Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Combustion synthesis of Ti3Si1−xAlxC2 solid solutions from TiC-, SiC-, and Al4C3-containing powder compacts

Preparation of the Ti₃Si_1−xAl_xC₂ solid solution with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS) using TiC-, SiC-, and Al₄C₃-containing powder compacts. Due to the variation of reaction exothermicity with sample stoichiometry, the combustion temperature and reaction front velocity decreased with increasing Al content of Ti₃Si_1−xAl_xC₂ for the TiC- and Al₄C₃-added samples, but increased for the samples with SiC. In contrast to the formation of Ti₃(Si,Al)C₂ as the dominant phase for the TiC- and SiC-added samples, TiC was identified as the major constituent in the final products of samples adopting Al₄C₃. In addition, the evolution of Ti₃(Si,Al)C₂ was improved by increasing the Al content of the TiC- and SiC-added powder compacts, but deteriorated considerably upon the increase of Al₄C₃ in the Al₄C₃-containing sample.     

Effects of TiC and Al4C3 addition on combustion synthesis of Ti2AlC

Preparation of the ternary carbide Ti₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of Ti:Al:C = 2:1:1, TiC-containing samples with TiC of 6.67–14.3 mol%, and Al₄C₃-containing samples with Al₄C₃ of 1.96–10 mol%. Effects of TiC and Al₄C₃ addition were studied on combustion characteristics and the degree of phase conversion. Due to the growth of laminated Ti₂AlC grains, the reactant compact was subjected to an axial elongation during the SHS process. Because the addition of TiC and Al₄C₃ led to a decrease in the reaction temperature, the flame-front propagation velocity was correspondingly reduced for the TiC- and Al₄C₃-containing samples when compared with the elemental reactants. Based upon the XRD analysis, formation of Ti₂AlC along with a secondary phase TiC was identified in the synthesized products. The grains of Ti₂AlC are typically plate-like with a size of 10–20 μm and several laminated Ti₂AlC grains form a layered structure. The content of Ti₂AlC yielded from the elemental powder compacts is about 85 wt%. The addition of TiC was found to facilitate the formation mechanism and therefore to enhance the extent of Ti₂AlC conversion approaching 90 wt%. As a result of the reduced exothermicity of the reaction, however, the content of Ti₂AlC decreased slightly in the products synthesized from the Al₄C₃-added samples.     

Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Oxidation of Cr2AlC at 1300 °C in air

High purity, dense Cr₂AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr₂O₃ but Al₂O₃. Since Al was consumed to produce the Al₂O₃, Al-depletion and Cr-enrichment occurred underneath the Al₂O₃ layer. This led to the formation of a Cr₇C₃ layer containing voids. These grew during oxidation, eventually destroying the Cr₇C₃ layer formed on the unoxidized Cr₂AlC matrix.     

Thermal stability of bulk Zr2Al4C5 ceramic at elevated temperatures

Thermal expansion analysis is applied to the study of phase segregation of Zr₂Al₄C₅ at elevated temperatures (20-2000 °C) under flowing argon. Such information would be useful for the detection of phase decomposition temperature and thermal stability of materials. The detected phase-decomposition temperature of Zr₂Al₄C₅ is approximately 1900 °C. The presented thermal expansion analysis results are in good agreement with the X-ray diffraction (XRD) and SEM results. The results indicate that Zr₂Al₄C₅ is susceptible to decomposition through sublimation of high vapor pressure of Al and weaker covalent bonds between ZrC slabs and Al₄C₃ layers. Thus, minimal amounts of Al, Zr₂Al₃C₅, Al₄C₃ and ZrC form on the surface layer. Zr₂Al₃C₅ further decomposes to ZrC_1 − x and Al₄C₃ at 2000 °C. However, the amount of decomposing phase slowly increases, and the structural shape of bulk Zr₂Al₄C₅ ceramic is always kept stable during heat treatment.     

Magnetic properties of Al2O3-Cr2O3 solid solutions

Solid solution ceramics (Al₂O₃)_x(Cr₂O₃)_1−x with different x in the range of 0 < x < 1 were synthesized via traditional ceramic production method. X-ray diffraction results and Rietveld refinements indicated that all samples possessed rhomb-centered structure and continuous solid solutions were synthesized. The samples were composed of irregular grains with several micrometers in diameter. Temperature dependence of magnetization measurements showed monotonous decreasing Néel temperature with increasing x and percolation effect happened with threshold of x = 0.65. As x became higher, weak ferromagnetism was observed in the samples. Field dependence of magnetization measurements further confirmed the weak ferromagnetism in the samples with x = 0.7, 0.8 and 0.9.     

Microstructure and adhesion of Cr3C2-NiCr vacuum plasma sprayed coatings

Vacuum plasma spraying (VPS) was used to spray a Cr₃C₂-NiCr coating of ∼ 150, 300 and 450 μm in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr₃C₂ phase, besides Cr₇C₃ and Cr₂O₃. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m^1/2 when the thickness increases from 150 to 450 μm. Also, it has been found that the parameter Kca_o, determined by linear regression from the Kca versus 1 / e² curve by means of the interfacial indentation model advanced by Chicot et al., has a value of ∼ 9.8 MPa m^1/2.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

Crystallization kinetics of amorphous Cr2AlC thin films

Amorphous Cr₂AlC thin films were produced by room temperature magnetron sputtering on NaCl substrates with subsequent dissolution of the NaCl. The crystallization kinetics of Cr₂AlC was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Two exothermal reactions are observed during DSC up to 1200 °C. Comparing lattice parameters obtained from XRD and ab initio calculations it is suggested that the first reaction is associated with the formation of hexagonal (Cr_,Al)₂C_x, while after the second reaction Cr₂AlC is formed. The activation energy for the phase transformations are 426 and 762 kJ/mol, respectively.     

Comparative study of Cr3C2-NiCr coatings obtained by HVOF and hard chromium coatings

In the present work the corrosion resistance of micro-cracked hard chromium and Cr₃C₂-NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (E_OC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around −0.50 and −0.25 V/(Ag∣AgCl∣KCl_sat) for hard chromium and Cr₃C₂-NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr₃C₂-NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 kΩ cm² were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr₃C₂-NiCr coating the total impedance diminished from around 750 to 25 kΩ cm² as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr₃C₂-NiCr coating than other samples studied.     

Oxidation behavior of arc-sprayed FeMnCrAl/Cr3C2-Ni9Al coatings deposited on low-carbon steel substrates

FeMnCr/Cr₃C₂ and FeMnCrAl/Cr₃C₂ coatings, using Ni9Al arc-sprayed coating as an interlayer on low-carbon steel substrates, were deposited by high velocity arc spraying (HVAS) on the cored wires. The high temperature oxidation behavior of the arc-sprayed FeMnCrAl/Cr₃C₂-Ni9Al and FeMnCr/Cr₃C₂ coatings on the low-carbon steel substrates was studied during isothermal exposures to air at 800 °C. The surface and interface morphologies of the coatings after isothermal oxidation after 100 h were observed and characterized by optical microscopy, field emission scanning electron microscope, energy dispersion spectrum, and X-ray diffraction. The results showed that the oxidation weight gains of the coatings were significantly lower than that of the low-carbon steel substrate. Moreover, the FeMnCrAl/Cr₃C₂-Ni9Al coating registered the lowest oxidation rate. This favorable oxidation resistance is due to the Al and Cr contents of the aforementioned coating that inhibits the generation of Fe and Mn oxides. This is attributed to the interdiffusion between the substrates and the Ni9Al arc-sprayed coating, which can convert the mechanical bonding between substrates and coatings into a metallurgical one, thereby inhibiting the oxidation of interface between the low-carbon steel and the coating.     

Variation of microstructure and composition of the Cr2AlC coating prepared by sputtering at 370 and 500 °C

Cr₂AlC coating was deposited at 370 and 500 °C by D.C. magnetron sputtering from an as-synthesized bulk Cr₂AlC target. The phase composition and preferential orientation of the coating were investigated using XRD, and the microstructure of the coating was characterized by TEM. Results indicated that Cr₂AlC coating with a strong (110) preferential orientation could be obtained. The coating microstructure was clearly affected by the deposition temperature. At 370 °C, the deposited coating possessed a triple-layered structure with an α-(Cr, Al)₂O₃ inner layer, an amorphous intermediate layer and a crystalline Cr₂AlC outer layer. However, the coating deposited at 500 °C had a single-layered structure consisting of crystalline Cr₂AlC layer. The growth mechanism of the Cr₂AlC coating at different deposition temperatures is discussed.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering

The influence of the addition of 0.3, 0.5 and 0.7 wt.% VC on the density, microstructure and mechanical properties of WC-Cr₃C₂-11 wt.% Co with 0, 0.2, 0.4 and 0.6 wt.% Cr₃C₂ hard metals prepared by spark plasma sintering (SPS) at a temperature of 1200 °C (5 min, 40 MPa) was investigated. Microstructure analysis revealed that the WC grain size in the sintered hard metals was strongly influenced by the VC and Cr₃C₂ content. With the addition of inhibitors and the increased amount of Cr₃C₂, the density is reduced, and on the contrary, the addition of VC as an inhibitor contributes to promoting the densification. The combined addition of Cr₃C₂ and VC could strongly reduce the WC grain growth to about 350 nm. Observation suggests that the fracture of WC-Co cemented carbide is brittle and intergranular. The amount of added VC/Cr₃C₂ should be controlled in a certain range. Samples with an appropriate proportion of VC/Cr₃C₂ added exhibit higher hardness which can be up to 1938 HV₃₀. Toughness, too, can reach 16.34 MPa m^1/2.     

Spectroscopic and crystal field analysis of absorption and photoluminescence properties of red phosphor CaAl12O19:Mn modified by MgO

The spectroscopic properties of a series of red phosphors with general composition of CaAl₁₂O₁₉:yMn⁴⁺ and (Ca_1−xAl₁₂O₁₉, xMgO):yMn⁴⁺ (x = 0-1, y = 0.001-1.5%) synthesized by a modified solid state method in air have been investigated in detail. Addition of MgO is necessary for Mn⁴⁺ charge compensation and leads to the formation of separate crystal phases of Al₂O₃ and MgAl₂O₄, which was confirmed by the XRD studies. Enhancement of Mn⁴⁺ luminescence with increasing content of MgO was observed and a mechanism for explanation of this phenomenon is suggested. For an analysis of the crystal phases and luminescent efficiency of the phosphors in the prepared series, crystal field calculations of the Mn⁴⁺ energy levels have been performed. This theoretical approach allowed for assigning the observed excitation and emission spectra. Red shift of the Mn⁴⁺ luminescence with increasing concentration of Mg ions is explained from the point of view of enhanced nephelauxetic effect after doping.     

Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.     

High-Q microwave dielectrics in the (Mg1−xZnx)Al2O4 (x = 0-0.1) system

The microwave dielectric properties and the microstructures of (Mg_1−xZn_x)Al₂O₄ (x = 0-0.1) ceramic system prepared by the conventional solid-state route were investigated. The forming of spinel-structured (Mg_1−xZn_x)Al₂O₄ (x = 0-0.1) solid solutions was confirmed by the XRD patterns and the measured lattice parameters, which linearly varied from a = b = c = 8.0815 Å for MgAl₂O₄ to a = b = c= 8.0828 Å for (Mg_0.9Zn_0.1)Al₂O₄. By increasing x, the Q × f of (Mg_1−xZn_x)Al₂O₄ can be tremendously boosted from 82,000 GHz at x = 0 to a maximum of 156,000 GHz at x = 0.05. The Zn substitution was effective in reducing the dielectric loss without detrimental effects on the ?_r and τ_f values of the ceramics.     

Competition between interfacial and interlayer exchange couplings in Co/Cr2O3/Fe trilayers

The temperature dependence of competition between interlayer and interfacial couplings is observed at different temperatures in Co (3 nm)/Cr₂O₃ (t)/Fe (10 nm) trilayers with t = 3 nm, 6 nm, 15 nm and 25 nm, respectively. The interlayer coupling enhances and the interfacial coupling weakens with increasing temperature. The balanceable temperature between interfacial and interlayer couplings shifts to low temperatures with increasing spacer thickness. Furthermore, the competition between interfacial and interlayer couplings greatly affects the magnetotransport properties of the trilayers. The negative magnetoresistance and the minimum resistance corresponding to balanceable temperature are found in trilayers.