Phase Composition Stability Research Articles

Stability of phase composition and properties of Zn4+xSb3 medium-temperature thermoelectric material

The phase composition and thermoelectric properties of a β-Zn4Sb3 based material before and after thermocycling tests at 273–723 K have been studied. Specimens with stoichiometric Zn and Sb molar ratios and with additions of different excess Zn quantities have been synthesized by direct component smelting followed by spark plasma sintering. The phase composition and structure of the materials have been studied using X-ray diffraction and transmission electron microscopy. The thermal conductivity and specific heat of the materials have been measured with laser flash and differential scanning calorimetry techniques. The phase composition of the as-synthesized, as-sintered and as-thermocycled Zn4+xSb3 thermoelectric material has been found to vary depending on excess Zn content in the charge. Excess Zn has been shown to dissolve in the β-Zn4Sb3phase upon spark plasma sintering and thermocycling. β-Zn4+xSb3 solid solution depletion of interstitial zinc atoms or ZnSb phase precipitation upon thermocycling increase the thermal conductivity and reduce the thermoelectric efficiency of the material. The Zn4.1Sb3 composition has shown high thermoelectric efficiency, the 715 K ZT being ∼1.23 and 1.18 before and after thermocycling, respectively.

Rapidly solidified Al-Cu-Co quasicrystalline alloy: microstructure and catalytic properties

This study presents the characterisation and evaluation of the catalytic potential of an alloy with a composition referring to the quasicrystal from the Al-Cu-Co system. The material was synthesised through a melt-spinning process, providing rapid solidification conditions and a favourable material form of thin ribbons. Microstructural and chemical analyses were performed using scanning electron microscopy, X-ray diffraction and transmission electron microscopy methods. The obtained results confirmed the presence of a major phase which was a decagonal quasicrystal in the form of globular grains and additional crystalline phases occurring along the grain boundaries. The catalytic potential of the materials was explored using phenylacetylene hydrogenation. The catalyst, obtained by pulverising the ribbons, demonstrated catalytic activity with over 40% phenylacetylene conversion and selectivity exceeding 60% for styrene production under mild reaction conditions. The recovered catalyst displayed a stable phase composition, as confirmed by X-ray diffraction, and unchanged morphology, indicating the possibility of catalyst reuse for subsequent reaction cycles. The presented results provide insight into the experimental verification of the catalytic properties of the developed environmentally friendly catalysts.

Influence of nano-sized WC addition on the microstructure, residual stress, and tribological properties of WC-Co HVAF-sprayed coatings

It has recently been proposed to modify conventional WC-Co coatings with nano-sized particles to improve their properties. Modification of the coatings with nano-sized carbides may provide higher wear resistance or phase composition stability. This work characterized the changes in the residual stresses induced by introducing WC nanoparticles into a WC-Co coating and the resulting change in their sliding wear behavior.A powder mixture consisting of agglomerated and sintered WC-17Co feedstock powder and 5 % nano-sized WC was deposited on a carbon steel substrate using the High Velocity Air Fuel (HVAF) process. Microstructural characterization using scanning electron microscopy and EBSD confirmed that nano-sized WC particles were embedded in the coatings after the spraying process, primarily along the boundaries of the WC-Co lamellae. This resulted in an enhancement of microhardness, indentation fracture toughness, and dry sliding wear resistance, evaluated via ball-on-disk tests against Al2O3 counterparts. Residual stresses in the coatings were also measured by XRD using the sin2ψ method. The normal stress along the torch movement direction decreased significantly when nano-sized WC was added. The addition of nanoparticles also caused a slight decrease in the normal stress perpendicular to the torch movement.

Influence of microwave heating on grain growth behavior and phase stability of nano Y2O3/La2O3 co-doped ZrO2 ceramics

Y2O3-doped ZrO2 (YSZ) is a significant structural material and excellent functional material, it has been widely used in fields such as ceramics, thermal barrier coatings, and medical, etc. The disadvantage of YSZ is that it undergoes phase transition occurs during high-temperature cooling, leading to cracks in zirconia. It is worth mentioning that the failure phase transition of YSZ can be suppressed by the addition of rare earth oxide such as lanthanum oxide. In this paper, ZrO2 nano-powders with 0.2 mol% La2O3-2.8 mol% Y2O3 were prepared by a high-energy ball milling method. The comparison of microwave sintering and conventional sintering temperatures (800 °C–1300 °C) on the phase transition transformation of La2O3-doped Y2O3-stabilized zirconia powders was investigated by the control variable method. Meanwhile, the effects of microwave sintering technology on the microstructure and grain growth behavior of the sintered powders were further analyzed. The results show that microwave sintering is superior to conventional sintering technology. Microwave sintering promotes the rate of conversion of zirconia from monoclinic to tetragonal phase. Furthermore, the average grain size of the samples increased linearly with increasing temperature, and the grain growth further contributed to the presence of stable phases. Adding La3+ and Y3+ can reduce the grain growth activation energy and increase the contents of both the t-ZrO2 and c-ZrO2 in the sample. In this experiment, we find that the samples sintered best at 1200 °C with stable phase composition, well-defined morphology and uniform particle distribution. It is well known that the synthesis of high-quality zirconia materials is essential for good zirconia raw materials in practical production. This study provides an experimental and theoretical basis for the synthesis of zirconia powder with excellent performance.

Increased monocarbonate formation during long-term hydration of CAC-limestone binder by calcium sulfate addition – A temperature-dependent XRD study

The long-term hydration of a mix with and without gypsum addition was investigated at 10, 23 and 40 °C by QXRD analysis. In addition, thermodynamic modelling was performed on all systems using GEMS to determine the stable phase composition and the influence of water loss during hydration. The results show that the formation of conversion-sensitive phases such as CAH10 is accelerated at 10 °C and hindered at 23 and 40 °C by the addition of gypsum. Moreover, at all different temperatures, monocarbonate forms next to AH3 as the main hydrate phase and the final phase composition is reached after at least 14 d (at 23 °C and 40 °C already after 2 d). Formation of low amounts ettringite only occurs at 10 and 23 °C during the entire storage time. At 40 °C, however, ettringite is not observed, which is probably due to water loss in the mix until 2 d.

Study of the Effect of Adding Nb2O5 on Calcium Titanate-Based Ferroelectric Ceramics

This paper considers the effect of adding niobium oxide (Nb2O5) to ferroelectric ceramics based on calcium titanate (CaTiO3), and establishes a connection between the observed alterations in strength and dielectric properties and the variation in the Nb2O5 dopant concentration in the ceramics’ composition. The method of mechanochemical solid-phase synthesis was used as the main method for obtaining the ceramics, followed by thermal sintering under specified conditions in order to form a stable phase composition of the ceramics, and to initialize phase transformations in the composition. Based on the assessment of the phase composition of the resulting ceramics, it was determined that a growth in the Nb2O5 dopant concentration beyond 0.10 mol results in the formation of an orthorhombic-phase CaNb2O4 of the Pbcm(57) spatial system, the weight contribution of which grows. A growth in the Nb2O5 additive concentration results in the formation of two-phase ceramics, the formation of which allows for an enhancement in the mechanical strength of ceramics and resistance to external influences. During the study of the dependence of the strength properties on the dopant concentration alteration, a three-stage change in hardness and crack resistance was established, regarding both structural ordering and phase transformations. The measurement of dielectric characteristics showed the direct dependence of dielectric losses and the dielectric constant on the phase composition of ceramics.

PRODUCTION OF FUNCTIONALLY GRADIENT MATERIALS BASED ON SILICON CARBIDE AND HIGH-ALLOY STEEL USING SPARK PLASMA SINTERING TECHNOLOGY

An important scientific task of practical materials science is the production of metal-ceramic composites in the form of functional gradient materials (FGM) for special-purpose products. In this regard, a study was conducted on the application of spark plasma sintering (IPS) technology for the effective diffusion connection of SiC ceramics and high-alloy steel (grade X18R15) to obtain a combined FGM composite. In a comprehensive experimental study, the dynamics of consolidation and changes in the phase composition of dispersed SiC under conditions of different temperatures and heating rates, pressing pressure, and holding time were studied. As a result, the IPS conditions were optimized for obtaining SiC ceramics of high relative density (82%) and microhardness (500 HV) of stable phase composition. The physicochemical foundations of the formation of a strong compound of a two-component SiC-ceramic and steel system under IPS conditions without additives and using a mixture of additives in the form of a binder, a reaction binder and a damper (Ti–Ag, Ti–TiH2, Ti–Ag–TiH2 and Ti–Ag/Mo additive systems) have been studied. The structure, composition of ceramics and intermediate (binding and damping) layers, as well as the diffusion of elements at the boundary of the formed compounds in FGM composites, were studied using XRF, SEM and EMF methods. It was found that the Ti–Ag/Mo additive in the ratio of 30 wt. % Ti–70 wt. % Ag and a dense layer of Mo (thickness ~ 2 mm), acting as a damper to compensate for the temperature coefficient of linear expansion, ensure the formation of a connected FGM composite of an integral shape. The presented studies have been implemented for the first time, are promising and require further development in order to gain scientific knowledge of the manufacture of composite products for special purposes.

Effect of rare-earths (La, Nd, Pr) on zirconate ceramics for thermal barrier coatings

Zirconium oxide-based powders with different concentrations of REE concentrate (La, Nd, Pr) in an amount from 25 to 35 mol.% were synthesized by the reverse chemical precipitation method. The formation of REE zirconates during consolidation has been studied by Raman spectroscopy, XRD and scanning electron microscopy. It has been established that the formation of REE zirconates with the pyrochlore structure occurs after annealing of synthesised powders at a temperature of 1000°C. It has been demonstrated that densifying processes during sintering at 1400 –1600°C occur most slowly in a ceramic material with 35 mol.% REE oxides, which has a stable phase composition at all sintering temperatures, consisting mainly of REE zirconates with the pyrochlore structure. The resulting ceramic material based on REE zirconates has phase stability at 1300°C.

Preparation of CMAS, LZS glass-ceramics and study of the glass ceramics anti-corrosion properties in liquid leadbismuth eutectic at 500°C

Liquid lead-bismuth eutectic (LBE) was an important candidate material for the fourth generation reactor coolant. Its high temperature and heavy metal corrosion conditions put forward higher requirements for the corrosion resistance of reactor structural materials. As a new kind of special material, glass-ceramic material, which possesses the excellent properties of glass and ceramics, has good mechanical property, wear-resisting property and corrosion resistance. The LBE corrosion behavior of glass-ceramics at typical operating temperatures of nuclear reactors was investigated. The lead bismuth corrosion test was carried out under the condition of oxygen saturation at 500°C for 500h. The lead bismuth corrosion resistance of the two glass ceramic samples in static LBE was analyzed by means of scanning electron microscope (SEM) and X-ray diffraction (XRD). After 500 h corrosion in oxygen saturated liquid LBE at 500°C, there was no obvious corrosion on the surface of CMAS glass ceramics. Its dense microstructure and stable phase composition effectively prevented the penetration of Pb and Bi elements, and had good resistance to corrosion; The surface of LZS glass ceramics was covered with a layer of loose and porous compounds, and the thickness of the corrosion zone was 10 μm, which occurred a certain degree of lead-bismuth corrosion, and its corrosion resistance was poor. The results showed that under the same corrosion conditions, no lead bismuth corrosion behavior was found on the surface of CaO-MgO-Al2O3-SiO2 (CMAS) glass-ceramic, while there was a corrosion layer with a thickness of about 10 μm on the surface of Li2O-ZnO-SiO2 (LZS) glass-ceramic, indicating that the lead bismuth corrosion resistance of CMAS glass ceramics was significantly better than that of LZS glass ceramics.

Mechanical property and microstructure development in alkali activated fly ash slag blends due to efflorescence

Efflorescence of alkali activated materials (AAMs) is caused by alkali leaching and precipitation of carbonated salts, which occurs concurrently with leaching and natural carbonation. Efflorescence is specifically driven by precursor and activator variability in AAMs vis-à-vis phase changes, microstructure, and mechanical properties are not well understood. To that end, this study analyses the effects of long-term (90 days) efflorescence on AAMs with eight varied calcium and activator contents and correlated with compressive and splitting tensile strengths. Microstructural features including N-A-S-H/C, N-A-S-H gel change are analysed using Fourier-transform infrared spectroscopy (FTIR), magic-angle spinning nuclear magnetic resonance (MAS-NMR) and thermogravimetric analysis (TGA). AAMs with 9 wt% Na2O and Ca/(Si + Al) ratio of 0.0 to 0.25 exhibit enhanced efflorescence and dealumination of Al[IV] in N-A-S-H/C,N-A-S-H gels, higher mobility of Na+ ions, and natural carbonation. AAMs with 5 wt% Na2O and Ca/(Si + Al) ratio of 0.0 to 0.25 exhibit lower efflorescence and higher stability, in contrast. Under efflorescence, the binder with 5 wt% Na2O and Ca/(Si + Al) ratio of 0.25 exhibited dealumination and carbonation, and the binders with 5 wt% Na2O and Ca/(Si + Al) of 0.0 to 0.10 showed less dealumination and higher concentration of Al[IV] in aluminosilicate gel, indicative of greater phase stability. This study highlights the criticality of calcium and activator doses in controlling the stability of phase composition and mechanical property, which is essential for the industrial application of AAMs.

One-Step Design of a Water-Resistant Green-to-Red Phosphor for Horticultural Sunlight Conversion

As a universal sunlight-conversion agent for agricultural shed films, the green-to-red phosphor CaS:Eu2+ suffers greatly from its poor moisture resistance. Unlike the conventional surface modifications upon application of postprocessing, this work proposes a one-step, two-additive strategy for synthesizing the CaS:Eu2+,CaBr2,CaF2 composite phosphor. For the first time, we realize remarkable improvements in both the chemical stability and the luminescence intensity of a CaS:Eu2+-based material synchronously. The dual halide-modified composite phosphor maintains a stable phase composition against moist air and water immersion. The red emission of the as-obtained CaS:Eu2+,CaBr2,CaF2 phosphor becomes 3 times as intense as that of nonmodified CaS:Eu2+. Moreover, the polymer film fabricated with the modified phosphor presents good weather resistance and brilliant light-conversion performance. Therefore, this one-step method gives the CaS:Eu2+,CaBr2,CaF2 phosphor a great advantage as a long life light-conversion material for agricultural shed films.

Study of long-term post-SHS phase kinetics in Ni/Al reactive multilayer nanofoils

We present the results of the recent experiments with two species of Ni/Al reactive multilayer nanofoils (RNMFs) supporting the self-propagating high-temperature synthesis (SHS) reaction. SEM method was used to evaluate their general structure and quantitative chemical composition before and after reaction. Phase composition was determined from XRD spectra. Special attention was paid to the stability of phase composition over relatively long time intervals after SHS reaction – order of the hours and days. In some cases, XRD spectra showed notable evolution of crystalline structure, which can have significant impact on practical application of SHS, for instance, in soldering technologies. To our knowledge, similar effect has been previously observed only for SHS-reactive powder mixtures.

Study on thermal shock behavior of YSZ abradable sealing coating prepared by mixed solution precursor plasma spraying

In this study, a yttria-stabilized zirconia (YSZ) ceramic matrix abradable sealing coating named 8YSZ-5 was successfully prepared by mixed solution precursor plasma spraying, and its thermal shock behavior was studied in detail. During the thermal shock cycles, the microstructure, residual stress and phase stability of the 8YSZ-5 coating were examined by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) technique. The results show that the 8YSZ-5 coating has excellent thermal shock resistance through a 200-cycle thermal shock test at 1150 °C. During the thermal shock cycles, the spallation only existed in a small scale in the edge of the 8YSZ-5 coating, and the coating always maintains a stable phase composition of tetragonal structure. The residual stress study shows that the in-plane residual stress of the 8YSZ-5 coating is detected to be compressive stress, which increases with increasing the thermal shock cycles. The microstructure analysis shows that as the thermal shock test progresses, the number of the pores in the coating gradually decreases due to their spontaneous closure and the average size of the grains gradually increases. The mechanical properties investigation shows that the hardness and surface friction coefficient of the 8YSZ-5 abradable sealing coating increase gradually, while the bond strength of the coating decreases by 50% after 200 thermal shock cycles. The implications of this study are important for the design of advanced abradable sealing coatings prepared by the mixed solution precursor plasma spraying technology.

Effect of Exposure to Air on the Phase Composition and Particle Size of Nanocrystalline Lead Sulfide

Nanocrystalline powders of lead sulfide with particle size from 5 to 105 nm have been synthesized by chemical deposition from aqueous solutions of lead acetate or nitrate using sodium sulfide as sulfidizing agent and in the presence of sodium citrate or Trilon B as complexing agents. The exposure of the nanopowders to air for six years has shown that PbS nanopowders obtained in the presence of sodium citrate Na3Cit, which behaves as both complexing and stabilizing agent, display the largest stability of phase composition. The stabilizing role of Na3Cit is due to its ability to form a shell on nanoparticle surface to prevent lead sulfide oxidation. It has been established that nanoparticle size remains constant and stable upon long-term keeping in air. The phase composition of PbS nanopowders prepared using Trilon B gradually changes owing to oxidation into lead sulfate upon long-term exposure to air.

Influence of rare earth elements on the structure and properties of powders based on zirconium dioxide during consolidation

Zirconium oxide powders with different contents of REE concentrate (La, Nd, Pr) in an amount from 25 to 35 mol. % were synthesized by the method of reverse chemical deposition. The effect of the annealing temperature in the range of 600–1200 ° C on the phase transformations of the synthesized powders has been investigated by Raman spectroscopy. It was found that the formation of REE zirconates with a pyrochlore structure occurs after annealing at a temperature of 1000 ° C. At higher annealing temperatures, the presence of tetragonal ZrO2 was noted in the samples with a lower concentrate content in the phase composition. A study of the influence of the sintering temperature of 1400-1600 °C on the densification processes, phase composition and microstructure of ceramic materials based on REE zirconates has been carried out. It has been shown that the densification processes proceed most slowly in a ceramic material with 35 mol.% of rare-earth oxides, and at all sintering temperatures it has a stable phase composition, consisting mainly of rare-earth zirconates with a pyrochlore structure.

Стойкость фазового состава и механических свойств наноструктурированных композиционных керамик на основе CaO–ZrO2 к гидротермальным воздействиям

Исследованы структура, фазовый состав и механические свойства (микротвердость в интервале глубин внедрения индентора 1200 нм ≤ h ≤ 6000 нм и вязкость разрушения) наноструктурированной циркониевой керамики (стабилизированной CaO), упрочненной корундом и SiO2 , в процессе ускоренного старения в гидротермальных условиях (температура Tag = 134°С, давление P = 3 атм, влажность B = 100%, 0 ≤ t ag ≤ 25 ч). Показано, что использование CaO в качестве стабилизатора тетрагональной фазы диоксида циркония (вместо “традиционного” Y2 O3 ) способствует повышению стойкости к гидротермальным воздействиям композиционных керамик ZrO2 + Al2 O3 и ZrO2 + Al2 O3 + SiO2 . Установлено, что достигнутое введением диоксида кремния (CSiO2 = 5 мол. %) увеличение вязкости разрушения (более чем на 40%) обеспечивает удовлетворительное соотношение твердость/вязкость разрушения (H = 12.3 ГПа, KC = 6.66 МПа м1/2) композиционной керамики CaO–ZrO2 + Al2 O3 + SiO2 даже после ее ускоренного состаривания в течение 25 ч.

Citronellal cyclisation to isopulegol over micro-mesoporous zsm-5 zeolite: effects of desilication temperature on textural and catalytic properties

In this work, desilication reassembly post treatment process was applied in synthesis of mesoporous zeolite with stable phase composition and applied it in citronellal cyclisation reaction for the production of isopulegol. The desilication and temperature effects were further investigated on physical and chemical characteristics of zeolite and compared them with catalytic activity. The desilicated zeolite samples have been characterized with the help of N2-adsorption, XRD, ICP-OES, pyridine adsorption and FTIR techniques. Its performance was explored by controlling operative parameters. Experimental outcomes exhibited that desilication of zeolite would led to formation of mesopores inside the stable zeolite framework structures without substantial damage of their internal composition. These changes facilitate mass transfer and catalytic activity with an increase in surface area, mesoporosity, pore size, pore volume, acidity, and Lewis acid sites. Optimum desilication temperature (80 °C) was found as a best for an comprising extra active and selective mesoporous zeolite catalyst for citronellal cyclisation. Thus, this type of zeolite material has shown 100% conversion (e.g. complete conversion of citronellal reactants to the desired products), 95% isopulegol selectivity and highest reaction rate (0.11 min−1). This study exhibits the mesoporous zeolite (MZ-80 °C) as one of the most effective catalyst for citronellal cyclization reaction based on assessment of catalytic performance relative to other commonly available options.

Thermal stability and recrystallization of semiconductor nanostructured sulfides and sulfide solid solutions

Metal sulfide semiconductor nanostructures represent the most promising group of nanostructured materials with excellent optical and electrical properties for such applications as photoelectrochemical cells, photocells, photoconductors, infrared detectors, nonvolatile memory devices and resistive switches. Much attention is drawn to nanostructured sulfides PbS, Ag2S, SnS, and CdS of lead, silver, tin and cadmium, and also sulfide solid solutions. As a rule, microelectronic devices, photocells, photoconductors and infrared detectors, containing a nanostructured sulfides, operate in air at elevated temperatures (up to ∼400–450 K), at which the growth and variation in the phase composition of sulfide nanoparticles may take place, leading to the degradation of their electrical and optical properties. The uncontrolled growth of sulfide quantum dots and nanoparticles at high temperatures results in the instability and degradation of properties of electronic devices and compositions with nanostructured sulfides. In present paper, the data on the recrystallization and thermal stability of phase composition and particle size of semiconductor nanostructured PbS, Ag2S, SnS, and CdS sulfides and sulfide solid solutions CdxPb1-xS are considered. For the first time, the regions of thermal stability of sulfide nanoparticle sizes are determined.

Thermal stability of nanoparticle size and phase composition of nanostructured Ag2S silver sulfide

The thermal stability of phase composition and particle size of nanocrystalline silver sulfide Ag2S powder has been examined for the first time by in situ high-temperature X-ray diffraction, scanning electron microscopy and Raman spectroscopy methods. It was shown that in oxidizing environment or in the presence of oxidizing impurities, heating of nanostructured silver sulfide is accompanied by its decomposition (dissociation) with isolation of metallic silver. Reduction of silver sulfide particles leads to decreasing the decomposition temperature of Ag2S. The decomposition of Ag2S nanopowder with isolation of silver metal as a result of heating in air by high-power laser radiation was confirmed by Raman scattering. In the absence of oxidizing environment, heating of silver sulfide does not lead to its decomposition and nanoparticle growth up to the temperature of ∼450 K. The nanoparticle size increases as a result of annealing at a temperature of 450–930 K corresponding to the region of collective recrystallization of silver sulfide nanopowder. For nanostructured silver sulfide, the activation energy Q of collective recrystallization in the temperature range 450–900 K is ∼0.12 eV⋅atom−1.

Low-temperature aging of ceramic on the basis of tetragonal zirconium dioxide stabilized by cations of yttrium and ytterbium

The effect of low-temperature aging of ceramics Zr0.97Y0.03O2 (Y-TZP) and Zr0.98Y0.02O2 (Yb-TZP) by the methods of the high-grade diffraction analysis and atomic force microckopy is studied. Formation of the phase M-ZrO2 in ceramic (Y-TZP) in a large quantity after hydrothermal impact in subsurface layers of ceramic samples is shown. A positive effect of the stabilizing cation Yb+3 on the phase composition stability and strength properties of ceramic based on T-ZrO2 is registered. Adaptation of the structural ceramic based on T-ZrO2 according to medical requirements is the aim of this study.