Full strength and toughness recovery after repeated cracking and healing in bone-like high temperature ceramics.

Bones of humans and animals combine two unique features, namely: they are brittle yet have a very high fracture toughness linked to the tortuosity of the crack path and they have the ability to repeatedly heal local fissures such that full recovery of overall mechanical properties is obtained even if the local bone structure is irreversibly changed by the healing process. Here it is demonstrated that Ti2AlC MAX phase metallo-ceramics also having a bone-like hierarchical microstructure and also failing along zig-zag fracture surfaces similarly demonstrate repeated full strength and toughness recovery at room temperature, even though the (high temperature) healing reaction involves the local formation of dense and brittle alumina within the crack. Full recovery of the fracture toughness depends on the healed zone thickness and process zone size formed in the alumina reaction product. A 3-dimensional finite element method (FEM) analysis of the data obtained from a newly designed wedge splitting test allowed full extraction of the local fracture properties of the healed cracks.

Butane Dry Reforming Catalyzed by Cobalt Oxide Supported on Ti2 AlC MAX Phase.

MAX (Mn+1 AXn ) phases are layered carbides or nitrides with a high thermal and mechanical bulk stability. Recently, it was shown that their surface structure can be modified to form a thin non-stoichiometric oxide layer, which can catalyze the oxidative dehydrogenation of butane. Here, the use of a Ti2 AlC MAX phase as a support for cobalt oxide was explored for the dry reforming of butane with CO2 , comparing this new catalyst to more traditional materials. The catalyst was active and selective to synthesis gas. Although the surface structure changed during the reaction, the activity remained stable. Under the same conditions, a titania-supported cobalt oxide catalyst gave low activity and stability due to the agglomeration of cobalt oxide particles. The Co3 O4 /Al2 O3 catalyst was active, but the acidic surface led to a faster deactivation. The less acidic surface of the Ti2 AlC was better at inhibiting coke formation. Thanks to their thermal stability and acid-base properties, MAX phases are promising supports for CO2 conversion reactions.

Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy.

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

Pyrococcus furiosus 4Fe-ferredoxin, chemisorbed on gold, exhibits gated reduction and ionic strength dependent dimerization.

Pyrococcus furiosus ferredoxin is a small metalloprotein that shuttles electrons between redox enzymes. In its native 4Fe-4S form the protein is highly thermostable. In addition to three cluster-ligating cysteines, two surface cysteine residues (C21 and C48) are present. We used the reactivity of these surface thiols to directly immobilize ferredoxin on a bare gold electrode, with an orientation in which the cluster is exposed to solution. Voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) studies established the immobilization of the 4Fe form. Native and recombinant wild-type ferredoxins were compared with the C48S, C21S, and C21S/C48S mutants. The variants with one and two surface cysteines can be directly chemisorbed on bare gold. Cyclic voltammetry demonstrated that the reduction potentials are similar to those in solution. The interfacial electron transfer kinetics revealed that the reduction is gated by the interconversion between two oxidized species. AFM images showed that dimers are chemisorbed at low ionic strength, while monomers are present at high ionic strength. XPS spectra revealed the presence of S, Fe, C, N, and O at the surface, which are assigned to the corresponding atoms in the peptide and the cofactor. Analysis of the sulfur spectrum corroborates that both C21 and C48 form gold-thiolate bonds. Moreover, two inorganic sulfide and two iron species were identified, suggesting an inhomogeneous charge distribution in the 4Fe-4S cluster. In conclusion, P. furiosus ferredoxin can be directly and vectorially chemisorbed on gold with retention of its properties. This may provide a biocompatible electrode surface with docking sites for redox enzymes.

The effect of Ge incorporation on the Brønsted acidity of ZSM-5.

The effect of the isomorphous substitution of some of the Si atoms in ZSM-5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)3SiO)3-Al-O(H)-T-(OSi(OH)3)3, with T=Si or Ge, and ((HO)3SiO)3-Al-O(H)-Si-(OGe(OH)3)(OSi(OH)3)2, ii) a 31P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X-ray photoelectron spectroscopy (XPS) study of ZSM-5 and Ge-ZSM-5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T-atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge-ZSM-5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM-5. This structural relaxation is not achievable in Ge-ZSM-5 with a low Ge content, which therefore has a slightly lower acidity than ZSM-5. The NMR study indicates no difference between the Brønsted acidity of ZSM-5(47) and Ge(0.09)ZSM-5(36). Instead, evidence for the presence of a substantial amount of Ge-OH groups in the Ge-containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM-5(47), instead, the results can be best interpreted by assuming the presence of additional Ge-OH and Si-OH groups near the surface of the Ge(0.08)ZSM-5(47) sample.