High Temperature Transmission Electron Microscopy Research Articles

Improving the Long-Term Stability of PbTe-Based Thermoelectric Modules: From Nanostructures to Packaged Module Architecture.

Nanostructured lead telluride PbTe is among the best-performing thermoelectric materials, for both p- and n-types, for intermediate temperature applications. However, the fabrication of power-generating modules based on nanostructured PbTe still faces challenges related to the stability of the materials, especially nanoprecipitates, and the bonding of electric contacts. In this study, in situ high-temperature transmission electron microscopy observation confirmed the stability of nanoprecipitates in p-type Pb0.973Na0.02Ge0.007Te up to at least ∼786 K. Then, a new architecture for a packaged module was developed for improving durability, preventing unwanted interaction between thermoelectric materials and electrodes, and for reducing thermal stress-induced crack formation. Finite element method simulations of thermal stresses and power generation characteristics were utilized to optimize the new module architecture. Legs of nanostructured p-type Pb0.973Na0.02Ge0.007Te (maximum zT ∼ 2.2 at 795 K) and nanostructured n-type Pb0.98Ga0.02Te (maximum zT ∼ 1.5 at 748 K) were stacked with flexible Fe-foil diffusion barrier layers and Ag-foil-interconnecting electrodes forming stable interfaces between electrodes and PbTe in the packaged module. For the bare module, a maximum conversion efficiency of ∼6.8% was obtained for a temperature difference of ∼480 K. Only ∼3% reduction in output power and efficiency was found after long-term operation of the bare module for ∼740 h (∼31 days) at a hot-side temperature of ∼673 K, demonstrating good long-term stability.

In situ high temperature transmission electron microscopy on melting mechanism of secondary copper smelting slag

A new technique for investigations in nano-scale is applied to secondary copper smelting slag with the aim to understand melting mechanism and phase development within the material: in situ transmission electron microscopy at high temperatures. Heating the material step by step with accompanying EDX analysis allows detailed tracking of phase transformations, including changes in chemical composition. The behavior of Cu nano-spheres as well as Fe and Zn in different phases is described in detail. Evaporation of Cu, Fe and Zn is compared to available thermodynamic data, showing relatively good agreement for Cu and Fe, but different results for Zn. The observation of liquid Fe formation is a new phenomenon, which needs further investigation. High temperature transmission electron microscopy is found to be a suitable method for certain research questions, when taking limiting factors like vacuum conditions and electron beam effects into account.

In situ high-temperature transmission electron microscopy investigation of dynamic element migration for Sm-Co-Fe-Cu-Zr sintered magnets

The investigation delved into the dynamic migration of elements and the evolution of interface structures within the Sm(CobalFe0.09Cu0.09Zr0.03)7.2 magnet as temperature increased, employing high-temperature transmission electron microscopy. Notably, the Cu distribution exhibited temperature susceptibility. Elevating the temperature from room temperature (RT) to 400 °C led to a notable expansion in the distribution width of Cu, ranging from 28.5 to 50.8 nm. Simultaneously, the average Cu content at the cell boundaries decreased from 14.4 to 11.6 at.%. According to the high-temperature in-situ experimental results of the transmission electron microscopy thin-film samples, the derived diffusion coefficients, D300°Cand D400°C, were determined as 0.0525 nm2/s and 1.414 nm2/s, respectively. The significantly higher D400°C compared to D300°C underscores the considerable influence of temperature on Cu element diffusion behavior. The simulation results demonstrate that the trend of magnetic properties variation at different temperatures closely aligns with the observed trend. Subsequent calculation of the temperature coefficient of intrinsic coercivity (β) reveals that |β300–400°C| is notably higher than |βRT-300°C|. This suggests a substantial impact of Cu diffusion-induced deterioration on the β. Moreover, micromagnetic simulation results delineated the reverse magnetization process for magnets at varying temperatures, revealing that the presence of the Cu diffusion region diminished the pinning effect of the cell boundary phase. Furthermore, a wider Cu diffusion region facilitated easier magnetic moment reversal at the cell boundary phase, resulting in a marked deterioration of intrinsic coercivity. This elucidates that the formation of the Cu diffusion region stands as one of the primary reasons for the sharp decline in magnetic properties of Sm2Co17-type magnets under high-temperature conditions.

Better understand the crystallization dynamics of ZrCu TFMGs: Benefits of combining global and local in situ approaches

This work describes crystallization mechanisms in a model ZrCu thin film metallic glass, synthesized through magnetron sputtering. Global-scale characterization techniques, including differential scanning calorimetry and X-ray diffraction, are compared with local-scale characterization obtained through in situ transmission electron microscopy during isothermal heating. This multi-scale approach establishes the crystallization sequence of ZrCu thin film metallic glasses. Furthermore, it highlights the role of oxidation as a nucleation site, initiating the crystallization process. Once initiated, crystallization progresses as a propagating front, scanning and transforming the amorphous matrix. The combination of both global and local approaches yields consistent key thermodynamic values. Additionally, monitoring the advancing crystallization front during in situ high-temperature transmission electron microscopy provides access to crucial kinetic parameters, such as diffusion coefficients.

Multiscale in-situ observations of the micro- and nanostructure of a PH 13-8 Mo maraging steel during austenitization

The aim of the present work is to elaborate on the microstructural evolution of a PH 13-8 Mo maraging steel during austenitization by using multiscale in-situ techniques, which range from high-temperature electron backscatter diffraction and high-energy X-ray diffraction to high-temperature transmission electron microscopy. In order to supplement these in-situ experiments, samples quenched from different temperatures after isochronal heating are subjected to an in-depth microstructural characterization by means of atom probe tomography.The results indicate diffusion-dominated kinetics for the α’ to γ transformation in the investigated heating rate range. Moreover, high-temperature electron backscatter diffraction experiments confirm the occurrence of the austenite memory effect. In addition to the inheritance of grain size and crystallographic orientation of the prior austenite grains, a lath-like substructure remains in austenite, which consists of thermally stable, geometrically necessary dislocations. It is suggested that these dislocations play the key role for enabling recrystallization at higher temperatures without prior cold deformation. This phenomenon is evidenced for the first time in a PH 13-8 Mo maraging steel. However, it is believed that excessive amounts of carbides at austenite grain boundaries or an alteration of the dislocation structure could impede this abnormal recrystallization.

Geometric defects induced by strain relaxation in thin film oxide superlattices

Functional thin film superlattices with stability in extreme environments can lead to transformative performance in optical and thermal applications such as thermophotovoltaics. In this work, key issues associated with defects that prevent layer-by-layer growth in epitaxial, low-miscibility oxide superlattices are investigated. Layer protrusions, approximately 8 nm wide and 3 nm thick, arise from a strain relaxation mechanism in 8 nm bilayer superlattices of Ba(Zr0.5Hf0.5)O3/MgO and propagate through the subsequent superlattice layers forming an inverted pyramid structure that is spatially phase offset from the matrix. The density and size of these defects scales with the number of interfaces in the sample, indicating that surface roughness during growth is a significant factor in the formation of these defects. In situ high temperature transmission electron microscopy (1000 °C, in vacuo) measurement reveals that phase decomposition of Ba(Zr0.5Hf0.5)O3 and decoherence of the superlattice is nucleated by these defects. This work highlights that achieving optimum growth conditions is imperative to the synthesis of single-crystalline superlattices with sharp interfaces for optimized performance in extreme environments.

In situ study of phase transformations in electron beam additive manufactured Ti-6Al-4 V titanium alloy by high temperature synchrotron X-ray diffraction and TEM

The microstructure and phase composition of the EBAM Ti-6Al-4 V alloy samples were studied by the methods of in situ high temperature synchrotron X-ray diffraction and transmission electron microscopy (TEM) during heating from room temperature to a temperature of 1373 K. In the initial state, the main phase of the EBAM Ti-6Al-4 V alloy the samples was the α phase, the second phases were β + α″. The volume fraction of the β phase does not exceed 5%, α″ - 3%. It was found that the new α″ phase was formed at the temperature of 1073 K due to the phase transition α→α″. The α″ phase volume fraction increased as the heating temperature increased to the temperature of 1223 K and then decreased with a further increase of the temperature due to the α″→β phase transition. The dependences of the lattice parameters of the α, β and α″ phases on the heating temperature were revealed. It was established that the crystal lattice of the α phase was distorted during heating. The α phase passed into the unstable state. An increase of the c/a parameters ratio was accompanied by the formation of the α″ and β phases.

In Situ High-Temperature TEM Observation of Inconel Corrosion by Molten Chloride Salts with N2, O2, or H2O

In situ transmission electron microscopy (TEM) diffraction and imaging techniques are used to monitor and quantify corrosion of Inconel-625 by pure molten chloride salts (MgCl2 − NaCl − KCl) at 500 °C–800 °C in 1.0 atm inert N2 or pure O2, or by salts which are controllably hydrated in a high vacuum chamber. The isothermal corrosion rate R in inert N2 increases from 203 ± 30 μm year−1 at 700 °C to 463 ± 30 μm year−1 at 800 °C. An oxygen ambient causes a six-fold increase to R = 1261 ± 170 μm year−1 at 700 °C. Salt hydration dramatically accelerates corrosion to R> 3 × 105 μm year−1 at 700 °C while it leads to a more moderate R = 95 ± 20 and 486 ± 30 μm year−1 at 500 °C and 600 °C, respectively. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600 °C, where corrosion is aggravated by increased generation and solubility of corrosive HCl gases. Hence, to reduce rate of corrosion it is important to both avoid incorporation of H2O into the system at each stage and ensure proper flushing of the system before increasing the temperature beyond 600 °C. Compositional analysis of the corroded cells indicate that corrosion in O2 ambient is dominated by oxidation of metals by O2 gas dissolved in the chloride melt, but corrosion in H2O ambients is caused by chlorination of metals by dissolved HCl gas and MgOH+ ions. So, to reduce rate of corrosion, steps should be taken to tailor chloride melt compositions that has low solubility for HCl and O2. All of our corroded samples exhibit passive-protective oxide layers of Cr, Mg, and Ni. In addition, distinct volatile compounds of Ni, Mo and Cr involving NiCl2, (Na,K)2MoO4 and CrO2(OH)2 are detected in N2, H2O, and O2 ambients, respectively. We believe that corrosion acceleration can be minimized by minimizing formation of volatile by-products or promoting reactions that could convert these volatile compounds to solid phases, as these volatile compounds led to destruction of protective oxide layers.

Synthesis of graphene–like phases by laser ablation of micro-crystalline graphite in water suspension

The aim of our work was to obtain graphene-like phases (defected graphene, graphene oxide and reduced graphene oxide) as fine suspensions by applying pulsed laser ablation (PLA) to microcrystalline graphite suspension in water medium. The fundamental (λ = 1064 nm), second (λ = 532 nm), third (λ = 355 nm) as well as the fourth harmonic (λ = 266 nm) of a Nd:YAG laser system, 15 ns pulse duration and 10 Hz pulse repetition rate were used in PLA processes. The morphology of the particles was studied by transmission electron microscopy (TEM). Their phase composition and structure were explored by Raman and X-ray Photoelectron spectroscopies, grazing incidence X-ray diffractometry (GIXRD) and high temperature powder X-ray diffraction (HTXRD) and TEM. It can be assumed, in accordance to the XPS, TEM and Raman studies, that the colloids obtained by all experiments contain predominantly rGO and defected graphene, but also some GO, graphite micro-particles, and graphene-like phases. Amorphous carbon is frequently observed in by PLA at λ = 355 nm as well as at λ = 266 nm at high laser beam fluences but it can be found in very small quantities in all specimens. The XPS analysis of the samples revealed a significant increment in the quantity of different oxygen–containing radicals as a function of the fluence, which is a result of the quantity of the sp3–hybridized carbon. Finally, the established PLA procedures have been found to be highly reproducible.

Mechanism of stress relaxation and phase transformation in additively manufactured Ti-6Al-4V via in situ high temperature XRD and TEM analyses

Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellent mechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of thermal residual stresses and the formation of martensitic structure across a wide range of typical processing conditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical performance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution (α′ martensite phase decomposition), accounting for the kinetics of phase transformation during the heat treatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation is still ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investigated up to 1000°C using both in situ high temperature X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommodation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported based on elemental concentration changes during heat treatment, in combination with changes in crystal structure observed using the high temperature XRD and TEM measurements. This study provides a deeper understanding on the mechanism of phase transformation through martensitic decomposition, as well as a deeper understanding of the influence of post-thermal treatment conditions on the alloy's crystal structure.

Structure and thermal stability of LiNi0.8Co0.15Al0.05O2 after long cycling at high temperature

LiNi0.8Co0.15Al0.05O2 (NCA) is widely used as cathode material in commercialized high energy density lithium ion battery due to its high specific capacity. However, the structure and thermal stability of NCA after long electrochemical cycles are remained unclear. Herein, we investigate the structure and thermal stability of aged NCA at both lithiated and delithiated states using in-situ charge/discharge and high temperature X-ray diffraction and transmission electron microscopy images. The thermal stability of delithiated NCA cathode after 400 cycles at 1C is obviously inferior to the lithiated and pristine one. The particle surface of aged NCA changes from layered to spinel phase, while the bulk area maintains the layered structure. The temperature inducing structure change of aged NCA is much lower than that of pristine NCA and the delithiated state of NCA is more vulnerable at high temperature. The drastic structure transformation of aged NCA occurs at 250 °C during heating, which accompanies with oxygen loss and the formation of intergranular cracks in the NCA secondary particles. Thus, this work provides significant information of structure stability of cycled NCA at high temperature for optimizing its surface structure to achieve excellent cycling performance under extreme conditions.

Orientation Relationships upon the Structural Transformation of Monoclinic and Cubic Phases in Silver Sulfide

The orientation relationships between the low-temperature monoclinic semiconductor acanthite α-Ag2S and the high-temperature body-centered argentite β-Ag2S are determined based on high-temperature X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) data on silver sulfide. It is found that, in contrast to acanthite, the possible distances between silver atoms in cubic argentite are too small to allow Ag atoms to occupy metal sublattice sites with a probability of 1. It is shown that the (010) and (001) atomic planes of acanthite are parallel to the (1 $$\bar {1}$$ 0) and (221) planes of argentite, respectively. The found orientation relationships between acanthite and argentite are important for understanding the physical operation of the Ag2S/Ag heteronanostructure, which is considered as a potential basis for designing resistive switches and nonvolatile memory devices.

In-situ high temperature XRD and TEM study of the thermal stability and sintering behavior of octacalcium phosphate

Currently, the characteristics of solid-state phase transformation corresponding to octacalcium phosphate (OCP) sintering is still not completely understood. Although it was known that Ca-deficient (Ca-def) hydroxyapatite (HA) is usually unstable at high temperature, the thermal stability and sintering behavior of OCP have been rarely reported. The objective of this study is to systematically investigate the thermal stability and sintering behavior of OCP powders by comprehensive characterization techniques including in-situ high temperature X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). In-situ XRD results showed that the “collapsed OCP”, Ca-def HA and β-tricalcium phosphate (TCP) crystal structures formed by heating OCP at 200–250 °C, 300–600 °C and 800–900 °C, respectively. Furthermore, DTA analysis revealed that the peak temperatures of the phase transitions of OCP to Ca-def HA and Ca-def HA to β-TCP were ∼170 °C and ∼720 °C, respectively. SEM examination indicated that a large number of pores were introduced during sintering OCP powders due to the release of structurally contained water at 200–250 °C. Therefore, a porous β-TCP ceramic can be directly prepared by sintering OCP powders at 900 °C. This phase transition induced foaming method may be extended for preparing porous bioceramics by sintering calcium phosphate phases as long as they contain crystalline water, which could provide guidelines for better utilizing the phase transformations of calcium phosphate to produce special bioceramics for biomedical applications.

Ag2S/Ag heteronanostructure

An Ag2S/Ag heteronanostructure has been prepared for the first time by hydrochemical deposition. The “acanthite α-Ag2S–argentite β-Ag2S” phase transformation has been studied in situ by high-temperature X-ray diffraction and transmission electron microscopy. The crystal structure of argentite has been revealed. It has been found that the concentration of vacant sites in the metal sublattice of argentite exceeds 92%. The reversible acanthite–argentite transformation in the Ag2S/Ag heteronanostructure at the application of the external bias voltage is considered.

Few-Layer Thin-Film Metallic Glass-Enhanced Optical Properties of ZnO Nanostructures.

A few layers of Cu-based (Cu47Zr42Al7Ti4) thin-film metallic glasses (TFMGs) were sputtered on hydrothermally synthesized ZnO nanowires/glass and ZnO nanotubes/glass to fabricate UV photodetectors. The few layers of Cu-based TFMG are ultrathin at ∼0.98 nm and have a noncrystalline metal structure according to X-ray diffraction, Raman, photoluminescence, and high-temperature transmission electron microscopy verification. The photoresponse performance of the coated few-layers Cu-TFMG samples was enhanced 1680-7700% compared with the noncoated sample. The few-layers Cu-TFMG has high transmittance ∼90% in the visible band and creates a large capacitor to absorb UV photocurrent and release dark current.

Structural properties of the thermoelectric material CuCrS2 and of deintercalated CuxCrS2 on different length scales: X-ray diffraction, pair distribution function and transmission electron microscopy studies

Structural properties of the thermoelectric materials CuCrS2 and CuxCrS2 on different length scales.

Reverse Transformation of Deformation-Induced Phases and Associated Changes in the Microstructure of Explosively Clad Ti-5Ta-2Nb and 304L SS

Ti-5Ta-2Nb alloy was joined to 304L austenitic stainless steel by explosive cladding technique. Explosive cladding resulted in the formation of deformation-induced martensite in 304L SS and fcc phase of Ti in the Ti-5Ta-2Nb side of the joint. The stability of these metastable phases was systematically studied using high-temperature X-ray diffraction technique and transmission electron microscopy, which enabled the optimization of the temperature window for post-cladding heat treatments.

In situ high-temperature TEM observation of material escape from a surface of CoFeNi/Cu/ZrAlO composite into the amorphous carbon layer

We observed the moment of material escape from a surface of CoFeNi/Cu/Zr(Al)O2 composite into the amorphous carbon layer when we studied the phase transformation of the structure using in situ transmission electron microscopy (TEM) technique at 800°C. To protect the top surface of the TEM specimen against focused-ion beam process damage, the specimen had been coated with an amorphous carbon layer, a thin Pt film and a W protective layer. During our high-temperature experiments at 800°C, we detected that the CoFeNi nanoparticles moved from the surface of the TEM specimen into the amorphous carbon layer. A porous amorphous carbon layer had a large impact on the visualization of this phenomenon. Liquid-like behavior of the CoFeNi phase, which possessed some crystalline order, was detected before the material escaped from the surface. After heating, the carbon layer became tightly packed with small particles and single atoms. The majority of the particles in the carbon layer were in the size range of 1–4nm. The particles were assigned to the CoFeNi, FePt, and W phases. The CoFeNi particles escaped directly from the specimen surface, while the FePt and W particles were formed in the carbon layer during heating as a result of atomic reactions. The single atoms observed in the carbon layer were attributed to the heavy elements Pt and W.

Hydrolysis of bis(dimethylamido)tin to tin (II) oxyhydroxide and its selective transformation into tin (II) or tin (IV) oxide

Sn6O4(OH)4, a hydrolysis product of Sn(NMe2)2, is transformed to tin (II) or tin (IV) oxide by solid and solution phase processing. Tin (II) oxide is formed by heating Sn6O4(OH)4 at ⩽200°C in air or under inert atmosphere. Tin (IV) oxide nanoparticles are formed in the presence of a carboxylic acid and base in air at room temperature. IR spectroscopy, Raman spectroscopy, thermogravimetry (coupled with infrared spectroscopy), powder X-ray diffraction, high temperature X-ray diffraction, scanning electron and transmission electron microscopy are used for the characterization of Sn6O4(OH)4 and the investigation of its selective decomposition into SnO or SnO2. Spectroscopic and X-ray diffraction results indicate that SnO is formed by the removal of water from crystalline Sn6O4(OH)4. SEM shows octahedral morphology of the Sn6O4(OH)4, SnO and SnO2 with particle size from 400nm–2μm during solid state conversion. Solution phase transformation of Sn6O4(OH)4 to SnO2 occurs in the presence of potassium glutarate and oxygen. SnO2 particles are 15–20nm in size.

In situ transmission electron microscopy studies of the kinetics of Pt-Mo alloy diffusion in ZrB2 thin films

Using in situ high-temperature (1073–1173 K) transmission electron microscopy, we investigated the thermal stability of Pt and Mo in contact with polycrystalline ZrB2 thin films deposited on Al2O3(0001). During annealing, we observed the diffusion of cubic-structured Pt1−xMox (with x = 0.2 ± 0.1) along the length of the ZrB2 layer. From the time-dependent increase in diffusion lengths, we determined that the Pt1−xMox does not react with ZrB2, but diffuses along the surface with a constant temperature-dependent velocity. We identify the rate-limiting step controlling the observed phenomenon as the flux of Mo atoms with an associated activation barrier of 3.8 ± 0.5 eV.