Materials For High-temperature Applications Research Articles

Molten salt synthesis of nano structured pyrochlore lanthanum zirconate: a potential material for high temperature applications

Molten salt synthesis of nanocrystalline pyrochlore lanthanum zirconate (La2Zr2O7) powders with NaCl–KCl alone as reaction aid was prepared in the present investigation with low synthesis temperature for the first time. The structural and morphological data of the synthesized lanthanum zirconate powder was characterized using XRD, FESEM—EDS and TEM. Functional group identification was carried out using FT-Ir and simultaneous themal analysis on the starting material was also carried out characterized by thermogravimetric analysis and differential thermal analysis. Simultaneous thermal analysis of the starting materials was carried out and the results show the suitability of the molten salt synthesis below 900 °C. The XRD pattern reveal the existence of pyrochlore structure La2Zr2O7 matches with the standard ICDD pattern. The average crystallite size was measured to be 34.66 nm by Debye Scherer formula and 35.53 nm using WH plot. The particle size measured using the particle size analyser, scanning electron microscope and transmission electron microscope shows in the order of 40–50 nm and the powders are seen clearly without agglomerations. Energy dispersive spectrum results show the clear presence of La–Zr–O without any other inclusions and confirms the results obtained through x-ray photo emission spectroscopy. SAED pattern obtained through TEM shows the polycrystalline nature of the synthesised nano La2Zr2O7. The structural functional group evolution was examined using FT-Ir confirms the formation of La–Zr–O chemical bonds. The results confirms that nano crystalline pyrocholore La2Zr2O7 can be successfully synthesised using relatively low temperature molten salt route at 850 °C with the aid of anhydrous NaCl–KCl eutectic salt mixture alone as reaction aid and anhydrous La2O3 and ZrO2 as precursors.

Effect of Si4+ substitution on mechanical and thermal expansion properties of Ca0.5Sr0.5Zr4P6O24 ceramics

AbstractThe Ca(1+x)/2Sr(1+x)/2Zr4P6‐2xSi2xO24 (CSZP6‐2xS2x, 0 ≤ x ≤ 0.3) ceramics were prepared using powders synthesized by co‐precipitation method, and the effects of Si4+ substitution on the structure, mechanical, and thermal expansion properties were investigated. The optimum sintering time was 4 hours, which greatly shortened the sintering process. By controlling the appropriate composition and sintering temperature, a single phase with the typical NaZr2P3O12 structure was obtained. As a function of x increasing, the parameter a increased and then decreased, while the parameter c and unit cell volume both increased. The mechanical property of CSZP6‐2xS2x ceramics was affected with increasing amount of Si4+ content. The Si4+ substitution could increase the thermal expansion anisotropy and lead to the generation of microcracks, which are responsible for the increase in the amount of hysteresis between heating and cooling curves. The thermal expansion decreased progressively from 1.8 to 0.8 × 10−6/°C. The investigations indicate that the CSZP6‐2xS2x ceramics are potential materials in high temperature applications

Development and characterization of nanoparticle metal matrix composites: (Al 2024-SiC np)

Composite materials get an importance in value as an advance materials such as automotive, aerospace, aircraft, defense, medical, marine, sports, recreation and various engineering applications due to their high strength-to- weight ratio, good cast ability and better tribological properties. Al-base 2024 alloy reinforced with SiC micron and nano particles were developed using stir casting technique due to low cast, ease in fabrication, recyclability and isotropic characteristics. The development of Aluminum metal matrix nano composites (AMMNCs) is most considered material for high temperature applications because of their excellent mechanical properties, increase performance and weight saving for more reduction of fuel consumption. The results indicate that Al–SiC composite has improved physical and mechanical properties such as tensile strength, charpy impact strength, brinell hardness, density and porosity percentage. Samples with micron particles and nano particles were developed to compare the properties of both. Microstructures of the samples were studied by means of x-ray Diffraction and Scanning Electron Microscopy. The elemental composition was also analyzed through Energy Dispersive Spectroscopy. It was found that composites of Al–SiC have better and improved physical properties in nano dimensions.

Structural, electronic, elastic and thermodynamic properties of Al1-xZxNi (Z=Cr, V and x= 0, 0.125, 0.25) alloys: First-principle calculations

The AlNi intermetallic compound is one of the most promising engineering materials for high temperature applications. Unfortunately, this compound has low tensile ductility at high temperature which limit its functional applications. We show that alloying the AlNi compound with the Cr or V will improve its tensile ductility and therefore its industrial applications. In this paper, the structural, electronic, elastic and thermodynamic properties of Al1-xZxNi (Z = Cr, V and x = 0, 0.125, 0.25) alloys have been studied by the first principles calculations in the framework of density functional theory. The calculated formation energy and cohesive energy of the alloys show that, all alloys have stable structures. The valence charge density distribution study reveals that, chemical bonds between the Z and Ni atoms in the alloys have the nature of covalent, which improve the brittleness of the AlNi alloys. The calculated Pugh’s ratio (or the Poisson’s ratio), shear and bulk modules show that, the increasing of hardness by increasing the x. Moreover, the ductility of the alloys decreased when the value of x is nonzero. The calculation results show that the Pugh’s (Poisson’s) ratio is higher than 1.75 (0.26), indicating a ductile nature of these alloys. Furthermore, thermodynamic properties of the alloys including Grüneisen parameter, thermal expansion coefficient, bulk modulus, Debye temperature and constant volume heat capacity have been investigated and discussed under different pressures and temperatures.

Chromium-nickel-alloys for wear application at elevated temperature

Chromium alloys are a promising class of materials for high temperature application. In this work we investigated the microstructure, hot hardness and sliding wear resistance against an Al2O3 counter-body of three Chromium-Nickel (CrNi) alloys. The microstructure of the investigated alloys consists of a Ni-rich and a Cr-rich phase and can be manipulated by heat treatments. In the solid solution annealed state the room temperature (RT) hardness of CrNi alloys exceeds 800 HV10. The most potent strengthening mechanism in this system is the super saturation of the Cr-rich phase with Ni. Consequently, solid solution annealed specimen experience a strong decline in hardness with the onset of diffusion and precipitation of Ni-rich precipitates within the Cr-rich phase. Aged specimens display a lower hardness at RT but show a lower relative decline in hardness at temperatures above 600 °C. Alloys with higher contents of Ni show a better performance at high temperature sliding wear due to the formation of stable mechanically mixed layers (MML).

Study and research on obtaining porous materials type Al3Ti by sintering reactive

" Aluminum-titanium compounds (Ti3Al, TiAl and Al3Ti) were identified as materials for high temperature applications in areas like automotive industry, materials for aircrafts or filter materials used at high temperatures and in corrosive environments. From these compounds, Al3Ti has a low density and good resistance to the oxidation especially in air. This compound was obtained in the form of porous material by reactive sintering starting from elementar powders. The powder mixture was pressed at 500 MPa, and, the sintering took place at a temperature of 700 °C, for 30 minutes under vacuum. Following the reaction only the Al3Ti compound was formed. The obtained samples had a pore size distribution in the 3-100 μm range."

Waste Stream Porous Alkali Activated Materials for High Temperature Application

The growing interest of using industrial waste as recycled raw materials for the production of new, innovative materials is associated with effective use of natural resources and circular (zero-waste) economy. The research object is waste stream materials coming from chemical and processing industries, such as aluminium scrap recycling waste, chamotte-like precursor, firebricks sawing residues, and their use in production of high-temperature resistant, porous insulation materials by using alkali activation technique with 6M NaOH solution. Adding aluminium scrap recycling waste to the composition of the tested alkali activated materials (AAM) contributed to the porous structure of the material with the pore size ranging from 1000 – 5000 µm (detected by Micro-XCT, SEM)). Lightweight (350-850 kg/m3) and heat-resistant (up to 1000°C) AAM with compressive strength from 1.0 to 3.0 MPa was obtained. The mineralogical composition of the obtained AAM was detected (XRD) and the heat resistant minerals in the structure of AAM were identified. It was concluded that the increased amount of Al2O3 in the raw material composition resulted in improved thermal stability of the AAM. In case where SiO2/Al2O3 ratio is <2, the formation of high-temperature resistant minerals, such as carnegeite and nepheline, was observed. The obtained AAM could resist up to 8 thermal shock cycles and it could be easily adapted to the industrial production and application such as thermal insulation layer in laboratory furnaces.

Influence of the W Content on the Characteristics of MoxW1−xSi2 Heating Elements Fabricated by Self-Propagating High-Temperature Synthesis

MoSi2-based composites are promising structural materials for high-temperature applications. MoxW1−xSi2 (x = 1, 0.9, 0.8, and 0.7) powders were successfully synthesized by self-propagating high-temperature synthesis. These powders were used to fabricate heating elements, which were then subjected to a post-annealing process to improve the structural quality. To determine the influence of the W content (0–30 mol%) on the characteristics of MoxW1−xSi2 composites, the surface morphologies, densities, and bending strengths of the MoxW1−xSi2 heating elements were investigated. Moreover, to validate the durability of the MoxW1−xSi2 heating elements, an accelerated degradation test (ADT) was conducted at temperatures up to 1700°C (heating rate = 5°C/min). The MoxW1−xSi2 heating element with 30 mol% of W exhibited a high density, a high bending strength, and superior durability after the ADT test. In contrast, the MoxW1−xSi2 heating element with 0 mol% of W showed inferior durability owing to the formation of bubbles at the MoxW1−xSi2 /SiO2 interface. Thus, the addition of W enhances the reliability and durability of MoxW1−xSi2 composites as heating elements.

Thermal and mechanical properties of Ta2O5 doped La2Ce2O7 thermal barrier coatings prepared by atmospheric plasma spraying

La2Ce2O7 (LC) is a new promising thermal barrier coating (TBC) material for high-temperature applications. However, the sudden decrease of thermal expansion coefficient (TEC) at ∼623 K limits its application. In this study, the plasma-sprayed La2Ce1.7Ta0.3O7.15 (LCT) coating was developed by partial substitution of Ce4+ in LC with Ta5+. LCT coating shows lower thermal conductivity between 298 K and 1273 K (0.54–0.71 W/(m·K)) than LC coating (0.65–0.85 W/(m·K)) and the traditional yttria partially stabilized zirconia (YSZ) coating (1.53–1.72 W/(m·K)). It also exhibits excellent thermal stability at least up to 1573 K for 1000 h. What is more, the sudden TEC drop is suppressed owing to the reduced oxygen vacancy concentration governed by Ta5+-substitution content. As a result, LCT TBC shows an improved thermal cycling lifetime in an air furnace as compared to LC TBC.

Microstructure and Room Temperature Mechanical Properties of Different 3 and 4 Element Medium Entropy Alloys from HfNbTaTiZr System.

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.

Crystal structure, mechanical and thermophysical properties of Ca0.5Sr0.5Zr4-xSnxP6O24 ceramics

Ca0.5Sr0.5Zr4-xSnxP6O24 (0 ≤ x ≤ 0.2) ceramics belonging to a new NZP family were prepared by coprecipitation, to determine the effect of Sn4+-substitution for Zr4+ on the crystal structure, mechanical and thermophysical properties of the ceramics. XRD patterns revealed that a single phase was formed for all compositions. Rietveld refinement was used to analyze the crystal structure and the lattice parameters exhibited a satisfactory structural convergence of R-factors. The substitution of Sn4+ for Zr4+ could effectively improve the sinterability, meanwhile, the flexural strength and Vickers hardness could reach to 60.2 MPa and 3.05 GPa, respectively. The thermal expansion varied from 1.72 to 2.78 × 10−6/oC with Sn4+-substitution increasing. Besides, the thermal conductivity of Ca0.5Sr0.5Zr3.8Sn0.2P6O24 (x = 0.2) reached a minimum value of 0.79 W m−1oC−1 at 1000 °C. The present investigation let provide that the Ca0.5Sr0.5Zr4-xSnxP6O24 ceramics with high mechanical strength and excellent thermophysical properties are promising candidate materials in high temperature applications.

The evolution of carbon fibers with Fe3+ doping and effects on the mechanical properties of Cf/BAS composites

The barium aluminosilicate glass ceramic (BAS) is a potential material for high temperature application due to its high melting point and low thermal expansion coefficient. However, the low mechanical performance has limited its widely using in industry. A lot of effort has been made to reinforce the BAS matrix by incorporating carbon fibers, but the enhancement is limited by the strong interface bonding. In this work, the Fe3+ was employed to promote graphization of carbon fibers and increase mechanical properties by weakening the interface bonding. The evolution of carbon fibers with Fe3+ doping and effects on the mechanical properties of Cf/BAS composites were investigated. The results indicate that the incorporation of 1.35 wt.% Fe3+ could increase the flexural strength and fracture toughness of the composites by 28.11% and 37.10%, respectively. This is attributed to the formation of brick micro-crystalline graphite interface layer and carbon nano-sized grains in the matrix. When Fe3+ doping content is higher than 1.35 wt.%, the mechanical performance of composites undergo a gradually decrease, resulting from the excessive erosion of carbon fiber, accompanied by the growth of carbon nanofiber on the fiber surface. The mechanisms of the evolution of carbon fiber and the effects on mechanical properties are discussed.

Piezoelectric, dielectric, and ferroelectric properties of high Curie temperature bismuth layer-structured bismuth titanate-tantalate (Bi&lt;sub&gt;3&lt;/sub&gt;TiTaO&lt;sub&gt;9&lt;/sub&gt;)

Piezoelectric materials have been extensively employed in numerous devices. With the rapid development of modern information technology, the high temperature piezoelectric materials that can work in extreme environments are in great demand. Therefore, it is urgent to investigate piezoelectric materials with high Curie temperature and strong piezoelectric performance. This paper reports the significantly improved piezoelectric properties of high temperature bismuth titanate-tantalate (Bi&lt;sub&gt;3&lt;/sub&gt;TiTaO&lt;sub&gt;9&lt;/sub&gt;, BTT) polycrystalline ceramics. In this work, the rare-earth cerium ions modified Bi&lt;sub&gt;3&lt;/sub&gt;TiTaO&lt;sub&gt;9&lt;/sub&gt; piezoelectric ceramics are prepared by the conventional ceramic technique. The introduction of Ce ions significantly enhances the piezoelectric performance of BTT ceramics. The BTT-6Ce (BTT+0.6 wt.% CeO&lt;sub&gt;2&lt;/sub&gt;) exhibits optimized piezoelectric properties with a piezoelectric coefficient &lt;i&gt;d&lt;/i&gt;&lt;sub&gt;33&lt;/sub&gt; of 16.2 pC/N, which is four times the value of unmodified BTT (&lt;i&gt;d&lt;/i&gt;&lt;sub&gt;33&lt;/sub&gt;~4.2 pC/N). The dielectric and ferroelectric measurements indicate that Ce ions remarkably reduce the dielectric loss tan&lt;i&gt;δ&lt;/i&gt; and increase polarizations, which are beneficial to the piezoelectric properties. The BTT and BTT-6Ce (&lt;i&gt;x&lt;/i&gt; = 0.6) ceramics each have a high Curie temperature &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;C&lt;/sub&gt;: ~890 ℃ and 879 ℃, respectively. The coercive field &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; of BTT and BTT-6Ce ceramics are 53.8 kV/cm and 57.5 kV/cm, respectively, while the remnant polarizations &lt;i&gt;P&lt;/i&gt;&lt;sub&gt;r&lt;/sub&gt; of BTT and BTT-6Ce ceramics are 3.4 μC/cm&lt;sup&gt;2&lt;/sup&gt; and 5.4 μC/cm&lt;sup&gt;2&lt;/sup&gt;, respectively, at a frequency of 1 Hz, temperature of 180 ℃, and drive field of 110 kV/cm. The thermal annealing measurements indicate that the BTT ceramics still possess stable piezoelectric properties after being annealed at 800 ℃. The results exhibit that the cerium-modified BTT ceramics are good materials for high temperature applications.

First-principles calculations and high thermoelectric performance of La–Nb doped SrTiO3ceramics

SrTiO3is a promising thermoelectric material for high temperature application of waste heat electrical generation.

High temperature electromagnetic wave absorption properties of SiCf/Si3N4 composite induced by different SiC fibers

Ceramic matrix composites are potential electromagnetic (EM) wave absorbing materials for high temperature applications, and the critical problem is to obtain temperature insensitive absorption characteristics in broadband. In this study, the double layer SiC fiber reinforced composite with ZMI-SiCf/Si3N4 as lossy layer and SLF-SiCf/Si3N4 as impedance matching layer, has been constructed via the chemical vapor infiltration method. The designed SiCf/Si3N4 composite with the thickness of 4.2 mm, which processes uniformly continuous microstructure and hierarchical permittivity, shows greatly enhanced EM wave absorbing performance of measured reflection loss less than −8 dB in whole X and Ku band at the room temperature. The thickness dependent absorbing properties of the SiCf/Si3N4 composite are investigated, which are found to be highly correlated to the combining mode of the lossy and matching layers. Particularly, the temperature dependent absorption characteristics indicate that the fabricated SiCf/Si3N4 composite keeps relatively reliable high temperature absorbing performances up to 600 °C. The temperature insensitive absorption characteristics should be mostly attributed to the enhanced compensating effect of the decreased interfacial polarization loss with increasing temperature induced by multiple interfaces, as well as the unchanged geometric structure of the designed composite.

Novel heat-resistant Al-Si-Ni-Fe alloy manufactured by selective laser melting

The novel Al-Si-Ni-Fe alloy with high compression strength at elevated temperatures has been successfully produced by selective laser melting at an average volumetric energy density of about 100 J/mm3 and has a relative density of 99,86%. The hardness of as fabricated Al-Si-Ni-Fe alloy is 186HV and it decreases during heat treatment to 124HV due to stress relief and silicon particle growth. The yield compression stress is 355 MPa at 200 °C. This high strength is provided by the fine structure formed by the Si, Al5Fe(Ni,Cu) and Al3(Ni,Cu) phases. The good strength properties of the alloy at elevated temperatures make it a quite promising lightweight material for high temperature applications.

Impression creep behaviour of TiB2 particles reinforced steel matrix composites

ABSTRACTImpression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873–973 K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149 kJ mol−1 and steel reinforced with 2 and 4 vol.-% TiB2 was found to be 298 and 338 kJ mol−1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application.

Thermal Analysis of NiAl30 Powder Mixture

NiAl alloys belong to the group of modern materials for high temperature application due to their low density, excellent thermal stability and oxidation resistance. They could be used as high temperature components which are usually made of nickel superalloys, heat-resistant steel or even cobalt superalloys. Production of these alloys is relatively simple and moreover, they could be used at higher temperatures exceeding 1000 °C. However, their high melting points and poor casting properties complicate the production by melt-metallurgy process. Powder metallurgy and reactive sintering were found to be a suitable replacement for melt-metallurgy. Many phases form during reactive sintering and reaction conditions of their formation are not known or are contradictory. In this work, thermal analysis of NiAl30 (in wt. %) powder mixture was studied. Results offer the effect of heating rate on reaction temperatures, microstructure and phase composition.

MORPHOLOGICAL, PHYSICAL AND THERMAL CHARACTERIZATION OF MICROFIBRILLATED CELLULOSE

ABSTRACT The aim of this study was to characterize microfibrillated cellulose (MFC) produced with a Masuko Sangyo Super Masscolloider using bleached and unbleached pulp of Eucalyptus spp. The MFC was characterized regarding morphology (TEM), crystallinity, viscosity, zeta potential and thermal properties (TGA). Regardless of the fiber type, the processing dramatically reduced the dimensions of the material, so as to obtain structures with nanometric dimensions. MFC produced with unbleached pulp preserved the original brown color of the pulp, which may be advantageous for some applications in the packaging sector, while films produced with bleached pulp were more translucent. The MFC showed lower values of viscosity and crystallinity index in relation to cellulosic pulp. The zeta potential was influenced by the type of fiber used. The main thermal transitions in MFC occurred after 200 ºC, demonstrating the potential of this material forhigh-temperature applications.

High temperature oxidation behavior of Al0.6CrFeCoNi and Al0.6CrFeCoNiSi0.3 high entropy alloys

Amongst the structural materials for high temperature applications that are currently in the focus of research, high entropy alloys (HEAs) exhibit very promising features, such as high strength, good fracture toughness, high temperature phase stabilities and excellent resistance against softening at elevated temperatures. Key influencing factors for high temperature applications are the mechanical properties at elevated temperatures as well as the oxidation behavior of a given material. AlxCrFeCoNi alloys have been mostly investigated among different HEAs. However, up to now there are limited reports dealing with the oxidation behavior of AlxCrFeCoNi–based HEAs, especially with respect to the effect of Si-alloying on the oxidation behavior at elevated temperatures. In this study, the oxidation behavior of two HEAs, Al0.6CrFeCoNi and Al0.6CrFeCoNiSi0.3, were comparatively investigated at T = 800 °C, 900 °C and 1000 °C in air to determine the effect of Si-alloying. The results show that oxidized layers typically consist of Cr2O3/spinel and Al2O3 with an underlying Al-depleted zone. AlN was found inside the Al-depleted zone with the exception of Al0.6CrFeCoNiSi0.3 oxidation at T = 800 °C. However, Si alloying only positively influenced the oxidation behavior at T = 800 °C. Here, it promoted the formation of a continuous Al2O3 layer. The Si-alloying did not enhance the oxidation resistance at T = 900 °C and 1000 °C.