Vacancy-dependent stability of cubic and wurtzite Ti1-x Al x N

Effect of chemical composition, defect structure, and stress state on the elastic properties of (V1-x Al x )1-y N y

For (V_1-x Al _x )_1-yN _y an extensive and theoretically unexplained spread in experimentally obtained elastic moduli ranging from 254 to 599 GPa is reported in literature. To identify its origin, the effect of chemical composition (0  ⩽  x  ⩽  0.75), non-metal to metal ratio (N/M-ratio: 0.48  ⩽  y   ⩽  0.52), and stress state (-6  ⩽  σ  ⩽  2 GPa) on the elastic modulus at room temperature is studied sytematically by density functional theory employing the Debye-Grüneisen model. As the Al concentration is increased from x  =  0 to x  =  0.75, strong Al-N sp³d² hybridization causes an increase in elastic modulus of 26%. The effect of the N/M-ratio on the elastic properties is also Al content dependent. As y  is increased from y   =  0.50 to y   =  0.52, decreasing bond distance upon vacancy formation causes an anomalous increase in the elastic modulus of 6% for V_1-yN _y, while a decrease in elastic modulus of up to 5% occurs for (V_1-x Al _x )_1-yN _y. A stress state variation from  +2 to  -6 GPa increases the elastic modulus e.g. for (V_0.5Al_0.5)_0.5N_0.5 by 70 GPa and hence 13% due to shifts in density of states towards lower energies implying bond strengthening. Thus, it is suggested that the extensive spread of 58% in reported elastic moduli for (V_1-x Al _x )_1-yN _y can at least in part be rationalized based on variations in chemical composition, off-stoichiometry induced point defects, and stress state.

Ab initio inspired design of ternary boride thin films

The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB₂, VB₂, etc.) tend to be chemically more stable in the AlB₂ structure type, whereas late transition metal diborides (WB₂, ReB₂, etc.) are preferably stabilized in the W₂B_5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB₂ or W₂B_5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB₂, TcB₂, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.

Crystallite size-dependent metastable phase formation of TiAlN coatings

It is well known that surface energy differences thermodynamically stabilize nanocrystalline γ-Al2O3 over α-Al2O3. Here, through correlative ab initio calculations and advanced material characterization at the nanometer scale, we demonstrate that the metastable phase formation of nanocrystalline TiAlN, an industrial benchmark coating material, is crystallite size-dependent. By relating calculated surface and volume energy contributions to the total energy, we predict the chemical composition-dependent phase boundary between the two metastable solid solution phases of cubic and wurzite Ti1-xAlxN. This phase boundary is characterized by the critical crystallite size d critical . Crystallite size-dependent phase stability predictions are in very good agreement with experimental phase formation data where x was varied by utilizing combinatorial vapor phase condensation. The wide range of critical Al solubilities for metastable cubic Ti1-xAlxN from x max = 0.4 to 0.9 reported in literature and the sobering disagreement thereof with DFT predictions can at least in part be rationalized based on the here identified crystallite size-dependent metastable phase formation. Furthermore, it is evident that predictions of critical Al solubilities in metastable cubic TiAlN are flawed, if the previously overlooked surface energy contribution to the total energy is not considered.

Hard magnetism in structurally engineered silica nanocomposite

Creation of structural complexity by simple experimental control will be an attractive approach for the preparation of nanomaterials, as a classical bottom-up method is supplemented by a more efficient and more direct artificial engineering method. In this study, structural manipulation of MCM-41 type mesoporous silica is investigated by generating and imbedding hard magnetic CoFe2O4 nanoparticles into mesoporous silica. Depending on the heating rate and target temperature, mesoporous silica undergoes a transformation in shape to form hollow silica, framed silica with interior voids, or melted silica with intact mesostructures. Magnetism is governed by the major CoFe2O4 phase, and it is affected by antiferromagnetic hematite (α-Fe2O3) and olivine-type cobalt silicate (Co2SiO4), as seen in its paramagnetic behavior at the annealing temperature of 430 °C. The early formation of Co2SiO4 than what is usually observed implies the effect of the partial substitution of Fe in the sites of Co. Under slow heating (2.5 °C min(-1)) mesostructures are preserved, but with significantly smaller mesopores (d100 = 1.5 nm). In addition, nonstoichiometric CoxFe1-xO with metal vacancies at 600 °C, and spinel Co3O4 at 700 °C accompany major CoFe2O4. The amorphous nature of silica matrix is thought to contribute significantly to these structurally diverse and rich phases, enabled by off-stoichiometry between Si and O, and accelerated by the diffusion of metal cations into SiO4 polyhedra at an elevated temperature.