High-temperature Dilatometry Research Articles

Sintering activation energies of anisotropic layered and particle alumina/zirconia-based composites and their mechanical response

Information on the sintering activation energy is currently focused on evaluation of single-phase ceramic systems. This work shows the results of high-temperature dilatometry measurements of layered and particle composites based on alumina and zirconia. Layered composites with different layer thickness ratios and particle composites with variable composition in the entire concentration range were prepared by electrophoretic deposition allowing manufacturing composites with precious design and strongly bonded interfaces. The phenomena observed during the high-temperature dilatometry measurements are discussed, and the data were used to calculate the sintering activation energies of composites using the modified Master Sintering Curve concept. By covering a wide range of composite designs, it was possible to determine differences in activation energies and to show their dependence on the direction in the case of laminate composites given by the directionally dependent sintering behaviour. Sintering activation energies of layered composites were always higher than for monoliths due to constrained sintering showing maximum sintering activation energies at lower volumes of zirconia in the layers for longitudinal and transversal orientation of the samples. A similar trend was identified in particle composites due to slowed down alumina densification by the pinning effect. Additionally, mechanical properties represented by Vickers hardness and indentation elastic modulus were related to the microstructure developed during sintering. The effects of interconnectivity of phases present in the composites together with other parameters of the microstructure were described.

Thermal expansion studies of Li2TiO3 by dilatometry and in-situ high-temperature X-ray diffraction

Li2TiO3 (lithium titanate) is a material of interest in the field of fusion reactors due to its potential application as a tritium breeder material. Because of the extreme working conditions and temperature gradients involved, thermal expansion properties are critical to consider when assessing a material for a fusion reactor. The reactor is subjected to severe neutron flux, high temperature and thermal cycling, which can cause significant stress on the material. A low coefficient of thermal expansion (CTE) of Li2TiO3 is important as it mitigates mechanical stress and minimized the risk of structural damage resulting from thermal cycling. Material degradation, cracking, or delamination might result from a mismatch in CTE between different materials in the reactor. Therefore the estimations of material's thermal expansion properties hold significance for the designing and fabrication of structural material. In this study, Li2TiO3 material was compacted into pellets and high-temperature X-ray diffraction (HT-XRD) was employed to obtain X-ray diffraction patterns from room temperature to 1273 K at 100 K intervals. The lattice parameters (a, b, and c) of Li2TiO3 polycrystals were determined and corresponding unit cell volumes were estimated at each temperature. The thermal expansion percentages for the lattice parameters a, b, and c between room temperature and 1273 K were found to be 1.21%, 1.84%, and 1.83% respectively. The calculated coefficient of thermal expansion (CTE) were determined as 12.5 × 10−6 K−1, 19 × 10−6 K−1, and 18.9 × 10−6 K−1for lattice parameters a, b and c respectively. The crystallographic density at room temperature was estimated to be 3.42 g/cm3. The molar volume was calculated to range between 32.12 and 33.72 cm3/mol from room temperature to 1273 K. Additionally, high-temperature dilatometry was employed to measure the thermal expansion of Li2TiO3, resulting in the estimation of coefficient of thermal expansion. From room temperature to 1290 K, the thermal expansion percentage and mean coefficient of thermal expansion were calculated to be 1.91% and 19.64 × 10−6 1/K, respectively. Furthermore, the temperature dependent mean and instantaneous thermal expansion values were also determined. The differential scanning calorimetry (DSC) method was used to estimate the temperature of the monoclinic to cubic transformation, which was determined to be 1416 K. This paper provides a comprehensive description of the conducted experiments, accompanied by the detailed analysis of the thermal expansion properties derived through the use of High-Temperature X-Ray Diffraction (HT-XRD) and HT-dilatometry (High-Temperature Dilatometry).

Modification of barium titanate sintering via rare earth oxides addition: Dilatometric and microstructural study

In this paper, the effect of rare-earth dopants (RE: Ce, Eu, Tb, Dy, and Er) in barium titanate (BT) on the shaping, sintering and microstructure was studied aimed mainly at selecting appropriate dopant for densification delaying while preserving optimal BT characteristics. The addition of dopant oxides into suspension resulted in the reduction of the deposition kinetics. The relative density of doped green bodies was lower than pure BT and decreased with dopant concentration. Only Ce enhanced green density up to 2%. The sintering process was studied using high-temperature dilatometry. The hindering densification caused by dopants was observed, but only 5 wt % of Tb and Er in BT shifted the onset of sintering to higher temperatures that has the potential applicability of these materials in layered ceramic harvesters. The significant positive impact of doping on the microstructure was attained almost in all cases manifesting themselves in higher sintered densities and markedly lower mean grain size compared to pure BT. The XRD and Raman spectroscopy analyses confirmed incorporation of the dopants into the perovskite structure and no significant existence of RE-enriched secondary phases in the microstructure.

Thermal expansion study on LnMgAl11O19 (Ln=La, Gd) by high temperature X-ray diffraction

Thermal expansion performance is essential for lanthanide magnesium hexaaluminate (LnMgAl11O19, Ln=La, Gd) top-coating as the thermal barrier coatings materials. In the present work, LnMgAl11O19 (Ln= La and Gd) with magnetoplumbite structure was synthesized by the chemical co-precipitation method. The lattice thermal expansion behavior of both the samples was investigated using a high-temperature X-ray diffraction test combined with Reitveld refinement. The results showed that the thermal expansion of the a-axis was smaller than that of the c-axis, which was related to the anisotropic elongation of Ln-O bonds in the [LaO12] clusters. Meanwhile, the lattice expansion coefficient of the LaMgAl11O19 was higher than that of GdMgAl11O19. This was confirmed by the result of the macroscopic thermal expansion which was measured using a high-temperature dilatometry. The underlying mechanism was discussed, which was due to the weaker bond energy of La–O than that of Gd–O.

High-temperature properties of titanium-substituted yttrium niobate

Abstract

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al2O3 and ZrO2 monoliths as well as layered Al2O3/ZrO2 composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.

Improved ceramic and electrical properties of CaZrO3-based proton-conducting materials prepared by a new convenient combustion synthesis method

The present work emphasizes the features of a new combustion synthesis method suitable for easy synthesis and sintering of CaZrO3-based materials as promising proton-conducting electrolytes for electrochemical applications in solid oxide electrochemical devices. Sc- and In-doped CaZrO3 compounds are selected to be the objects of investigation and their crystal structure properties as well as microstructural, thermal and electrical characteristics are studied by the means of X-ray diffraction analysis, high-temperature dilatometry, scanning electron spectroscopy, impedance and 4-probe DC conductivity techniques. The established properties are critically compared with previously obtained data to confirm the prospects of the proposed method synthesis for the preparation of Zr-based proton-conducting ceramic materials. It is found that CaZr0.95Sc0.05O3–δ exhibits predominantly protonic conductivity under wet air atmospheres below 700°C; while, CaZr0.9In0.1O3–δ demonstrates a certain value of electronic conductivity along with ionic one.

Phase equilibria, crystal structure and properties of complex oxides in the Nd2O3–SrO–CoO system

The phase equilibria in the ½Nd2O3–SrO–CoO system were systematically studied at 1373K in air. The intermediate phases formed in the ½Nd2O3–SrO–CoO system at 1373K in air are: Nd1-xSrxCoO3-δ (0.0≤x≤0.5 with orthorhombic structure, sp. gr. Pbnm and 0.6≤x≤0.95 whose structure was detected as cubic according to XRD sp. gr. Pm3m, but shown to be tetragonal by TEM due to the oxygen vacancy ordering), Nd2-ySryCoO4-δ (0.6≤y≤1.1 with tetragonal K2NiF4-type structure, sp. gr. I4/mmm) and Nd2-zSrzO3 (0.0≤z≤0.15 with hexagonal structure, sp. gr. P-3m1). The unit cell parameters for the single phase samples were refined by the Rietveld analysis. The changes of oxygen content in Nd1-xSrxCoO3-δ (0.6≤x≤0.95) and Ruddlesden-Popper oxide Nd2-ySryCoO4-δ were examined by TGA. All were found to be oxygen deficient phases. High-temperature dilatometry allows calculating the thermal expansion coefficient and evaluating the chemical expansion coefficient at high temperature. The projection of isothermal-isobaric phase diagram for the Nd–Sr–Co–O system at 1373K in air to the compositional triangle of metallic components has been constructed. The phase equilibria in the studied Nd–Sr–Co–O system were compared to La–Sr–Co–O and Nd–M–Co–O (M=Ca and Ba).

High-temperature glassy-ceramic sealants SiO2—Al2O3—BaO—MgO and SiO2—Al2O3—ZrO2—CaO—Na2O for solid oxide electrochemical devices

Glasses of the SiO2–Al2O3–BaO–MgO and SiO2–Al2O3–ZrO2–CaO–Na2O systems were synthesized in the perspective to apply them as sealants in SOFC at operating temperatures of 700–900 °C. Thermal properties of the chosen glass compositions and their compatibility with the SOFC materials (YSZ-electrolyte and alloy-interconnector Crofer22APU, 15×25T) were investigated by means of synchronic thermal analysis and high-temperature dilatometry. The elemental analysis was performed by atomic emission spectroscopy. The average values of the temperature coefficients of the linear extension are 10.0×10−6 °C−1 for glass 45%SiO2– 15%Al2O3–25%BaO–15%MgO and 9.5×10−6 °C−1 for glass 60%SiO2–10%Al2O3–10%ZrO2–5%CaO–15%Na2O. The gluing microstructure in YSZ/glass/Crofer22APU was studied by scanning electron microscopy. The crystallization process of silicate phases was revealed to occur in the SiO2–Al2O3–BaO–MgO glass. The analysis of the crystallization products was performed by Raman spectroscopy and X-ray diffraction. Glassy ceramics was proved to possess better parameters in comparison with amorphous glass to be used as a sealant in electrochemical sensors and oxygen sensors. The SiO2–Al2O3–ZrO2–CaO–Na2O low-temperature amorphous glass can be applied in SOFC.

Physicо-chemical characterization and transport features of proton-conducting Sr-doped LaYO3 electrolyte ceramics

Structure, thermal and transport properties of chemically stable proton-conducting La0.9Sr0.1YO3–δ electrolyte are investigated in the present work. The features of ceramics preparation, thermal expansion and electrical conductivity are thoroughly examined by the aid of X-ray diffraction analysis, thermogravimetry coupled with differential scanning calorimetry, scanning electron microscopy, high-temperature dilatometry and 4-probe dc-conductivity measurements. From the experimental results it is demonstrated that Sr-doped LaYO3 material can be easily prepared by citrate-combustion synthesis method. It is found that the sintered ceramic samples are highly dense and possess acceptable thermal expansion coefficients. Despite the fact that their absolute ionic conductivity level is found to be rather low, the ionic transport dominates for a wide range of oxygen partial pressures at the interval between 600 and 900°C, suggesting the use of the as prepared LaYO3-based materials for pumping and sensing devices.

High temperature dilatometry of simulated oxide nuclear fuel

High temperature dilatometry of model systems based on uranium dioxide with additives of burnable neutron absorbers both as Gd2O3 and as AlGdO3, and fission products simulators (FPS) was performed. It shown that in some cases instead of high temperature samples shrinkage there is a sharp transition to the expansion, which is associated with an increase of the samples volume due to the formation of liquid phases. The beginning of a complex composition eutectic melting starts at temperatures from 1950 to 2250 °C in the uranium dioxide samples containing significant amounts of Al, Gd, and FPS. Thus, in the analysis of oxide nuclear fuel behavior at high temperatures should be considered that the formation of liquid phases is possible at a temperature of 1000 °C lower than a melting point of pure stoichiometric uranium dioxide if its initial composition became more complex.

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia refractory

The optimized sintering conditions for a 3.5wt% magnesia partially stabilized zirconia (Mg-PSZ) refractory were proposed in our recent research. The influence of the sintering temperature on the development of phase composition, microstructure, densification, thermal expansion and mechanical strength was studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM), He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at 1670°C had the highest bend strength, the maximum densification, the lowest thermal expansion coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.

Densification of fine-grained alumina ceramics doped by magnesia, yttria and zirconia evaluated by two different sintering models

Abstract The influence of various dopants (500 ppm MgO and Y2O3; 250 ppm ZrO2) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y2O3 and ZrO2 significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.

Evaluation of Industrial Rejects of Mineral and Metallurgical Processing as Ceramic Synthetic Proppants

Hydraulic fracturing is a very important technique to increase the productivity and recovery of oil reservoirs with low pressure or high depth. The hydraulic fracturing technique is a method of well stimulation in which liquid under high pressure is pumped down a well to fracture the reservoir rock adjacent to the wellbore. Propping agents are used to keep the fractures/cracks open. They can be made from the reject of mineral extraction processes when they present specifics characteristic. The aim of this paper is the evaluation of the use of industrial rejects to the development of ceramic synthetic proppants that are resistant to chemical corrosion and high mechanical strength in aqueous acid and salt water environment. In this study, industrial rejects from nickel sulfide ore flotation (grey mud - GM), red mud (RM) from Bayer process and black mud (BM) from Caron processing of nickel was used as raw material for the production of ceramic synthetic proppants. The material processing was performed through drying, milling, pressing, sintering and characterizing raw material. Several characterization techniques such as X-ray powder diffraction (XRD), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), high temperature dilatometry and compression strength test were performed after sintering to evaluate the technological potential of recycling these metallurgical and mineral rejects. Mechanical properties were improved after sample sintering. The results show that it is possible to manufacture ceramics of high strength and resistance to acidic/salt water environment as a result of processing minerals reject.

Layered ceramic composites via control of electrophoretic deposition kinetics

The article summarises optimisation of electrophoretic deposition of alumina and zirconia from concentrated isopropanolic suspensions stabilised by monochloracetic acid. Method for in situ monitoring of deposited mass was developed and experimentally checked. Prediction of electrophoretic deposition kinetics was based on the voltage drop measurement and electrophoretic mobility estimation from experimental kinetics data. It was shown that voltage change during electrophoretic deposition did not affect relative density of the deposited layers in the laminates prepared. For this purpose a method of the relative density measurement using the microscopic measurement of relative shrinkage of individual layers was developed. Accuracy of the method was confirmed by comparison with shrinkage measurement using high temperature dilatometry. Adopting obtained results the alumina/zirconia laminate with precise control of layers thicknesses and final densities was prepared.

Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings

The effect of doping on the structural, morphological and thermal properties of ZrO 2–XO 1.5 (X = Y, La, Sm, Er) solid solutions for thermal barrier (TBC) applications was investigated. Oxide powders of various compositions from 9.7 to 40 mol% XO 1.5 (X = Y, La, Sm, Er) were synthesised by the sol–gel route. The structural analysis of the powders was performed using X-ray diffraction analysis coupled with Rietveld refinements and the measurement of their specific surface area with the BET method. For each rare earth dopant, the morphology of the powders varies from monoliths to agglomerates of thinner particles when the doping amount increases. In order to determine the specific heat, the thermal diffusivity at room temperature and the thermal expansion coefficient of some selected compositions, DSC, laser thermal diffusivity and high-temperature dilatometry measurements were performed on samples densified by Spark Plasma Sintering. Working thermal characterisation indicated that zirconia doped with 30 mol% SmO 1.5 and ErO 1.5 have better insulation properties and a lower thermal expansion coefficient than our reference YSZ ceramic. These various compositions are very promising for the elaboration of multilayer TBCs by the sol–gel process.

Effect of Ti substitution for Zr on the thermal expansion property of fluorite-type Gd 2Zr 2O 7

Gd 2(Zr 1− x Ti x ) 2O 7 (0 ⩽ x ⩽ 1.00) solid solutions were synthesized by solid state reaction at 1550–1650 °C for 10 h in air. The relative density, microstructure and thermal expansion property of Gd 2(Zr 1− x Ti x ) 2O 7 were investigated by the Archimedes method, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy and high-temperature dilatometry. Gd 2Zr 2O 7 exhibits a defect fluorite-type structure; however, Gd 2(Zr 1− x Ti x ) 2O 7 (0.25 ⩽ x ⩽ 1.00) solid solutions have an ordered pyrochlore-type structure. Gd 2Zr 2O 7 and Gd 2Ti 2O 7 are completely soluble. The degree of structural order for Gd 2(Zr 1− x Ti x ) 2O 7 solid solutions increases with increasing Ti content. The thermal expansion coefficients of Gd 2(Zr 1− x Ti x ) 2O 7 increase with increasing temperature from 30 to 1250 °C. The thermal expansion coefficients of Gd 2(Zr 1− x Ti x ) 2O 7 gradually decrease with increasing Ti content at identical temperature levels.

Development of zirconia–magnesia/zirconia–yttria composite solid electrolytes

Composite solid electrolytes were prepared by thoroughly mixing ZrO 2:8 mol% MgO (Z8Mg) and ZrO 2:3 mol% Y 2O 3 (Z3Y) ceramic powders followed by pressing and sintering at 1500 °C/1 h. The properties of the sintered pellets were studied by X-ray diffraction for evaluation of the structural phases by the Rietveld method, by high-temperature dilatometry for analysis of the thermal shrinkage/expansion behavior, and by impedance spectroscopy for determination of the oxide ion conductivity. The x(Z8Mg) + (1 − x)(Z3Y) specimens, x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, are partially stabilized (monoclinic, cubic and tetragonal phases) with density > 94% of the theoretical density and show thermal shock resistance and electrical conductivity values suitable for high-temperature oxygen gas detection. One-end closed tube samples of the composite solid electrolytes were assembled in Pt/Z8Mg + Z3Y/Cr + Cr 2O 3/Pt electrochemical cells for exposure to different levels of oxygen in the 1–850 ppm range. The total electrical conductivity increases for increasing the relative Z3Y content. Addition of Z3Y to Z8Mg (80 wt.%–20 wt.%) suppresses the electronic contribution to the electrical conductivity at 620 °C.

A Promising LaSmZr<sub>2</sub>O<sub>7</sub> Ceramic with Pyrochlore Structure for Thermal Barrier Coatings

Based on La2Zr2O7 ceramic for thermal barrier coatings, LaSmZr2O7 ceramic doped with samarium ion was successfully prepared using solid state reaction method. The pellets were sintered at 1600°C for 10 hours in air. The phase structure, thermal conductivity and thermal expansion coefficient of LaSmZr2O7 ceramic and La2Zr2O7 ceramic were measured by XRD, laser-flash device, high-temperature dilatometry, respectively. The results show that the crystal structure of LaSmZr2O7 ceramic is not affected by the doped samarium ion in the zirconium lattice. The thermophysical results show that the thermal conductivity of the LaSmZr2O7 ceramic is lower than that of La2Zr2O7 ceramic, while the thermal expansion coefficient is higher than that of La2Zr2O7 ceramic. These results indicate that LaSmZr2O7 ceramic or Ln2Zr2O7 ceramics doped with other rare earth ions could be candidate materials for future thermal barrier coatings.

Sintering densification curve: A practical approach for its construction from dilatometric shrinkage data

This article summarizes the usage of high-temperature dilatometry in ceramic processing and powder technology with special attention on the description of the sintering process. A practical method for transformation of dilatometric shrinkage data into densification curves (the dependence of the sample density on sintering temperature or time) is described in detail. A new automatic procedure to recalculate sintering shrinkage data allowing the plot of the densification curve has been developed, which is presented here.