Metal–ceramic interfaces studied with high-resolution transmission electron microscopy

How the core structure of an interface dislocation network depends on both misfit and bond strength across the interface is investigated. It is shown that, in principle at least, it is possible to assess the bond strength by investigating the atomic structure of the dislocation cores. As examples, the misfit-dislocation structures at Ag/Mn₃O₄, Cu/MnO interfaces formed by parallel close-packed planes of Ag or Cu and O obtained by internal oxidation were studied using HRTEM and lattice static calculations. The lattice static calculations are instrumental in indicating the possible dislocation network and their results served as input for HRTEM image simulations which are then compared with experimental HRTEM images. In addition, the influence of dissolution of a segregating element (Sb) in these systems was also studied using HRTEM. The influence on Mn₃O₄ precipitates in Ag is distinct, namely: (i) the initial precipitates, sharply facetted by solely {111}, are changed into a globular shape with sometimes also short {220} and (002) facets, (ii) a partial reduction of Mn₃O₄ into MnO occurs for a part of the precipitates. Further Sb appeared to prevent Oswald ripening of the precipitates.     

Mn2O3掺杂对MnO催化氧还原性能的提高

本文主要研究了纯相MnO以及Mn₂O₃掺杂MnO氧还原催化剂的结构和性能,XRD、SEM、HRTEM等测试表明,氢气还原条件下可以得到Mn₂O₃掺杂的MnO,氨气还原得到纯相MnO。循环伏安(CV)法、Tafal曲线法、时间电流曲线和线性扫描伏安等方法对其催化氧还原性能的研究表明：MnO催化氧还原的峰值电压在-0.1 V到-0.5 V之间,Mn₂O₃的掺杂提高了氧还原峰值电流强度和电压;RDE与RRDE测试表明：Mn₂O₃掺杂MnO的催化氧还原反应主要是4电子反应,而纯相MnO催化氧还原主要是2电子反应。通过本研究表明：Mn₂O₃掺杂提高了MnO的催化氧还原性能,元素不同价态离子的共存提高了催化氧还原反应的活性。     

Enhancement of lithium storage capacity and rate performance of Se-modified MnO/Mn3O4 hybrid anode material via pseudocapacitive behavior

To improve rate and cycling performance of manganese oxide anode material, a precipitation method was combined with thermal annealing to prepare the MnO/Mn₃O₄/SeO_x (x=0, 2) hybrid anode by controlling the reaction temperature of Mn₂O₃ and Se powders. At 3 A/g, the synthesized MnO/Mn₃O₄/SeO_x anode delivers a discharge capacity of 1007 mA·h/g after 560 cycles. A cyclic voltammetry quantitative analysis reveals that 89.5% pseudocapacitive contribution is gained at a scanning rate of 2.0 mV/s, and the test results show that there is a significant synergistic effect between MnO and Mn₃O₄ phases.     

Oxidation behavior of an austenitic stainless FeMnSiCrNi shape memory alloy

The oxidation behavior at 800 °C in air of an austenitic stainless Fe14.29Mn5.57Si8.23Cr4.96Ni (wt.%) shape memory alloy was investigated by thermogravimetry, OM, SEM (EDS), XRD, GAXRD, TEM, and DSC. The results showed that its oxidation process obeys a parabolic law. After 100 h exposure, the oxide scales are composed of outer Mn₂O₃, middle Mn₃O₄, and inner MnCr₂O₄. A ferrite layer exists between the oxidation layer and matrix. The ferrite results from the transformation of oxidation-induced Mn-depleted layer, and its thickness increases with increasing oxidation time. Profuse chi phase precipitates inside matrix and at the interface between matrix and ferrite layer.     

Influence of misfit and interfacial binding energy on the shape of the oxide precipitates in metals: ; Interfaces between Mn3O4 precipitates and Pd studied with HRTEM

Transmission electron microcopy (TEM) revealed Mn₃O₄ precipitates with two types of dominant shape in Pd–3 at.% Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn₃O₄. These observations were compared with earlier observations in the Ag/Mn₃O₄ system and the octahedrons show a relatively larger truncation by (002) in Pd than in Ag. Further, the second type of precipitate shape, comprising about 1/3 of all of the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure, showing an orientation relationship where the tetragonal axes of Mn₃O₄ are parallel to the cube axes of Pd, with the c-axis of Mn₃O₄ as habit plane normal. High-resolution TEM observations revealed the presence of a square misfit dislocation network with line direction 〈110〉 and Burgers vector 1/2〈110〉 at these interfaces with (002)Mn₃O₄6{200}Pd. The general conclusions of the present analysis are: (1) the anisotropy in interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well-defined shapes associated with the Wulff construction; (2) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in mismatch is present and if the matrix is sufficiently stiff; and (3) the stronger coupling strength due to electronic binding effects across the interface in Pd compared with Ag is responsible for formation of the dislocation network structures at larger misfit.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

CO+CO2 mixtures and diffusional transport in MnO

Studies of MnO at high temperatures (1000–1200?C) suggest that diffusional transport can be different when the oxide is exposed to carbon-free environments and to CO/CO₂ mixtures, respectively. In the phase field of MnO near the MnO/Mn₃O₄ boundary it is concluded that defect clusters (consisting of four manganese vacancies + one interstitial manganese ion) are the important defects. Under these conditions it is proposed that carbon can dissolve in the oxide in association with the defect clusters. A defect structure model is proposed to account for the differences in properties. In keeping with this interpretation it is shown that parabolic rate constant for growth of MnO scales in CO/CO₂ mixtures is not only dependent upon the oxygen activity, but also up on the carbon activity in the gas. The electrical conductivity is also affected by changes in the carbon activity.     

Oxidation of copper-manganese alloys under pure oxygen

Oxidation, in oxygen gas at atmospheric pressure, of copper-manganese alloys (Mn content less than 40 at.%) has been investigated between 600 and 850° C. The reaction kinetics, determined by thermogravimetry, follow a parabolic law for alloys having a low manganese content (less than 10 at.% Mn) but are more complex for higher concentrations, particularly in the first stages of the oxidation process. Whereas in the early stages of oxidation the kinetics are controlled by surface reactions which accompany the formation of the different oxide layers, they are later controlled by the diffusion of a mobile species when the parabolic law is followed. In this condition an apparent activation energy may be determined from the rate constants. These energies are of the order of 120–140 kJ mol^–1, comparable with that for oxidation of pure copper (134 kj mol^–1), indicating a similar oxidation mechanism.The oxide layers formed were identified by cross-checking results of X-ray diffraction, electron microprobe analysis, and from glow discharge spectrometry. External layers of CuO and Cu₂O formed on alloys of lower manganese concentration, evolving towards one or several mixed copper-manganese oxide layers with increasing manganese content. Under the external layers, which were weakly adherent to the sample, an internal-oxidation layer formed, which was adherent and consisted of precipitates of Mn₃O₄/MnO dispersed in the copper lattice. For alloys richer in manganese (36 at. % Mn) and at temperatures above 850°C (20 at.% Mn), the internal-oxidation layer evolved into two zones: MnO particles beneath a zone of Mn₃U₄ particles.     

Corrosion Resistance of a High-Silicon Alloy in Metal-Dusting Environments

A Si-containing, high-temperature alloy (Fe–17Cr–9Ni–8Mn–4Si) was exposed to high-carbon activity and low-oxygen partial pressure environments (CO–H₂) over a temperature range from 650 to 950 °C. No metal dusting corrosion was observed in this alloy. The structure and composition of the surface films formed were characterized in detail at the nanometer level. At a temperature of 650 °C, the surface-oxide films formed are made up of an inner, continuous, amorphous-silica (SiO₂) layer and an outer crystalline manganese chromate (MnCr₂O₄) spinel layer with manganese oxide (MnO) crystals on the surface. By contrast, at a higher temperature of 950 °C, a more-complex layered structure is developed, comprising inner, continuous, amorphous SiO₂ and crystalline manganese-silicate (Mn₂SiO₄) layers and an outer crystalline Cr₂O₃/MnCr₂O₄ duplex layer with MnO crystals having variable textures on the surface.     

Effect of MnO doping on the structure, microstructure and electrical properties of the (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics

Mn²⁺-doped (K,Na,Li)(Nb,Ta,Sb)O₃ lead-free piezoelectric ceramics have been prepared by a conventional sintering technique. The effects of Mn²⁺ doping on the phase structure, microstructure and ferro-piezoelectric properties of the ceramics have been evaluated. MnO doping modifies the (K,Na,Li)(Nb,Ta,Sb)O₃ structure, giving rise to the appearance of a TTB-like secondary phase and to changes on the orthorhombic to tetragonal phase transition temperature. The modification of this temperature induces a reduction of the piezoelectric constants, which is accompanied by an increase on the mechanical quality factor. Mn²⁺ ions incorporate into the perovskite structure in different off ways depending on their concentration.     

Nucleation and growth of selective oxide particles on ferritic steel

In the continuous annealing process, steel sheets are annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) containing low partial water pressure (20-50 Pa). Under these conditions, the most oxidizable alloying elements in the steel segregate towards the surface where they form oxide particles. The nucleation and growth of those oxides were examined. Oxide nucleation mainly occurs between 650 and 750 °C. During their growth, the oxides take the form of a spherical cap and are composed of MnO, Mn₂SiO₄ (or MnSiO₃), MnAl₂O₄, SiO₂, Al₂O₃ and B₂O₃. Particle nucleation and growth are favored on grain boundaries.     

Hydrothermal synthesis of MnCO3 nanorods and their thermal transformation into Mn2O3 and Mn3O4 nanorods with single crystalline structure

MnCO₃ nanorods with diameters of 50-150 nm and lengths of about 1-2 μm have been prepared for the first time by a facile hydrothermal method. Mn₂O₃ and Mn₃O₄ nanorods were obtained via the heat-treatment of the MnCO₃ nanorods in air and nitrogen atmosphere, respectively. The morphology and structure of the as-synthesized MnCO₃, Mn₂O₃ and Mn₃O₄ nanorods were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. It was found that the MnCO₃ nanorods are single-crystalline, and their morphology and single-crystalline characteristic can be sustained after thermal transformation into Mn₂O₃ and Mn₃O₄. The corresponding growth directions for MnCO₃, Mn₂O₃ and Mn₃O₄ nanorods were [2 1 4], [1 0 0] and [1 1 2], respectively. When applied as anode materials for lithium ion batteries, the Mn₂O₃ and Mn₃O₄ nanorods exhibited a reversible lithium storage capacity of 998 and 1050 mAh/g, respectively, in the first cycles.     

Alumina Scale Formation on a Powder Metallurgical FeCrAl Alloy (Kanthal APMT) at 900–1,100 °C in Dry O2 and in O2 + H2O

A Rapidly Solidified Powder (RSP) metallurgical FeCrAl alloy, Kanthal APMT, was exposed in dry and humid O₂ for 72 h at 900–1,100 °C. The formed oxide scales were characterized using gravimetry in combination with advanced analysis techniques (SEM, EDX, TEM, XRD, AES and SIMS). The oxide scales were at all exposures composed of two-layered α-Al₂O₃ scales exhibiting a top layer of equiaxed grains and a bottom layer containing elongated grains. A Cr-rich zone, originating in the native oxide present before exposure, separated these two layers. The top α-Al₂O₃ layer is suggested to have formed by transformation of outwardly grown metastable alumina, while the inward-grown bottom α-Al₂O₃ layer had incorporated small Zr-, Hf- and Ti-rich oxide particles present in the alloy matrix. The scale also contained larger Y-rich oxide particles. Furthermore, in the temperature range studied, the presence of water vapour accelerated alloy oxidation somewhat and affected scale morphology.     

Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy

Oxidation of the Cr₂₀Mn₂₀Fe₂₀Co₂₀Ni₂₀ (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn₂O₃ with a thin inner Cr₂O₃ layer; at 700 and 800 °C it was mainly Mn₂O₃ with some Cr enrichment; at 900 °C it was Mn₃O₄. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process.     

Structural and electrochemical characteristics of Li x Mn2O4 spinel synthesized using a microwave-assisted method

The structural and electrochemical features of Li _x Mn₂O₄ spinel synthesized by the microwave-assisted method are discussed. The thermal gravimetric analysis shows that almost 84% of all the initial reactants undergo chemical transformation associated with the weight loss in the course of the microwave-assisted synthesis. The composition of the initial components influences the heating rate negligibly, and the temperature reaches 400°C within around 15 minutes. The X-ray phase analysis shows that the Li _x Mn₂O₄ samples contain impurities such as MnO₂ and Mn₂O₃, the amount of which decreases with the increase in the lithium content. The electrochemical characteristics of the Li _x Mn₂O₄ are strongly influenced by the composition of the starting components, which is particularly noticeable upon high discharge rates.     

Assessment of the Sr-Mn-O system

A thermodynamic assessment of the Sr-Mn-O system is presented. The main practical relevance of this system is that it contains the perovskite phase SrMnO₃, which is the Sr-rich end member of the phase (La,Sr) MnO₃, that finds widespread use as a cathode material for solid oxide fuel cells (SOFCs) and has recently attracted a lot of attention due to its interesting giant magnetoresistive properties. The thermodynamic parameters are optimized by applying the CALPHAD method. The SrMnO_3−z phase exists in two modifications, a layered hexagonal modification at low temperatures and a perovskite modification at high temperatures. Both modifications show considerable oxygen deficiencies, which are modeled using the compound energy model. The sublattice occupation of the phases is (Sr²⁺) (Mn³⁺, Mn⁴⁺)(O²⁻, Va)₃. On reducing Mn⁴⁺ to Mn³⁺, oxygen vacanices are formed. The phase SrMn₃O_6−z also shows an oxygen deficiency, which is modeled in an identical way. The Ruddlesden-Popper phases Sr₂MnO₄ and Sr₃Mn₂O₇, and the phases Sr₇Mn₄O₁₅ and Sr₄Mn₃O₁₀ are modeled as stoichiometric phases. The ionic liquid is modeled using the two-sublattice model for ionic liquids. The stability and thermodynamic data on many of the phases in this system are poorly known. For this reason, some aspects of this assessment must be regarded as tentative.     

Structure and Electrical Properties of Mn-Cu-O Spinels

The study presents the results of structural and electrical conductivity investigations of a Cu_1.3Mn_1.7O₄ spinel obtained using EDTA gel processes. An amorphous gel was synthesized and calcinated for 5 h in air at temperatures of 673, 773, 873, and 973 K. When calcinating the gel at temperatures below 973 K, the obtained powders consisted of two phases—the regular Cu_1.5Mn_1.5O₄ spinel and manganese(III) oxide. At 973 K, Mn₂O₃ was no longer observed, but a new Mn₃O₄ phase appeared in addition to the Cu_1.5Mn_1.5O₄ spinel. Green bodies prepared from these powders were sintered for 2 h in air at 1393 K. The obtained sinters had a porosity of around 12% and were composed predominantly of the spinel phase, with minor amounts of Mn₃O₄ and, in the case of three of four sinters—CuO. Electrical conductivity measurements were taken over the temperature range of 300-1073 K. A change in the character of conductivity of the studied sinters was observed in the range of 400-430 K, and it was associated with an increase in activation energy from 0.20 to 0.56 eV. The electrical conductivity of the studied sinters ranged from 74.8 to 88.4 S cm^?1, which makes the Cu_1.3Mn_1.7O₄ material suitable for application as a protective-conducting coating in IT-SOFC ferritic stainless steel interconnects.     

The interaction of dissolved H with internally oxidized Pd–Rh alloys

Homogeneous fcc Pd–Rh alloys have been internally oxidized in the atmosphere at several temperatures from 1023 to 1123 K forming oxide precipitates within a Pd matrix. As shown from electron diffraction patterns of the internally oxidized Pd_0.97Rh_0.03 alloy, the oxide that forms is a mixed oxide, PdRhO₂. Internally oxidized Pd–Rh alloys can be reduced with H₂ (573–623 K) forming PdRh precipitates within the Pd matrix. This is a technique for segregating components of a substitutional solid solution binary alloy. The segregated alloy can be returned to the homogeneous Pd–Rh alloy by annealing at an elevated temperature and thus, in contrast to the internal oxidation of, e.g., Pd–Al alloys, the process can be readily reversed. The oxidation and (reduction+H₂O loss) were monitored from weight changes.After internal oxidation/reduction “diagnostic” hydrogen isotherms (323 K) were measured to follow the extent of oxidation and to determine the extent of H trapped at the internal interfaces from the positive intercepts along the H/Pd axis in the dilute phase. The H capacities in the steeply rising hydride region of the H₂ isotherms for the internally oxidized alloys are smaller than for Pd unless the Pd in the PdRh precipitates is considered to be inactive for H₂ absorption for the calculation of H/Pd values.In the two-phase region the absorption plateau pressures were significantly greater than for Pd–H, however, they were much closer to the Pd plateau p_H2 than to that of the unoxidized Pd–Rh alloys. The desorption plateaux were very close to those of Pd–H for X_Rh=0.01 to 0.05 alloys and therefore it can be concluded that the matrix is pure Pd whose absorption plateaux are affected by the presence of the precipitates.The techniques of TEM, SEM and SANS (small angle neutron scattering) were employed to examine the alloys after internal oxidation both before and after reduction. SANS confirmed directly that internal precipitates form from the internal oxidation and that they are rather large with an appreciable fraction having diameters greater than 100 nm.     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.