MAX phases

The MAX phases are classified according to their n-number for example, Ti2AlN (n = l) is referred to as 211, Ti3SiC2 n = 2) as 312, and TL1AIN3 (n = 4) as 413. The unit cells are hexagonal (space group PSs/ntmc) and contain near close-packed transition metal carbides and nitrides, interleaved with planes of pure A-atoms, every third layer for 211, every fourth layer for 312, and every fifth layer for 413. 

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Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... 

Fig. 4. The HNCO-TROSY experiment for recording solely interresidual 1HN, 15N, 13C correlations in 13C/15N/2H labelled proteins. All 90° (180°) pulses for the 13C and 13C spins are applied with a strength of 2/ /l5 (p/ /3), where 2 is the frequency difference between the centres of the 13C and 13Ca regions. All 13Ca pulses are applied off-resonance with phase modulation by Q. A = 1/(4/hn) Tn = l/(4/NC ) S = gradient + field recovery delay 0 < k < TN/z2,max- Phase cycling i = y 4>2 = x, — x + States-TPPI 03 = x 0rec = x, — x.

Fig. 14. SeqHNCA-TROSY experiment for establishing sequential 1HN(i), 15N(i), 13Ca(i— 1) correlations in 13C/15N/2H enriched proteins. Durations of transfer delays A = 1/(4/Hn) 2Ta = 20-27 ms, depending on rotational correlation time of protein 2Tc = 5-7 ms S = gradient + field recovery delay 0 < k < Ta/t2,max- Phase cycling i = y (j>2 = y, — y + States-TPPI 0 = x 0ret. = x, — x. Semi-selective decoupling of 13C spins is attained using a SEDUCE-1 decoupling sequence.95...

Hoffman, E.N., Carbide-derived carbon from MAX phases and their separation applications. PhD Thesis, Drexel University, Philadelphia, PA, 2006. 

In summary, the difiusion of K into C o films produces a minimum resistivity phase that is barely metallic with p = 2.2 milliohm-cm at composition K3.oo o.o5C6o, and a high resistivity (max) phase at composition Ks.ooio.osCjo. Intermediate compositions yield activated conduction with x-dependent activation energy, which can be explained by a simple model with immiscible phases where the metallic phase maximally avoids percolation. The recognition that the normal state has a granular microstructure will also be important in understanding superconductivity in thin films. 

Another material of this kind is tungsten disulphide, originally developed by NASA for aerospace applications, now also applicable to specialty industries [58]. Ti3SiC2 is a thermodynamically stable, nano-layered, ternary carbide and part of a family of over 50 ternary carbides and nitrides, the MAX phases [62]. These phases are a new class of solids possessing unique combinations of properties they are readily machinable, relatively soft for ceramics, but elastically stiff, and electrically and thermally conductive. They combine the good properties of both metals and ceramics that could lead to this technology contributing to future lubricant developments. 

Dr. Barsoum is currently a Distinguished Professor at Drexel University. He and his research group were the first to fabricate and fully characterize an important new class of machinable ternary carbides and nitrides, the M v+iAXv (so-called MAX) phases. Since 1996, Dr. Barsoum and his collaborators have published over 60 refereed papers on these ternary carbides and nitrides, including ones in Nature and Science. Dr. Barsoum has authored or co-authored over 100 refereed publications, 6 US patents awarded and 4 pending. In 2000 he was awarded a Humboldt-Max Planck Research award for Senior US Research Scientists. He spent his 2000-2001 sabbatical year at the Max Planck Research Institute in Stuttgart. Germany. 

The MAX phase TijSiC has been synthesized from starting powder mixtures which do not include pure titanium. The presence of pure titanium in a powder is problematic because of its oxidizing, and in the form of a finely divided powder, explosive nature. The aim of this study was to evaluate the synthesis of bulk polycrystalline samples of TisSiCj from a starting powder mixture which is more suited for large scale production. 

Titanium silicon carbide MAX phase was synthesized by pressureless sintering of ball milled TiC and Si powders of six different compositions. The sintering reactions were evaluated in situ by dilatometer analysis under flowing argon gas. The as-sintered samples were evaluated using mainly x-ray diffraction (XRD) analysis. This study showed that titanium carbide, silicon carbide and titanium disilicide were present as intermediate or secondary phases in the samples. 

Our results indicate that TiSij is an intermediate phase to the formation of Ti3SiC2 when excess Si is present. The exce.ss of silicon also proved beneficial for the synthesis of the MAX (diase and there is a Si content which is optimal with respect to the maximum MAX phase content of the final product. The TijSiCj was found to decompose into TiC and gaseous Si at hi temperatures. 

Figure 3 shows x-ray diffractograms of the samples sintered under vacuum for 2 hours and 30 min in 1250 C. The B sample with the lowest amount of silicon in the starting powder differs from the other samples. It is the only sample which does not contain silicon carbide (SiC) and titanium disilicide (TiSi2). All samples contained titanium silicon carbide MAX phase (TisSiCj) and titanium carbide (TiC). 

Figure 4 Effect of initial silicon content on the amount of titanium silicon carbide MAX phase (Ti3SiC2) and titanium carbide (TiC) obtained in the final product after sintering for 2.5 hours at...

Much more MAX phase was achieved in these samples with excess silicon and most MAX phase was obtained with the 3TiC/2.6Si starting powder. This amount of silicon is likely to compensate for a certain amount of evaporation in such a way that the sample composition is shifted most closely to the Ti3SiC2 phase area. 

As no intermediate phases or unexpected secondary phases were observed in sample B, with no excess silicon, the MAX phase forming reaction is assumed to be a direct displacement reaction as proposed by Radhakrishnan et al. ... 

As the sample holders and the crucibles used in this study were made of graphite, carbon was readily available during the sintering reactions. All samples with excess silicon had comparable and relatively low amounts of TiSij. Previous results have shown that the silicide is an intermediate phase to MAX phase formation lixim these powders. TiSi2 is then likely consumed in a second, TijSiCj forming reaction ... 

Excess silicon is beneficial for the production of TisSiCj. The iargest amount of the MAX phase was achieved in the samples with a TiC/Si ratio of 3 2.6, i.e. about 46% Si. The excess silicon is likely to compensate for losses due to evaporation. 

The MAX phase decomposes into TiC and gaseous Si at temperatures above 1500 C. 

Michel W. Barsoum and Tamer El-Raghy, The MAX Phases Unique New Carbide and Nitride Materials, American Scienlisl 89 (2001). 

Materials for Extreme Environments Ultrahigh Temperamre Ceramics (UHTCs) and Nanolaminated Ternary Carbides and Nitrides (MAX Phases)... 

In this chapter, the physical and mechanical properties of bulk M +iAX phases are summarized. The chapter is subdivided into six sections, in the first two of which the structure and bonding (including theoretical) characteristics of these materials are reviewed. Their elastic properties are summarized in Section 7.3., while in Sections 7.4 and 7.5 their electrical properties and thermal properties are reviewed, respectively. The mechanical properties are dealt with in Section 7.6. What is not detailed in this chapter are the MAX-phase thin films, for which several excellent groups, especially in Europe [6-9], have provided details. Another topic that has encouraged much activity, but which will not be reviewed here, is that of the processing of the MAX phases rather, at this point emphasis is placed on the properties of bulk materials. 

Table 7.1 lists most of the known MAX phases - most of which were discovered by Nowotny et al. [10] during the 1960 s - together with their lattice parameters and theoretical densities. The A-group elements are mostly IIIA and IVA, and all but five compounds are 211s. The most versatile element is Al, as it forms nine compounds, including two nitrides, one 312 phase, and four 413 phases. Ga also forms nine 211 phases, six of which are carbides and three are nitrides. 

Given the close chemical and structural similarities of the MAX and MX phases, much can be learned about the former from what is known about the latter. For example, for the most part the M—M distances in the MAX phases are strongly correlated to, and almost equal to, the same distance in the MX phases [1,10]. Like the MX compounds [27, 28], it is useful to consider the ternaries to be interstitial compounds in which the A- and X-atoms fill the interstitial sites between the M atoms. In such a scheme, the c-parameter of the 211 phases - comprised of four M-layers per unit cell - should be approximately four times the a-parameter. Similar arguments for the 312 and 413 phases, with six and eight M layers per unit cell, respectively, predict ratios of about 6 and about 8 [29]. The actual c/a ratios of approximately 4, 5.8-6, and 7.8, are consistent with this simple structural notion. 

Currently, much effort is being expended in theoretical modeling of the MAX phases [30-37]. Not unlike the MX phases, the bonding in the MAX phases is... 

In the M2AIC phases, there is a net transfer of charge from the A-group element to the X-atoms [32]. Whether this is true of other MAX phases as well awaits further study. 

As shown in Table 7.2 and Figure 7.3, the M +iAX phases are, for the most part, elastically quite stiff. When combined with the fact that the densities of some of MAX phases are relatively low (ca. 4.1-5.Ogcm see Table 7.1) their specific stiffness... 

Figure 7.2 Typical total and partial DOS at p in Ti2AC compounds with increasing p electron for select MAX phases, (a) Total DOS forTi2AlN, concentrations [40]. The DOS are color-coded Ti3AIN2 (which does not exist) andTi4AIN3 [38] as indicated in the bottom panel (c) Partial and (b) Partial density of states ofTi, A and C atoms total DOS for Ti3SiC2 [30],...

Until the discovery of the MAX phases, the price paid for high specific stiffness values has been a lack, or at least a difficulty, of machinability. It is important, therefore, to note that one of the most characteristic properties of the MAX phases is the ease with which they can be machined, with nothing more sophisticated than... 

Table 7.2 Young s modulus ( ), shear modulus (C), and Poisson ratio (v) of select MAX phases.

Poisson s ratios for most of the MAX phases hover around 0.2, which is lower than the 0.3 of Ti, and closer to the 0.19 of near-stoichiometric TiC. 

Figure 7.3 Comparison of expert mental and theoretical bulk (6) and Young s ( ) moduli of select MAX phases.

An important, but subtle, factor influencing the B-values of the MAX phases is their stoichiometry, and more specifically their vacancy concentrations. This effect is best seen in the B-values of Ti2AlN, where theory and experiment show a decrease in lattice parameters as C is substituted for by N. Given that the lattice parameters shrink, it is not surprising that theory predicts that this substitution should increase the B-value, when, in fact, experimentally it decreases with increasing N-content [42]. This paradox is resolved when it is appreciated that B is a strong function of vacancies, and that the addition of N results in the formation of vacancies on the A1 and/or N sites. As discussed below, the presence of these defects also influence other properties [43, 44]. 

In contradistinction to other layered soUds sucdi as graphite and mica (which are elastically quite anisotropic), the MAX phases are mildly so. For example. Holm et al. predicted that C33 and Cn forTi3SiC2 wotdd be almost equal [31], a prediction that was later confirmed experimentally [74]. The same is true of some of the M2AIC... 

The Al-containing MAX phases and Ti3SiC2 have another useful attribute, namely that their elastic properties are not a strong function of temperature. For example, at 1273 K the shear and Young s moduli of Ti3AlC2 are about 88% of their room-temperature values [50, 67]. In that respect, their resemblance to the MX binaries is notable. 

For the Al-containing MAX phases, for example, in some cases, the M—A1 bonds are stronger - at least in the c-direction - than the M—X bonds (see Section 7.5.2). 

The electrical conductivities of the MAX phases, like those of their M and MX counterparts, are metallic-like in that the resistivity, q, increases linearly with increasing temperature, T (Figure 7.5a). The temperature dependencies of (g — Qo), where Qq is the residual resistivity at 0 K, show clearly that dg/dT depends on the transition metal in the MAX phase (Figure 7.5a). The actual room temperature resistivities, Qrx, are listed in Table 7.3, together with their residual resistivity ratios (RRRs), defined as 6rt/64k. vvhere Q4k is the resistivity at 4 K. The RRR values are a measure of the quality of a given material, with higher RRR values indicating less defective solids. 

In order to understand the electronic transport of a solid, it is necessary to know its charge carrier densities and mobilities. For most solids, the Hall coefficient (Rh) is used to determine the concentration and sign of the majority charge carriers. Once known, the mobility is determined from the conductivity values, a. The MAX phases, however, are unlike most other metallic conductors in that their Hall and Seebeck coefficients are quite small - in some cases vanishingly small - and a weak function of temperature [52, 84—87]. Furthermore, the magnetoresistance (MR) (Aq/q = q(B) — q(B = 0)/q(B = 0)]), where B, the applied magnetic field intensity, is positive, parabolic, and nonsaturating. Said otherwise, the MAX phases are compensated conductors, and a two-band conduction model is needed to understand their electronic transport. In the low-field, B, limit of the two-band model, the... 

Figure 7.5 Temperature-dependence of (p - p<,) for select MAX phases, where is resistivity at 4 K (b) Functional dependenceofelectronic charge mobilities at4 Kon RRRandthedensityofstates at the Fermi level, N( f).

Table 7.3 Summaty of electrical transport parameters calculated from the resistivity ( ), Hall coefficient (Rh) and magnetoresistance coefficient (a), for select MAX phases. Unless otherwise noted, = [ip = is assumed. Note that this approach can, and does, result in slightly different values than assuming n—p. The residual resistivity ratio is listed in column 4. 

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