M. W. Barsoum

Nanoporous carbide derived carbon with tunable pore size

Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk1. Although large pores can be produced and well controlled in a variety of materials2, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. To date, major efforts in the field of porous materials have been directed towards control of the size, shape and uniformity of the pores. Here we demonstrate that porosity of carbide-derived carbons (CDCs)3,4,5,6,7,8,9 can be tuned with subångström accuracy in a wide range by controlling the chlorination temperature. CDC produced from Ti3SiC2 has a narrower pore-size distribution than single-wall carbon nanotubes or activated carbons; its pore-size distribution is comparable to that of zeolites. CDCs are produced at temperatures from 200–1,200 °C as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.

Low temperature dependencies of the elastic properties of Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2

In this article we report on the temperature dependencies of the longitudinal and shear sound velocities in Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2. The velocities are measured using a phase sensitive pulse-echo ultrasonic technique in the 90–300 K temperature range. At room temperature, Young’s, ERT, and shear, μRT, moduli and Poisson’s ratio of Ti4AlN3 are 310±2, 127±2 GPa, and 0.22, respectively. The corresponding values for Ti3AlC2 are 297.5±2, 124±2 GPa, and 0.2. Both moduli increase slowly with decreasing temperature and plateau out at temperatures below ≈125 K. A least squares fit of the temperature dependencies of the shear and Young’s moduli of Ti4AlN3 yield: μ/μRT=1−1.5×10−4(T−298), and E/ERT=1−0.74×10−4(T−298), for T>125 K. The corresponding relationships for Ti3Al1.1C1.8 are: μ/μRT=1−1.2×10−4(T−298), and E/ERT=1−0.84×10−4(T−298) for T>125 K. The acoustic Debye temperatures calculated for Ti4AlN3 and Ti3AlC2, as well as Ti3SiC2, are all above 700 K, in agreement with values calculated from low tempe...

Fatigue-crack growth and fracture properties of coarse and fine-grained Ti3SiC2

An experimental study of fracture and cyclic fatigue-crack growth behavior was made in a reactively hot-pressed monolithic Ti{sub 3}SiC{sub 2} ceramic with both fine (3--10 {mu}m) and coarse-grained (50--200 {mu}m) microstructures. Whereas the fine-grained microstructure exhibited a steady-state (plateau) R-curve fracture toughness of K{sub c} {approximately} 9.5 MPa{radical}m, the coarse-grained Ti{sub 3}SiC{sub 2} exhibited a K{sub c} {approximately}16 MPa{radical}m. The latter value is though to be one of the highest fracture toughnesses ever observed in a monolithic, non-transforming ceramic, consistent with the profusion of crack-bridging processes active in the crack wake. Moreover, fatigue-crack growth thresholds, {Delta}K{sub TH}, were comparatively high for ceramic materials, as indicated by the coarse-grained microstructure which had a threshold {Delta}K{sub TH} of {approximately}9 MPa{radical}m. Such fatigue crack growth was associated with substantial evidence for wear degradation at active bridging sites behind the crack tip.

Elastic and electronic properties of select M2AX phases

In this letter we report on the low-temperature specific heat of several M2AX phases: Ti2AlC, V2AlC, V2AsC, Nb2SnC, Ti2AlN, Hf2InC, Nb2AlC, and Cr2AlC. The Debye temperatures are quite high. The density of states at the Fermi level, N(EF) varies from ≈1.4 (eV formula unit)−1 to 6 (eV formula unit)−1. Ab initio calculations show that N(EF) is dictated by the transition metal d–d bands; the A-group element has little effect. We also measured the velocity of sound in V2AlC, V2AsC, Ti2AlC, and Ti2AlN. The average bulk modulus of these materials is over 100 GPa, with a high of ≈140 GPa for Ti2AlN. Our theoretical calculations correctly predict the trend in both the density of states and the bulk modulus, although there is some disagreement in the actual values.

High-pressure x-ray diffraction study of Ta4AlC3

Using a synchrotron radiation source and a diamond anvil cell, we measured the pressure dependence of the lattice parameters of a recently discovered phase, Ta4AlC3. This phase adopts a hexagonal structure with the space group P63∕mmc; at room conditions, the a and c parameters are 3.087(5) and 23.70(4)A, respectively. Up to a pressure of 47GPa, no phase transformations were observed. Like Ta2AlC, but unlike many related phases such as Ti4AlN3, Ti3SiC2, Ti3GeC2, and Zr2InC, the compressibility of Ta4AlC3 along the c and a axes are almost identical. The bulk modulus of Ta4AlC3, 261±2GPa, is ≈4% greater than that of Ta2AlC. Both, however, are ≈37% lower than the 345±9GPa of TaC.

Structure of Ti4AlN3—a layered Mn+1AXn nitride

Recent high resolution transmission electron microscopy and electron probe X-ray microanalysis show that the compound originally thought to be Ti3Al2N2 is Ti4AlN3. In this paper we report on the crystal structure determination by Rietveld refinement on neutron and X-ray powder diffraction data. Ti4AlN3 crystallizes with a hexagonal unit cell, space group P63/mmc, and with lattice parameters a = 2.9880(2) and c = 23.372(2) A. The stacking sequence is such that every four layers of Ti atoms is separated by a layer of Al atoms. The N atoms occupy octahedral sites between the Ti atoms making up a network of corner shared octahedra. This compound is closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely, M2AX and M3AX2, where M is an early transition metal, A is a A-group element and X is either C and/or N.

Oxidation of Ti n + 1AlX n ( n = 1 ­ 3 and X = C , N): II. Experimental Results

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds Ti 2 AlC, Ti 2 AlC 0.5 N 0.5 , Ti 4 AlN 2.9 , and Ti 3 AlC 2 are reported. For the first two compounds, in the 1000-1100°C temperature range and for short times (20 h) the oxidation kinetics are parabolic. The parabolic rate constants are k x (m 2 /s) = 2.68 × 10 5 exp -491.5 (kJ/mol)/RT for Ti 2 AlC. and 2.55 × 10 5 exp - 458.7 (kJ/mol)/RT for Ti 2 AlC 0.5 N 0.5 . At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective. For the second pair, at short times ( 10) times. The presence of oxygen also reduces the decomposition (into TiX x and Al) temperatures of Ti 4 AlN 2.9 and Ti 3 AlC 2 from a T> 1400°C, to one less than 1100°C.

Long Time Oxidation Study of Ti3SiC2 , Ti3SiC2 / SiC , and Ti3SiC2 / TiC Composites in Air

We report herein on the oxidation kinetics and morphology of the oxide phases that form after long term (up to 1500 h) oxidation in ambient air of fine and coarse-grained samples of Ti 3 SiC 2 , Ti 3 SiC 2 , with 30 vol % TiC and Ti 3 SiC with 30 vol %SiC in the 875-1200°C temperature range. In all cases, the oxidation resulted in a duplex scale, an outer rutile and an inner rutile/ silica layer. At 875°C, and up to at least 100 h, the oxidation kinetics of the Ti 3 SiC 2 /30 vol % TiC samples are parabolic: at 925°C, the oxidation kinetics of the Ti 3 SiC 2 /30 vol % SiC are subparabolic, up to at least 500 h. The oxidation kinetics of all other runs are initially parabolic, but at times >30 h they become linear. The reason for the transition is not entirely clear, but could he due to the buildup of stresses in the external oxide scales. Comparison with previously published results indicate that the rate limiting step, when the oxidation kinetics are parabolic, is most probably the inward diffusion of oxygen and the simultaneous outward diffusion of titanium. The results also strongly suggest that the activation energies for diffusion of oxygen and titanium in rutile are almost identical over at least the 1000-1200°C temperature range.

Synthesis and characterization of Hf2PbC, Zr2PbC and M2SnC (M=Ti, Hf, Nb or Zr)

Abstract Predominantly single phase (92–94 vol.%), fully dense samples of Hf2SnC, Zr2SnC, Nb2SnC, Ti2SnC, Hf2PbC and Zr2PbC were fabricated by reactively HIPing the stoichiometric mixture of the corresponding elemental powders in the 1200–1325°C temperature range for 4–48 h. The latter two, fabricated here for the first time, required a further anneal of 48–96 h to increase the volume fraction of ternary phases. Hf2PbC and Zr2PbC are unstable in ambient atmospheres at room temperature. As a family these compounds are good electrical conductors; the lowest and highest values of the electrical conductivities were, respectively, 2.2×106 (Ω.m)−1 for Hf2SnC and 13.4×106 (Ω.m)−1 for Hf2PbC. The Vickers hardness values range from 3 to 4 GPa. All compounds are readily machinable. The Youngs moduli of Zr2SnC, Nb2SnC and Hf2SnC are, respectively, 178, 216 and 237 GPa. The thermal coefficients of expansion, TCEs, of the ternaries scale with those of the corresponding binaries, and are relatively low for such readily machinable solids. The lowest TCE belonged to Nb2SnC [(7.8±0.2)×10−6 K−1], and the highest to Ti2SnC [(10±0.2)×10−6 K−1]. The TCEs of Hf and Zr containing ternaries cluster around (8.2±0.2)×10−6 K−1. All the synthesized ternary carbides were found to dissociate into the transition metal carbide and the A-group element in the 1250–1390 °C temperature range.

Reaction of Al with Ti3SiC2 in the 800–1000°C temperature range

Abstract In this work, we report on the reaction of molten aluminum with Ti3SiC2 in the 800–1000°C temperature range. The reaction kinetics are linear and follow the relation: Kx (m s−1)=7.16 exp−(207 kJ/RT). The reaction layer consists of two interconnected or interpenetrating networks of TiC0.67 and molten Al. The samples preserve their original shape and dimensions after the reaction. Some TiC particles, as well as TiAl3 platelets are sometimes observed in the Al bath. The rate-limiting step is believed to be the dissolution of the Si from the Ti3SiC2 into the molten Al.