Room‐Temperature Carbide‐Derived Carbon Synthesis by Electrochemical Etching of MAX Phases

Porous carbons are widely used in energy storage and gas separation applications, but their synthesis always involves high temperatures. Herein we electrochemically selectively extract, at ambient temperature, the metal atoms from the ternary layered carbides, Ti₃AlC₂, Ti₂AlC and Ti₃SiC₂ (MAX phases). The result is a predominantly amorphous carbide‐derived carbon, with a narrow distribution of micropores. The latter is produced by placing the carbides in HF, HCl or NaCl solutions and applying anodic potentials. The pores that form when Ti₃AlC₂ is etched in dilute HF are around 0.5 nm in diameter. This approach forgoes energy‐intensive thermal treatments and presents a novel method for developing carbons with finely tuned pores for a variety of applications, such as supercapacitor, battery electrodes or CO₂ capture.     

Driving force and mechanism for spontaneous metal whisker formation

The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free technology. To date, this 50+ year old problem has resisted interpretation. Herein we show that the driving force is essentially a reaction between oxygen and the sprouting metal. The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains our observations on In and Sn whiskers and many past observations. The solution is in principle simple: diffusion of oxygen into the metal must be prevented or slowed down. This was demonstrated by coating the active surfaces with a polymer coating.     

Nanoscale continuum calculation of basal dislocation core structures in graphite

In this work, we calculate the core structures of basal dislocations in graphite in a nanoscale continuum framework. The model consists of a stack of buffered Kirchhoff plates where the plates represent the covalent interactions within individual graphene sheets and the buffer layers represent the secondary interactions between them. In the mid-plane of the buffer layers, cohesive surfaces are introduced to account for the nonlinear deformations due to basal dislocations. The cohesive surface separation is governed by using an empirical 4-8 Lennard–Jones potential. Meanwhile, their relative shear sliding is governed by using a newly proposed empirical periodic stacking-fault potential. With these potentials, the core structures of full dislocations and partials are calculated and examined. It is shown that the full dislocations automatically split into partials that repel each other. The core sizes of individual partials, measured between peak stresses, are about 5?nm wide for the edge component and slightly narrower for the screw component. Since these sizes are about 10 times the lattice constant, they lend credence to our continuum model of basal dislocation cores in graphite. It is also shown that when the dislocations are densely packed on the same glide plane, i.e. in a pile-up, with spacing one to two times the core size, the split partials retain their individual identity with well-defined and well-separated stress peaks. Meanwhile, the membrane normal stresses in the graphene sheets rise considerably at the pile-up tips which, in turn, may provoke further deformation and damage modes such as kinking and delamination.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

First PAC experiments in MAX-phases

MAX-phases are hexagonal ternary carbides and nitrides with the general formula: M _n?+?1 AX _n and n = 1 to 3. ¹¹¹In was implanted into the two MAX compounds Ti₂InC and Zr₂InC. Based on the general knowledge of previous ¹¹¹In implantations one expects to find the probes on the indium lattice-site in these compounds. First experiments on the annealing behaviour and the thermal stability of the indium-containing MAX-phases are reported. The observed EFGs are interpreted and first PAC-measurements under compressive stress are shown.     

Self-assembled growth of nanostructural Ge islands on bromine-passivated Si(111) surfaces at room temperature

We have deposited relatively thick (∼60 nm) Ge layers on Br-passivated Si(111) substrates by thermal evaporation under high vacuum conditions at room temperature. Ge has grown in a layer-plus-island mode although it is different from the Stranski-Krastanov growth mode observed in epitaxial growth. Both the islands and the layer are nanocrystalline. This appears to be a consequence of reduction of surface free energy of the Si(111) substrate by Br-passivation. The size distribution of the Ge nanoislands has been determined. The Br-Si(111) substrates were prepared by a liquid treatment, which may not produce exactly reproducible surfaces. Nevertheless, some basic features of the nanostructural island growth are reasonably reproducible, while there are variations in the details of the island size distribution.     

The Ti3AlC2 MAX Phase as an Efficient Catalyst for Oxidative Dehydrogenation of n‐Butane

Dehydrogenation or oxidative dehydrogenation (ODH) of alkanes to produce alkenes directly from natural gas/shale gas is gaining in importance. Ti₃AlC₂, a MAX phase, which hitherto had not been used in catalysis, efficiently catalyzes the ODH of n‐butane to butenes and butadiene, which are important intermediates for the synthesis of polymers and other compounds. The catalyst, which combines both metallic and ceramic properties, is stable for at least 30 h on stream, even at low O₂:butane ratios, without suffering from coking. This material has neither lattice oxygens nor noble metals, yet a unique combination of numerous defects and a thin surface Ti_1?yAl_yO_2?y/2 layer that is rich in oxygen vacancies makes it an active catalyst. Given the large number of compositions available, MAX phases may find applications in several heterogeneously catalyzed reactions.     

Studies on the potentiometric thallium(III)-selective carbon paste electrode and its possible applications

Construction, performance characteristics and applications of a carbon paste thallium(III) ion-selective electrode are described. The electrode, which is based on ion-associate compounds formed between cetylpyridinium and chlorothallate(III) complexes dissolved in tricresyl phosphate as pasting liquid, showed near-Nernstian response over the concentration range of 5.8 × 10^–6– 2.9 × 10^–3 mol/L. Potentiometric titrations of thallium(III) with cetylpyridinium chloride were affected by higher concentrations of excess halides, probably due to the formation of higher halogenothallates.