Reactive spark plasma sintering of Ti3SnC2, Zr3SnC2 and Hf3SnC2 using Fe,Co or Ni additives

This work studied the effect of adding 10 at% Fe, Co or Ni to M-Sn-C mixtures with M = Ti, Zr or Hf on MAX phases synthesis by reactive spark plasma sintering. Adding Fe, Co or Ni assisted the formation of 312 MAX phases, i.e., Ti₃SnC₂, Zr₃SnC₂ and Hf₃SnC₂, while their 211 counterparts Ti₂SnC, Zr₂SnC and Hf₂SnC formed in the undoped M-Sn-C mixtures. The lattice parameters of the newly synthesized Zr₃SnC₂ and Hf₃SnC₂ MAX phases were determined by X-ray diffraction. Binary MC carbides were present in all ceramics, whereas the formation of intermetallics was largely determined by the selected additive. The effect of adding Fe, Co or Ni on the MAX phase crystal structure and the microstructure of the produced ceramics was investigated in greater detail for the case of M = Zr. A mechanism is herein proposed for the formation of M₃SnC₂ MAX phases.     

First‐principles investigations on elevated temperature elastic and thermodynamic properties of ZrB2 and HfB2

As promising candidates for ultrahigh temperature applications, high‐temperature properties, which are quite rare and fragmentary, have great significance to ZrB₂ and HfB₂. In this work, thermodynamic and mechanical properties of ZrB₂ and HfB₂ from 0 K to 2000 K were investigated by a combination of first principles calculations and quasi‐harmonic approximations. The ground‐state properties, including lattice parameters, elastic constants, phonon dispersion, and mode‐Grüneisen parameters are calculated. The theoretical thermal expansion, elastic and thermodynamic properties at elevated temperatures show good agreement with experiments. By discussing Grüneisen parameters anisotropy, the mechanism for the thermal expansion anisotropy of ZrB₂ and HfB₂ is uncovered. The influence of direction‐dependent sound velocities on the anisotropy of thermal conductivity is also discussed.     

A High‐Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

Herein, we report on the crystal structures of Nb₂AlC and TiNbAlC—actual composition (Ti_0.45,Nb_0.55)₂AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb₂AlC sample in the a and c directions are, respectively, 7.9(5) × 10^?6 and 7.7(5) × 10^?6 K^?1 on one neutron diffractometer and 7.3(3) × 10^?6 and 7.0(2) × 10^?6 K^?1 on a second diffractometer. The respective values for the (Ti_0.45,Nb_0.55)₂AlC composition—only tested on one diffractometer—are 8.5(3) × 10^?6 and 7.5(5) × 10^?6 K^?1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.     

Elastic,mechanical, electronic,and defective properties of Zr–Al–C nanolaminates from first principles

By means of first principles calculations, Zr–Al–C nanolaminates have been studied in the aspects of chemical bonding, elastic properties, mechanical properties, electronic structures, and vacancy stabilities. Although the investigated Zr–Al–C nanolaminates show crystallographic similarities, their predicated properties are very different. For (ZrC)_nAl₃C₂ (n = 2, 3, 4), the Zr–C bond adjacent to the Al–C slab with the C atom intercalated in the Zr layers is the strongest, but the one with the C atom intercalated between the Zr layer and Al layer is the weakest. In contrast, the situation for (ZrC)_nAl₄C₃ (n = 2, 3) is just the opposite. For Zr–Al–C nanolaminates, the calculated bulk, shear and Young's modulus increase in the sequence of Zr₂AlC < Zr₃AlC₂ < Zr₂Al₄C₅ < Zr₃Al₄C₆ < Zr₂Al₃C₄ < Zr₃Al₃C₅ < Zr₄Al₃C₆. The (ZrC)_nAl₃C₂ (n = 2, 3, 4) series exhibit the most outstanding elastic properties. In the presence of the external pressure, the bulk and shear moduli exhibit a linear response to the pressure, except for Zr₂AlC and Zr₃AlC₂, both of which belong to the so‐called MAX phases. The two materials also exhibit very distinct properties in the strain‐stress relationship, electronic structures and vacancy stabilities. As the intercalated Al layers increase, the formation energy of V_Zr and V_Al increases, while the formation energy of V_C decreases.     

Pulsed 2‐Dimensional Raman Spectroscopy on Microdroplets

In this study, we used a technique based on pulsed Raman spectroscopy to determine the average concentration in the liquid phase and concentration distribution in the gas phase surrounding desorbing linear arrays of binary microdroplets. For this study, monodisperse microdroplets consisting of acetylene dissolved in acetone were generated by a modified vibrating orifice aerosol generator (VOAG). The 2‐dimensional concentration fields of acetylene desorbed from the droplets were determined with a spatial resolution of about 40 μm both along the direction of the droplet chain and perpendicular to it. The detection limits for the gas and liquid phase concentrations were about 100 ppm and 1 mmol/l, respectively.     

Layered Structure Induced Anisotropic Low‐Energy Recoils in Ti3SiC2

Low‐energy recoil events in Ti₃SiC₂ are studied using ab initio molecular dynamics simulations. We find that the threshold displacement energies are orientation dependent because of anisotropic structural and/or bonding characteristic. For Ti and Si in the Ti–Si layer with weak bonds that have mixed covalent, ionic, and metallic characteristic, the threshold displacement energies for recoils perpendicular to the basal planes are larger than those parallel to the basal planes, which is an obvious layered‐structure‐related behavior. The calculated minimum threshold displacement energies are 7 eV for the C recoil along the direction, 26 eV for the Si recoil along the direction, 24 eV for the Ti in the Ti–C layer along the direction and 23 eV for the Ti in the Ti–Si layer along the direction. These results will advance the understanding of the cascade processes of Ti₃SiC₂ under irradiation and are expected to yield new perspective on the MAX phase family that includes more than 100 compounds.     

Anisotropic corrosion of Ti2AlC and Ti3AlC2 in supercritical water at 500 ºC

Textured and untextured M_n+1AX_n compounds, Ti₂AlC and Ti₃AlC₂, namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 °C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c-plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (00l) faces consist of strongly bonded Ti₃C₂ and Ti₂C layers in Ti₃AlC₂ and Ti₂AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO₂ from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.     

Oxidation resistance of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases: A comparison

Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phase powders are densified using Spark Plasma Sintering (SPS) technique to obtain dense bulk materials. Oxidation tests are then performed over the temperature range 800°C-1000°C under synthetic air on the two different materials in order to compare their oxidation resistance. It is demonstrated that, in the case of the Ti₃Al_0.8Sn_0.2C₂ solid solution, the oxide layers consist in TiO₂, Al₂O₃, and SnO₂. The presence of Sn atoms in the A planes of the solid solution leads to an easy diffusion of Sn out of the MAX phase which promote the formation of the nonprotective and fast growing SnO₂ oxide. Moreover, the small Al/Ti atom's ratio promotes the growth of a nonprotective rutile-TiO₂ scale as well. In the case of the Ti₃AlC₂ MAX phase, the oxide layer consists in a protective alumina scale; a few TiO₂ grains being observed on the top of the Al₂O₃ layer. The parabolic oxidation rate constants are about 3 orders of magnitude smaller for Ti₃AlC₂ compared to Ti₃Al_0.8Sn_0.2C₂.     

Fabrication and thermal stability of two-dimensional carbide Ti3C2 nanosheets

Two-dimensional Ti₃C₂ nanosheets were obtained by exfoliation of synthesized Ti₃AlC₂ powders in 40% HF solution at room temperature. The influence of etching time on the synthesis, structural evolution during heating, and thermal stability of as-prepared Ti₃C₂ nanosheets were studied. The graphene-like structure of as-prepared Ti₃C₂ nanosheets was confirmed by XRD and SEM. TG–DTA and elemental analysis suggested that the OH and F groups attached on the surface of Ti₃C₂ nanosheets could be eliminated by heat treatment. It is noteworthy that Ti₃C₂ nanosheets are thermal stable up to 1200 °C in Ar atmosphere. In the heating process, a small quantity of TiO₂ anatase emerges at 500 °C and then reacts with Ti₃C₂ to form a solid solution of TiC_xO_1−x (0<x<1) with the increase of temperature.     

Thermal shock behavior of Ti2AlC from 200°C to 1400°C

The thermal shock behavior of Ti₂AlC synthesized by means of self‐propagating high‐temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti₂AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al₂O₃. Although the flexural strength decreases quickly in the temperature range of 300°C‐500°C, no discontinuous decrease in the retained strength is observed in Ti₂AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti₂AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared with the first quench. Quenching at 300°C and above, voids after the pullout of grains and cracks are present, which however are absent in the un‐quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Electronic structure and thermal properties of Sm3+-doped La2Zr2O7: First-principles calculations and experimental study

By applying the first-principles calculation, the electronic structure, mechanical and thermal properties of Sm³⁺-doped La₂Zr₂O₇ were investigated, and experiments were carried out to verify the theoretical results. As the Sm³⁺ doping rate increases, the lattice parameters decrease while the theoretical density increases. The doping of Sm³⁺ promotes the transformation from pyrochlore structure to defective fluorite structure. The Young's modulus of pure La₂Zr₂O₇ shows obvious anisotropy, while it tends to be isotropy with the doping of Sm³⁺. The calculated theoretical hardness is positively correlated with the doping rate, yet due to the solid solution strengthening effect, the materials with doping rate of 50% get the highest hardness. Based on the calculations and experiments, the optimal Sm³⁺ doping rate of La₂Zr₂O₇ is 50%. LaSmZr₂O₇ has hardness of 11.35 GPa, the thermal conductivity of 1.35 W/(m·K) at 1173 K, and the thermal expansion coefficient of 10.12 × 10⁻⁶/K at 1173 K. The above results indicate that LaSmZr₂O₇ has good mechanical and thermal properties, which provides new ideas for the selection of thermal barrier coating materials.     

FTIR and Raman Spectroscopy of Carbon Nanoparticles in SiO2, ZnO and NiO Matrices

Coatings of carbon nanoparticles dispersed in SiO₂, ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings with a view to apply these as selective solar absorber surfaces in solar thermal collectors. Cross-sectional high resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Raman spectroscopy was used to estimate the grain size and crystallite size of the carbon clusters of the composite coatings. X-HRTEM studies revealed a nanometric grain size for all types of samples. The C–SiO₂, C–ZnO and C–NiO coatings contained amorphous carbon nanoparticles embedded in nanocrystalline SiO₂, ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The thermal emittances of the samples were 10% for C–SiO₂, 6% for the C–ZnO and 4% for the C–NiO samples. The solar absorptances were 95%, 71% and 84% for the C–SiO₂, C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have the best solar selectivity behaviour followed by the C–ZnO, and last were the C–SiO₂ samples. Raman spectroscopy studies revealed that both the C–ZnO and C–NiO samples have grain sizes for the carbon clusters in the range 55–62 nm and a crystallite size of 6 nm.     

Molten salt dynamic sealing synthesis of MAX phases (Ti3AlC2, Ti3SiC2 et al.) powder in air

Since the synthesis of non-oxidized ceramic and alloy powders requires both high temperature and oxygen insulation conditions, here we demonstrate a cost-efficient molten salt sealing/shielded synthesis method with dynamic gas tightness. Compared to conventional synthesis method, it can prevent the loss of reaction materials at high temperature, cut off the connection between reacting material and outside air, and does not require long-time ball milling mixing treatment or provision of applied pressing before or during heating. Only low-cost salts (e.g., NaCl, KCl), a few minutes of raw material mixing, and regular heating molds are required to obtain high-purity (>96 wt%), micron-sized Ti₃AlC₂ and Ti₃SiC₂ powders with narrow size distribution, which significantly decreased the complexity and production costs in the synthesis process. The effect of temperature and raw material content on the products were investigated. The mechanism of diffusion reaction between reactants in molten salt environment was analyzed. The new method developed here was also applicable to Ti₂AlC, V₂AlC and Cr₂AlC MAX phases, as well as provided new ideas for the preparation of other MXenes precursors with certain stoichiometric ratios, air-sensitive materials and nanopowders.     

Synthesis and characterization of a new (Ti1-ε,Cuε)3(Al,Cu)C2 MAX phase solid solution

A new (Ti_1-εCu_ε)₃(Al,Cu)C₂ MAX phase solid solution has been synthesized by sintering at 760 °C compacted Ti₃AlC₂-40 vol.% Cu composite particles produced by mechanical milling. Using XRD and TEM-EDXS, it has been demonstrated that Cu can enter the crystallographic structure of the Ti₃AlC₂ MAX phase, whereas a Cu(Al,Ti) solid solution is also formed during thermal treatment. TEM-EELS analyses have demonstrated that Cu is mainly located on the A site of the MAX phase. The composition of the MAX phase solid solution, determined after selective chemical etching of the Cu(Al,Ti) matrix, by analyzing the filtrate and the solid phase using ICP-OES end EDXS methods respectively, is (Ti_0.93–0.97Cu_0.07–0.03)₃(Al_0.49–0.52Cu_0.51–0.48)C_2.     

First‐principles investigation of the thermodynamic stability of MB2 materials surfaces (M = Ti/Zr/Hf)

Some of the renewed interest in transition metal diborides (MB₂, M = Ti/Zr/Hf) arises from their potential use as matrices in ultrahigh‐temperature ceramic matrix composites (UHTCMCs). Crucial to the understanding of such composites is the study of the fiber/matrix interfaces, which in turn requires a deep knowledge of the surface structures and the thermodynamics of the matrix material. Here we investigate the surface stability of MB₂ compounds by first‐principles calculations. Five surfaces are stabilized when going from a M‐rich to a B‐rich environment, respectively (0001)_M, (100)_M, (101)_B(M), (113)_M and (0001)_B, with the highly stable (100)_M, (101)_B(M) and (113)_M surfaces being discussed here for the first time. The mechanism behind the surface stability is analyzed in terms of cleavage energy, surface strain and surface bonding states. Our results provide important information for a better understanding of the most likely surfaces exposed to the fibers in UHTCMCs, thereby for the construction of reliable interfaces and ultimately UHTCMCs models.     

First‐principles investigation of chemical stability and proton conductivity of M‐doped BaZrO3 (M=K,Rb, and Cs)

First‐principles calculations were carried out to examine the chemical stability and proton conductivity of A‐site‐doped BaZrO₃. In a previous study, we investigated these two properties of B‐site‐doped BaZrO₃. [RSC Adv. 3 [10] 3333‐3341 (2013)] Here, we investigated BaZrO₃ doped with K, Rb, and Cs at A‐sites. We found that K‐doped BaZrO₃ showed the highest proton conductivity and chemical stability among the examined A‐site dopants with respect to the carbonate formation reaction. Remarkably, there is a lack of a trade‐off relationship between chemical stability and proton conductivity in A‐site‐doped BaZrO₃.     

Combined experimental and ab initio based determination of the thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3

The thermal expansion of La_0.5Sr_0.5Co_0.25Fe_0.75O₃ (LSCF55) is investigated both by first principles phonon calculations combined with the quasi‐harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as α_V,GGA = 50.34 × 10^?6 K^?1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in situ high‐temperature X‐ray diffractometer (HT‐XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with α_a,hex = 10.89 × 10^?6 K^?1 and α_c,hex = 21.18 × 10^?6 K^?1 is obtained. The volumetric thermal expansion coefficient is measured as α_V,HT‐XRD = 43.17 × 10^?6 K^?1. In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield α_{l,bar specimen} = 17.37 × 10^?6 K^?1 and α_{V,bar specimen} = 52.11 × 10^?6 K^?1.     

Experimental and DFT insights into elastic,magnetic, electrical,and thermodynamic properties of MAB-phase Fe2AlB2

The elastic, magnetic, electrical, and thermodynamic properties of bulk polycrystalline Fe₂AlB₂ are reported, with further theoretical insights provided through first-principles calculations. DFT simulations, along with an appropriate empirical method to treat magnetic contributions, reproduce well the experimental heat capacity, phonon thermal conductivity, and thermal expansion. The shear, Young's and bulk moduli all decrease linearly from 107.7, 264.5, and 162.4 GPa at 298 K to 91.4, 227.7, and 149.6 GPa at 1273 K, respectively, while the Poisson's ratio shows a weak dependence on temperature, hovering around 0.23-0.26. The mechanical damping, Q⁻¹, is found to be a weak function of temperature up to 973 K, but above this temperature increases sharply, peaking at a specific temperature that becomes higher with increasing resonant frequency. The phonon contribution plays a dominant role in the measured heat capacity from 4.2 to 1000 K, while the magnetic contribution becomes significant around the Curie temperature (301 K). The electronic contribution increases with temperature above 100 K. Furthermore, the standard enthalpy, entropy, and Gibbs free energy are calculated as 16.60 kJ/mol, 92.92 J/(mol·K) and −11.09 kJ/mol, respectively. The thermal conductivity increases linearly with increasing temperature from RT to 1173 K, up to around 13.0 W/(m·K) at 1323 K after hovering at around 7.8 W/(m·K) in the low-temperature range. The electrons play a dominant role in heat conduction, while the phonons contribute only a little to the thermal conductivity. The dilatometric coefficient of thermal expansion of Fe₂AlB₂ is measured as 13.5 × 10⁻⁶ K⁻¹ in the temperature range RT-1348 K.     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.