Hans-Joachim Kleebe

Hans-Joachim Kleebe
Technische Universität Darmstadt | TU · Institute of Applied Geosciences

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247
Publications
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Introduction

Publications

Publications (247)
Article
It has been an enormous challenge to obtain double P-E loops and identify structure evolution in NaNbO3 based ceramics. BiMg2/3Ta1/3O3(BMT) modified NaNbO3 ceramics were fabricated by solid-state methods, showing slimmer double P-E loops compared with other reported work. Combined with the Raman and refined XRD results, we confirmed that the coexis...
Article
Palladium-containing silicon oxycarbide (SiPdOC) ceramics were synthesized using polymethylsilsesquioxane modified with palladium acetate as a single-source precursor. Thus, pyrolysis in Argon at 1100 °C led to nanocomposites consisting of Pd2Si nanocrystallites dispersed in an amorphous SiOC matrix. Exposure of SiPdOC to higher temperatures result...
Article
A ZrB2‐based ceramic, containing short Hi‐Nicalon SiC fibers, was fabricated with a Mo‐impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2‐MoSi2 layer, in order to prevent inward Mo‐diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with th...
Article
Full-text available
In this Part II of the paper on the evolution of carbon-rich Si–O–C polymer-derived ceramics (PDCs), emphasis is placed on the strengths and limitations of transmission electron microscopy (TEM) techniques with high lateral resolution, i. e., electron energy-loss spectroscopy (EELS), high-resolution TEM (HRTEM) and energy-filtered TEM (EFTEM). Here...
Article
Carbon-rich Si –O–C polymer-derived ceramics (PDCs) were investigated by various spectroscopic techniques, in order to characterize the evolution of their predominantly amorphous microstructure upon thermal treatment up to 1450 °C. Particular attention was addressed to modifications of the excess free carbon phase present in these materials. Surpri...
Article
Quenching relaxor ferroelectric 0.94(Na1/2Bi1/2)TiO3‐0.06BaTiO3 (NBT‐6BT) enhances the depolarization temperature (Td), linked to the stabilization of ferroelectric order. The thermal evolution of the domain structure and phase assemblage provides insights about the onset of ferroelectric order in quenched materials. Unpoled furnace cooled and quen...
Article
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SiFeO(N,C)‐based ceramic papers were prepared via a one‐pot synthesis approach by dip‐coating a cellulose‐based paper template with a polymeric perhydropolysilazane (PHPS) precursor modified with iron(III)acetylacetonate. The preceramic composites were subsequently pyrolyzed in ammonia atmosphere at 500, 700, and 1000°C, respectively, and the chara...
Article
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Domain wall motion in ferroics, similar to dislocation motion in metals, can be tuned by well‐concepted microstructural elements. In demanding high‐power applications of piezoelectric materials, the domain wall motion is considered as a lossy hysteretic mechanism that should be restricted. Current applications for so‐called hard piezoelectrics are...
Article
Cellulose‐based paper samples were surface‐modified by a polymeric single‐source precursor prepared from perhydropolysilazane (PHPS) and iron(III)acetylacetonate (Fe(acac)3) and ammonolyzed at 500, 700, 900 and 1000°C, leading to C/SiFe(N,C)O‐based ceramic papers with in‐situ generated hierarchical micro/nano‐morphology. As reference, cellulose‐fre...
Article
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Silicon oxycarbides synthesized through a conventional polymeric route show characteristic nanodomains that consist of sp2 hybridized carbon, tetrahedrally coordinated SiO4, and tetrahedrally coordinated silicon with carbon substitution for oxygen, called “mixed bonds.” Here we synthesize two preceramic polymers possessing both phenyl substituents...
Article
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Naturally occurring single crystals of bixbyite, (Fe,Mn)2O3, from the Thomas Mountain Range in Utah, U.S.A., were studied via (scanning) transmission electron microscopy (S)TEM. With up to 5 cm edge length, these mineral specimens are the largest bixbyite crystals found worldwide. Their hexahedral shapes are often modified by {211} facets at the co...
Article
In the present work, dense ZrB2‐based monoliths were produced via hot‐pressing of ZrB2 powders coated with amorphous polymer‐derived SiCN, SiZrCN or SiZrBCN at relatively moderate temperatures (i.e., 1800 °C). Thus, ZrB2 particles were embedded in polymer‐derived ceramic shell were realized by coating their surface with the corresponding polymeric...
Article
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In this paper, we report on electron microscopy studies of single crystals of the natural mineral lorándite, TlAsS2. The main focus of this investigation was to address the question as to whether those lorándite crystals are chemically and structurally homogeneous, in order to be utilized as an effective neutrino detector within the lorándite exper...
Article
Within the studies of the LiF-MgAl2O4 system, a dissolution - reprecipitation process is postulated at high pressure synthesis, involving LiAlO2 and MgF2 as transient phases. Heating experiments at atmospheric pressure, combined with a dedicated sample geometry enabled the identification of LiAlO2 and MgO via XRD and, for the first time, with elect...
Article
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Reversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO3‐modified NaNbO3 lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configu...
Article
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Quenching has been established as a viable method to increase the depolarization temperature in (100-x) Na1/2Bi1/2TiO3–xBaTiO3 (NBT–xBT). The proposed hypothesis of a stabilized ferroelectric order would entail changes in the polarized volume. To this end, air-quenched and furnace cooled samples of four compositions of NBT–xBT with x = 3, 6, 9, and...
Article
Ferroelectrics have become indispensable electroceramic materials in modern electronic applications. While point-defect engineering is a common tool to modify the properties of ferroelectrics, dislocations have not been widely considered, although they were previously shown to alter electrical properties in other oxides. Here, we study the room tem...
Article
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Relaxor ferroelectric Na1/2Bi1/2TiO3-based materials have gained considerable attention as a potential lead-free alternative in recent years and can be tailored to exhibit giant strain or superior high power properties. Quenching (1-x) (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-BT) ceramics in air from the sintering temperature is beneficial in enhancing the de...
Article
Electric-field-induced phase transitions are the most important characteristics of antiferroelectric mate- rials. However, in several prototype antiferroelectrics, these transitions are irreversible and the origin of this behavior is poorly understood. This prevents their widespread use, for example, in energy storage and memory applications. Here,...
Article
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The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. Thi...
Article
Herein we report on the synthesis of novel 0D-nanocarbon-based silicon-containing ceramic composites by a facile salt-free synthesis method followed by polymer-to-ceramic transformation. The 0D-nanocarbon-silica composites were synthesized via a one-pot sol-gel process using tetramethyl orthosilicate (TMOS) and functionalized nanodiamonds and conve...
Article
Conversion of cobalt‐doped layered zinc acetate into zinc oxide: Twinning is an often‐observed process during crystallization or mineralization of ZnO. However, if minor Co doping of a precursor is allowed, a twin free formation of highly regular hexagonal platelets of ZnO microcrystals displaying piezoelectric behavior is observed. Tessellation of...
Article
A zirconium and a zirconium / boron containing single‐source precursor were synthesized via chemical modification of a commercially available polysilazane (Durazane 1800) with tetrakis (dimethylamido) zirconium (IV) (TDMAZ) as well as with both TDMAZ and borane dimethyl sulfide complex, respectively. The polymer‐to‐ceramic transformation of the pre...
Article
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Functionalized ordered mesoporous materials are relevant in technologies, such as drug release, sensing, and separation. To design functionality, the silica framework can be functionalized with responsive molecules or polymers. Often, the pH value in those hybrid materials determines performance. Even though pH/pKa differences between polymers in b...
Article
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Cobalt‐doped zinc oxide single crystals with the shape of hexagonal platelets are synthesized by thermohydrolysis of zinc acetate, cobalt acetate and hexamethylenetetramine in mixtures of ethanol and water. The mineralization proceeds by a low temperature dissolution–reprecipitation process from the liquid phase via the formation of basic cobalt zi...
Chapter
In this article, an introduction to the manifold characterization technique of transmission electron microscopy (TEM) is given, presenting the various methods available for the investigation of solid-state materials such as ceramics and glasses. The high local resolution capacity down to the atomic level allows a detailed examination of structural...
Article
The search for tunable, size-dependent properties and unique processability has triggered the development of new synthetic routes for transition metal borides. MnB is a soft to semi-hard ferromagnetic material. This boride is now available by bottom-up, low-temperature solution chemistry. It is obtained as an unexpected metastable α′-variant that c...
Article
TEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain‐boundary structure, segregation and electrical transport perpendicular to the interface. To this end, varistor‐like ZnO bicrystals with piezotronic characteristics were chosen with (0001̅)║(0001̅) tail‐to‐tail orientation with...
Article
The understanding and design of wetting-transport and wetting-charge-transport interplay in nanometer-sized pores is a still not fully understood key step in improving nanopore transport-related applications. A control of mesopore wettability accompanied by pore filling and ionic mesopore accessibility analysis is expected to deliver major insights...
Article
Dislocations in oxides can be described as charged line defects and means for 1‐dimensional doping, which can tune electrical and thermal properties. Furthermore, theoretically it was shown that dislocation can pin ferroelectric domain walls. Broader application of this concept hinges on the development of a methodology to avail this approach to po...
Article
Ca and/or B modified silicon oxycarbides were synthesized via pyrolysis of suitable polysilsesquioxane‐based single‐source precursors. Their polymer‐to‐ceramic transformation was investigated with thermogravimetric analysis, coupled with in situ evolved gas analysis. The prepared silicon oxycarbides were investigated with respect to their crystalli...
Article
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Synthetic materials based on calcium phosphate (CaP) are frequently used as bone graft substitutes when natural bone grafts are not available or not suitable. Chemical similarity to bone guarantees the biocompatibility of synthetic CaP materials, whereas macroporosity enables their integration into the natural bone tissue. To restore optimum mechan...
Article
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Porous calcium phosphate (CaP) materials as bone graft substitutes can be prepared from Ca carbonate biomineral structures by hydrothermal conversion into pseudomorphic CaP scaffolds. The present study aims at furnishing such phosphatized Ca carbonate biomineral (PCCB) materials with antibacterial Ag ions in order to avoid perisurgical wound infect...
Article
In the present study, we compare the high-temperature creep behavior of three SiOC glass ceramics with different phase compositions. All three SiOC glass ceramics have a vitreous silica matrix in common but comprise different homogeneously dispersed phases: (i) only spherical β-SiC nanoparticles (sample denoted hereafter SiC/SiO2), (ii) only high-a...
Article
Full-text available
The quest to improve the thermoelectric figure of merit has mainly followed the roadmap of lowering the thermal conductivity while keeping unaltered the power factor of the material. Ideally an electron-crystal phonon-glass system is desired. In this work, we report an extraordinary reduction of the cross-plane thermal conductivity in crystalline (...
Article
This basic research investigates the microstructure evolution of a composite based on ZrB2-MoSi2 from the as-sintered features to the changes occurring upon oxidation at ultra-high temperatures, 1650 and 1800 °C, in a bottom-up loading furnace for 15 min. Scanning and transmission electron microscopy evidenced the formation of a matrix typified by...
Article
Nanostructured chromium nitride (CrN), both a hard material and a high-melting compound that is used in the medical industry and for new energy-harvesting applications, was synthesized phase-pure for the first time via low-temperature solution synthesis in liquid ammonia. TEM analysis confirms the nanoscale character of CrN. The antiferromagnetic p...
Article
In a physical vapor deposition process for fabrication of perovskite absorber layers it is an important prerequisite for large scale application to understand the reaction steps and the impact of the deposition parameters on the structural and compositional characteristics of vacuum deposited lead halide perovskite layers. In the sequential deposit...
Cover Page
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Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and funct 2018 Journal of Materials Chemistry C HO...
Article
In this work we investigate the electrochemical behaviour of two silicon oxycarbide (SiOC) glasses synthesized from the same starting precursor. In one case we perform the pyrolysis in Ar flow, while in the second case, the glass is synthesized under CO2 flow. The microstructural characterization of the glasses unambiguously demonstrates that the A...
Article
Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on the impact of cation vacancies on electroceramics, especially on (Na1/2Bix)TiO3 (NBT)-based material. Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional properties in the exemplary ferroelect...
Article
Metal nanowires (NWs) represent a prominent nanomaterial class, the interest in which is fueled by their tunable properties as well as their excellent performance in e.g. sensing, catalysis and plasmonics. Synthetic approaches to obtaining metal NWs mostly produce colloids or rely on templates. Integrating such nanowires into devices necessitates a...
Article
Full-text available
A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm2Co17-type pinning-controlled permanent magnets. These are of importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model ma...
Article
Hafnium oxide (HfOx)-based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two-terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfOx/TiN...
Article
In this work, we present a comprehensive study on the role of metal species in MOF-based Me-N-C (mono and bimetallic) catalysts for the hydrogen evolution reaction (HER). The catalysts are investigated with respect to HER activity and stability in alkaline electrolyte. Based on structural analysis by X-ray diffraction, X-ray induced photoelectron s...
Article
Transmission electron microscopy was performed on a model system of a novel CuMn2O4 spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn2O4 spinel into separated MnO and Cu-rich phases, which are CuO or Cu depending on the reduction p...
Article
Full-text available
Randall’s plaques (RP) are preferred sites for the formation of calcium oxalate monohydrate (COM) kidney stones. However, although processes of interstitial calcium phosphate (CaP) plaque formation are not well understood, the potential of plaque microstructures as indicators of CaP precipitation conditions received only limited attention. We inves...
Article
We report on the influence of oxygen impurities on the gas sensing properties of gallium nitride (GaN) chemiresistors. As shown by XRD, elemental analysis and TEM characterization, surface oxidation of GaN – for example, upon contact to ambient air atmosphere - creates an oxidative amorphous layer which provides the sites for the sensing towards CO...
Article
Currently, Me-N-C catalysts are the most prominent alternative to Pt/C catalysts for the oxygen reduction reaction in acidic media. It is well known that the achievable activity and selectivity strongly correlates with the nature of metal species. However, so far the effect of the metal species on the stability of these catalysts was not investigat...
Article
Dense SiHf(B)CN-based ceramic nanocomposites were prepared by spark plasma sintering (SPS) using high heating rates (∼450 °C/min.) and high pressures (≥100MPa). The obtained nanocomposites were investigated by X-ray diffraction, Raman spectroscopy and electron microscopy concerning their phase evolution and microstructure.The hardness and the elast...
Article
In the present study, a C/SiFeN(O)-based ceramic paper with in situ generated hierarchical micro/nano-morphology was prepared upon thermal treatment of a cellulose-base paper surface-modified with a polymeric single-source precursor prepared from perhydropolysilazane (PHPS) and iron(II) acetylacetonate (Fe(acac)2). The ammonolysis at 1000 °C of the...
Article
Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distributi...
Article
This basic research deals with the microstructure evolution of a W-doped ZrB2 ceramic, as-sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B2 solid solutions surround the original ZrB2 nuclei, whereas...
Article
The crystal structure and disorder phenomena of Al4B2O9, an aluminum borate from the mullite-type family, were studied using automated diffraction tomography (ADT), a recently established method for collection and analysis of electron diffraction data. Al4B2O9, prepared by sol-gel approach, crystallizes in the monoclinic space group C2/m. The ab in...
Article
The microstructure evolution of ZrB2 hot pressed with 15 vol% TaSi2 was studied in the as-sintered state and after oxidation for 15 min at 1500 and 1650 °C in stagnant air. In the pristine material, the original ZrB2 nuclei are surrounded by a mixed (Zr,Ta)B2 solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain b...
Article
Full-text available
Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high...
Article
ε-Fe3N shows interesting magnetism but is difficult to obtain as a pure and single-phase sample. We report a new preparation method using the reduction of iron(II) bromide with elemental sodium in liquid ammonia at −78 °C, followed by annealing at 573 K. Nanostructured and monophasic oxygen-free iron nitride, ε-Fe3N, is produced according to X-ray...
Chapter
Ferroelectrics play an important role in today's modern life. A large variety of applications including piezoelectric actuators, sensors, dielectric capacitors, memory devices, etc. are based on these materials. Recently, scientific interest has been given to the Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) perovskite ferroelectric, that exhibits...
Article
Full-text available
In paper-based devices, capillary fluid flow is based on length-scale selective functional control within a hierarchical porous system. The fluid flow can be tuned by altering the paper preparation process, which controls parameters such as the paper grammage. Interestingly, the fiber morphology and nanoporosity are often neglected. In this work, p...
Article
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Nanotube assemblies represent an emerging class of advanced functional materials, whose utility is however hampered by intricate production processes. In this work, three classes of nanotube networks (monometallic, bimetallic, metal oxide) are synthesized solely using facile redox reactions and commercially available ion track membranes. First, the...
Article
We replicate the unique pore structure of ion-track etched polymers with carbon using an organic sol-gel precursor strategy. First, phenol resin is cross-linked in the membrane pores, followed by selective dissolution of the template matrix and carbonization at a temperature of 550 °C. The potential to create nanomaterials of novel morphology is de...