising candidates for liquid volumetric absorbers in concentrated solar power systems. To characterize absorption and heat transfer performance in high temperature applications, their optical properties are required. Thus a method for experimentally determining the absorption coefficient of non-scattering high temperature semi-transparent liquids for large (∼1m-deep) direct absorption solar receiver applications was developed. It was used to measure the absorption coefficient in liquids over a broad spectral range and temperatures up to 800°C in a 40wt.% KNO3:60wt.% NaNO3 binary nitrate molten salt mixture (solar salt) and a 50wt.% KCl:50wt.% NaCl binary chloride molten salt mixture. The binary nitrate and binary chloride both demonstrated well distributed solar absorption (>95% absorption through 1m and 2m, respectively). At 400°C, the binary nitrate is optically thick in its re-emission spectrum and behaves as a blackbody radiator. The effects of thermal decomposition were also shown to have significant consequences on the overall performance of the binary nitrate mixture, transforming it into an opaque surface absorber following thermal degradation (>95% in <0.25m). The implications of these results for solar receiver design are discussed in terms of volumetric absorption, total effective emissivity, and capture efficiency. The measurement technique developed and results are relevant in a variety of high temperature applications including heat transfer systems, materials processing, pharmaceuticals, and food processing facilities.

Dielectric properties, relaxor behavior and temperature stability of (1−x)(K0.4425Na0.52Li0.0375)(Nb0.87Ta0.06Sb0.07)O3-xBa0.4Sr0.6TiO3 ceramics

In this paper, Ba0.4Sr0.6TiO3 was added in the (K0.4425Na0.52Li0.0375)(Nb0.87Ta0.06Sb0.07)O3 ceramics to improve the temperature stability. The phase structure, microstructure, dielectric, diffuse phase transition, relaxor behavior and temperature stability of (1−x)(K0.4425Na0.52Li0.0375) (Nb0.87Ta0.06Sb0.07)O3-xBa0.4Sr0.6TiO3 (abbreviated as (1−x)KNLNTS-xBST)ceramics were investigated. The phase structure of these ceramics changed from tetragonal phase to cubic phase with Ba0.4Sr0.6TiO3 additives. A significant reduction in the temperature dependence of relative permittivity occurred at x = 0.15, with ΔC/C298K was around ±15% over the temperature range 273–528 K, and loss tangent, tanδ ≤ 0.03. The result indicated that this material may have great potential for a variety of high temperature applications. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting toward higher temperature with increasing frequency, which were observed in the (1−x)KNLNTS-xBST solid solution. The dielectric relaxor behavior obeyed a modified Curie-Weiss law. The dielectric relaxor was attributed to the appearance of polar nanoregions owing to the formation of random fields. The ferroelectric characteristic decreased with the addition of BST. These results confirmed that the (1−x)KNLNTS-xBST ceramics can be regarded as an alternative for high temperature lead-free piezoelectrics.

Improved microstructure and fracture properties of short carbon fiber-toughened ZrB2-based UHTC composites via colloidal process

Ultra-high temperature ceramics are potential materials for a variety of high temperature applications because of excellent thermo-mechanical properties and oxidation resistance. To further improve their fracture properties, a novel colloidal process was proposed to fabricate the short carbon fiber-toughened ZrB2–ZrSi2 composites. Microstructure analysis found that the colloidal processing route could avoid the fibers' agglomeration and alleviate the fibers' damage, which minimizes the structural defects and retains the fibers' strength. The relative density of composites achieves 98.35% and the distribution of fibers in matrix is homogeneous. Mechanical tests indicate that the flexural strength is 458MPa and the fracture toughness is 6.9MPa·m1/2. In comparison to the composite obtained by conventional processing route, the fracture toughness increases by 47%. The main mechanisms for improved fracture properties could be attributed to the crack deflection, fiber sliding and fiber bridging.

Femtosecond-laser-inscribed sampled fiber Bragg grating with ultrahigh thermal stability.

We have successfully fabricated a series of sampled fiber Bragg gratings with easily adjustable sampling periods and duty cycles using an 800 nm femtosecond laser point-by-point inscription. The thermal stability of the fabricated fiber gratings was investigated using isochronal annealing tests, which indicated that the fiber gratings are capable of maintaining high reflectivity at temperatures of up to 1000°C for 8 h. This demonstrates the potential of the developed sampled fiber Bragg gratings for use in multi-wavelength fiber lasers and a variety of high temperature applications.

Decomposition Free Al&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;TiO&amp;lt;sub&amp;gt;5-&amp;lt;/sub&amp;gt;MgTi&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; Ceramics with Low-Thermal Expansion Coefficient

Solid solutions of (1 - x)Al2TiO5-xMgTi2O5 (x = 0 - 1) doped with alkaline feldspar were prepared. Thermal decomposition largely depends on the feldspar doping as same as the x value. It was found that decomposition free ceramics over 500 hours of heat treatment at 1100°C in an ambient atmosphere could be obtained for the feldspar-doped ceramics at x > 0.5 with a fracture strength of 33 - 40 MPa and coefficient of thermal expansion of 2.4 - 4.1 ×10-6K-1. A partial decomposition was observed for a compositional range of around x = 0.75. Both composition of the solid solution and an addition of the alkaline feldspar contributed synergistically to improve such thermal and mechanical properties. The decomposition free Al2TiO5-MgTi2O5 ceramics is expected for variety of high temperature applications including diesel particulate filters.

Ferroelectric Relaxor Behavior and Spectroscopic Properties of Ba&lt;SUP&gt;2+&lt;/SUP&gt; and Zr&lt;SUP&gt;4+&lt;/SUP&gt; Modified Sodium Bismuth Titanate

Polycrystalline ceramics (Na<SUB>0.5</SUB> Bi<SUB>0.5</SUB>)<SUB>1-x</SUB>Ba<SUB>x </SUB>Zr<SUB>y</SUB>Ti<SUB>1-y</SUB>O<SUB>3,</SUB> (BNBZT) (for x=0.10, 0.12; y=0.04), has been synthesized by conventional solid-state sintering. X-ray diffraction analysis indicates the formation of a single phase with tetragonal symmetry with pure perovskite structure. Scanning electron micrograph of the studied materials shows a distribution of grains. A broad dielectric peak with maximum permittivity has been observed near 1200 (for x=0.10, y=0.04) and 1600 (for x=0.12, y=0.04) respectively in the temperature range, RT–600℃. This result indicates that these materials may have great potential for a variety of high temperature applications. These ceramics show diffuse phase transition and the transition temperature shifting toward higher temperature with increasing frequency, which represents the relaxor behvaiour. The relaxor materials obey modified Curie–Weiss law and Vogel–Fulcher relationship. The values of the diffuseness parameter γ=2 for x = 0.10 and 1.67 for x = 0.12, obtained from the fit of a modified Curie-Weiss law established the relaxor type nature. For a more detailed interpretation of the ac data, the complex impedance (<i>Z*)</i> and electric modulus (<i>M*)</i> as a<i> </i>function of frequency <i>f</i> (i.e., 45 Hz–5 MHz) has been simultaneously analysed. Impedance study reveals that there exists a temperature dependent electrical relaxation phenomenon in the materials. Modulus represents hopping of ions and localized motion in studied compositions. Conductivity obey’s Jonscher law

Phase structure, dielectric properties, and relaxor behavior of (K0.5Na0.5)NbO3–(Ba0.5Sr0.5)TiO3 lead-free solid solution for high temperature applications

The (1−x)(K0.5Na0.5)NbO3–x(Ba0.5Sr0.5)TiO3 (KNN-BST) solid solution has been synthesized by conventional solid-state sintering in order to search for the new lead-free relaxor ferroelectrics for high temperature applications. The phase structure, dielectric properties, and relaxor behavior of the (1−x)KNN-xBST solid solution are systematically investigated. The phase structure of the (1−x)KNN-xBST solid solution gradually changes from pure perovskite phase with an orthorhombic symmetry to the tetragonal symmetry, then to the pseudocubic phase, and to the cubic phase with increasing addition of BST. The 0.90KNN-0.10BST solid solution shows a broad dielectric peak with permittivity maximum near 2500 and low dielectric loss (&amp;lt;4%) in the temperature range of 100–250 °C. The result indicates that this material may have great potential for a variety of high temperature applications. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting toward higher temperature with increasing frequency, which are two typical characteristics for relaxor ferroelectrics, are observed in the (1−x)KNN-xBST solid solution. The dielectric relaxor behavior obeys a modified Curie–Weiss law and a Vogel–Fulcher relationship. The relaxor nature is attributed to the appearance of polar nanoregions owing to the formation of randon fields including local electric fields and elastic fields. These results confirm that the KNN-based relaxor ferroelectrics can be regarded as an alternative direction for the development of high temperature lead-free relaxor ferroelectrics.

Effect of different thermomechanical processing on structure and mechanical properties of electron beam melted niobium

Among the refractory metals and alloys, niobium and niobium alloys are used in variety of high temperature applications ranging from light bulbs to rocket engines because of its high melting and boiling point, lower density, good ductility at room temperature and high corrosion resistance. In this paper the effects of different thermomechanical processing on structure and mechanical properties of electron beam melted niobium ingot was investigated. The correlation among the different processing conditions and microstructure as well as mechanical properties have been investigated using optical microscope, SEM, UTM and microhardness testing. The results show that the cold forging response of EB melted ingot was very poor, where as oxidation resistant coated ingot and ingot sealed in evacuated stainless steel jacket were successfully forged at 900 °C. The cast and hot forged EBM niobium ingot was cold rolled without any intermediate annealing. The hot forged, cold rolled and annealed niobium sheets exhibit better strength as compared to cold rolled and annealed niobium sheets. The mechanical properties of all the niobium sheets processed by using different processing conditions are superior to the properties specified by ASTM standard.

Self-healing mechanisms of a SiC fiber reinforced multi-layered ceramic matrix composite in high pressure steam environments

Non-oxide ceramic matrix composites are potential candidates to replace the current nickel-based alloys for a variety of high temperature applications in the aerospace field. The durability of a SiC ( f) /PyC ( i) /[Si,C,B] ( m) composite with a sequenced self-sealing matrix and Hi-Nicalon fibers was investigated at 600 °C for exposure durations up to 600 h. The specimens are aged in a variety of slow-flowing air/steam gas mixtures and total pressures, ranging from atmospheric pressure with a 10–50% water vapor content to 1 MPa with 10–20% water vapor content. The degradation of the composite was determined from the measurement of residual strength and strain to failure on post-exposure specimens and correlated with microstructural observations, weight changes and characterizations of the generated oxides. All of the post-exposure characterizations demonstrate the ability of the sequenced [Si,C,B] matrix to protect the PyC interphase from environmental attacks. Two different oxidation modes of the matrix, depending on the total pressure are discussed in terms of the reactivity of the boron-containing layers, and their relative positions in the sequenced matrix. In high pressure environments, a strong localized dissolving of a small amount of SiC fibers in the boria-containing oxide is evidenced at 600 °C.

Degradation mechanisms of a SiC fiber reinforced self-sealing matrix composite in simulated combustor environments

Non-oxide ceramic matrix composites are potential candidates to replace the current nickel-based alloys for a variety of high temperature applications in the aerospace field. The durability of a SiC (f)/PyC (i)/[Si, C, B] (m) composite with a multi-layered self-sealing matrix and Hi-Nicalon fibers was investigated at 1200 °C for exposure durations up to 600 h. The specimens are aged in a variety of slow-flowing air/steam gas mixtures and total pressures, ranging from atmospheric pressure with a 10–50% water content to 1 MPa with 10–20% water content. The degradation of the composite was determined from the measurement of residual strength and strain to failure on post-exposure specimens and correlated with microstructural observation of the damaged tows. The most severe degradation of the composite occurred at 1 MPa in an air/steam (80/20) gas mixture. Correlation between this degradation and the dissolving of the SiC fibers in the generated boria-containing glass, is discussed.

Superalloys and coatings for high-temperature applications

Superalloys remain the material of choice for a wide variety of high-temperature applications, especially for hot section components required for the gas turbines that provide jet propulsion and electricity generation. Indeed, around the world, new breeds of superalloy are being actively sought for use within the latest generations of aircraft engines and industrial gas turbines, whose performance (fuel economy, power output, safety, and emissions) will push once again these remarkable materials close to the limits of their capabilities. Thus, as working temperatures continue to rise and operating conditions become Superalloys and Coatings for High-Temperature Applications

Powder Injection Molding of Niobium

Niobium and niobium-based alloys are used in a variety of high temperature applications ranging from light bulbs to rocket engines. Niobium has excellent formability and the lowest specific weight among refractory metals (Nb, Ta, Mo, W, and Re). Powder injection molding of niobium powder was investigated for efficiency of the process. The sintering of injection molded bars was conducted up to 2000°C in vacuum and low oxygen partial pressure atmosphere. This paper investigates the effect of sintering time, temperature and atmosphere on processing of pure niobium.

Robust Joining and Assembly of Ceramic Matrix Composites for High Temperature Applications

Advanced ceramic matrix composites (CMCs) are under active consideration for use in a wide variety of high temperature applications within the aerospace, energy, and nuclear industries. The engineering designs of CMC components require fabrication and manufacturing of large and complex shaped parts of various thicknesses. In many instances, it is more economical to build up complex shapes by joining simple geometrical shapes. Thus, joining and attachment have been recognized as enabling technologies for successful utilization of ceramic components in various demanding applications. In this presentation, various challenges and opportunities in design, fabrication, and testing of high temperature joints in ceramic matrix composites will be presented. A wide variety of ceramic composites, in different shapes and sizes, have been joined using an affordable, robust ceramic joining technology (ARCJoinT). Microstructure and mechanical properties of joints in melt infiltrated and CVI Sic matrix composites will be reported. Various joint design philosophies and design issues in joining of composites will be discussed.

Metal Free Tungsten Carbide Ceramics

Over the past few decades, cemented carbides have been used in applications requiring high hardness, toughness, and strength. However, the metallic binder phase present in cemented ceramics can result in poor mechanical properties at high temperatures, and has proved susceptible to corrosion and erosion in aggressive environments. Metal free tungsten carbide ceramics developed by VM Advanced Carbides provide improved mechanical properties and chemical stability, and can be used in a variety of high temperature applications.

Morphological characterization of microporous carbon materials

The development of microporous carbon materials has attracted a great deal of attention in recent years. The potential applications of these materials include catalyst support, adsorbents, molecular sieves, porous electrodes and other battery components [1–4]. In addition, these microporous carbon materials have been used as preforms for infiltration with molten silicon or silicon alloys to produce silicon carbide based advanced ceramics for a variety of high temperature applications [5–10]. The melt infiltration kinetics and mechanism of reactions between liquid infiltrants and the porous carbon depend on a number of factors including the preform permeability, which is quite sensitive to small changes in pore structure. The permeability of the porous preform and fluid flow of the infiltrant changes due to volume change accompanied by the silicon-carbon reaction during the infiltration process, with the potential for choking-off and other undesirable results. In order to produce silicon carbide based ceramics with homogeneous second phase distribution and to maintain good control of the melt infiltration process, it is necessary to fabricate microporous carbon preforms with controlled morphology and pore size distributions. The objective of this paper is to study the pore size distribution, pore diameter and other microstructural parameters of three types of microporous carbon materials. In addition, evaluation of the phase separation and the effect of molybdenum on the local graphitization in the microporous carbon materials are presented. A mixture of furfuryl alcohol resin, diethylene and triethylene glycol liquid pore-forming agents, and ptoluene sulfonic acid catalyst were polymerized to form a soild polymer. The solid polymer was heated slowly to 1000 8C to pyrolyse the polymer forming a microporous carbon preform. By varying the ratios of the constituents in the polymer system, a wide range of pore volumes, pore sizes, and morphologies of the microporous carbon materials were obtained [7–10]. Microstructural characterization of the preforms was carried out on a Jeol-JSM 35 scanning electron microscope. The physical density measurements were carried out by measuring the dimensions and weight of the sample. Samples were heated in an oven for 6 –8 h at 160 8C before each measurement. This treatment removes any moisture from the material. The pore size distribution in the preforms was determined by mercury porosimetry using a Micromeritics Auto-Pore 9200 porosimeter. Two porous carbon preforms were selected for study using transmission electron microscopy. Carbon pieces ,3 mm maximum dimension were ground using 600 grit SiC paper and polished with 3 im diamond mylar to produce a flat disc. The carbon was mounted on a copper grid and then ion milled at 5 kV to perforation. Samples were examined at 120 kV using a Philips 400T microscope equipped with a Kevex UTW X-ray spectrometer. Three types of microporous carbon materials were used in this study. For simplicity, they are denoted as A, B, and C in the text and have density of r ˆ 0.71, 0.86, and 0.91 g=cm3, respectively. A scanning electron micrograph of fracture surface of specimen A (Fig. 1a) shows a homogeneous distribution of pores and carbon struts. The average diameter of pores in this material is about ,0.13 im. The microstructure of specimen B (Fig. 1b) has an average pore diameter of ,1.3 im. The fracture surface of specimen C (Fig. 1c) shows an average pore diameter of ,2 im and the carbon struts themselves contain submicron pores. Mercury porosimetry results for specimens A, B, and C are shown in Fig. 2. Specimen A shows no measurable intrusion volume at sizes above 0.19 im. All the pore volume is available between 0.003 and 0.19 im. The physical density of this material is 0.71 g=cm3, while the density obtained by mercury porosimetry is 0.72 g=cm3. Its pore surface area is 40.1 m2=g. In specimen B, the amount of pores with pore diameters higher than 1.75 im is about 3%. The steep rise in the curve indicates a very narrow pore size distribution for these materials. The measured density of specimen B is 0.86 g=cm3 while the density obtained by mercury intrusion is 0.87 g=cm3. The pore surface area of this material is 39.95 m2=g. It is interesting to note that there is no measurable micropore volume in samples A and B. The pore volume deduced from the mercury porosimetry data is almost equal to that obtained by physical density measurements. The density data obtained by mercury porosimetry and physical measurements, respectively are 0.72 g=cm3 and 0.71 g=cm3 for sample A and 0.87 g=cm3 and 0.86 g=cm3 for sample B. Their skeletal densities are also about 1.5 g=cm3. On the other hand, in sample C there is a slight difference in

Melt Rheology of Some Specialty Polymers

Speciality engineering thermoplastics are steadily gaining importance in a wide variety of high temperature applications due to their excellent performance char acteristics at elevated temperatures. A knowledge of the rheological behaviour of these speciality polymers over an industrially relevant range of shear rate and temperature is essential m the design of polymer processing equipment, process optimization and trouble-shooting. A method proposed earlier has been extended to estimate the complete flow curves for some of the important speciality poly mers such as polyphenylene oxide, polyphenylene sulfide, polyether sulfone, polyaryl sulfone, polyether ether ketone, polyetherimide and polyarylates, through the use of the developed grade- and temperature-independent unified curves along with the appropriate melt flow index value. The master curves have been fitted by appropriate rheological models and then using a single integral constitutive equation of the BKZ type the unified normal stress difference curves have been predicted. These unified normal stress difference curves would be useful for determining the elastic flow parameters for the speciality polymer melts merely from the knowledge of the melt flow index.