Emanuel Ionescu

Ceramic Nanocomposites from Tailor-Made Preceramic Polymers

The present Review addresses current developments related to polymer-derived ceramic nanocomposites (PDC-NCs). Different classes of preceramic polymers are briefly introduced and their conversion into ceramic materials with adjustable phase compositions and microstructures is presented. Emphasis is set on discussing the intimate relationship between the chemistry and structural architecture of the precursor and the structural features and properties of the resulting ceramic nanocomposites. Various structural and functional properties of silicon-containing ceramic nanocomposites as well as different preparative strategies to achieve nano-scaled PDC-NC-based ordered structures are highlighted, based on selected ceramic nanocomposite systems. Furthermore, prospective applications of the PDC-NCs such as high-temperature stable materials for thermal protection systems, membranes for hot gas separation purposes, materials for heterogeneous catalysis, nano-confinement materials for hydrogen storage applications as well as anode materials for secondary ion batteries are introduced and discussed in detail.

Single-Source-Precursor Synthesis of Hafnium-Containing Ultrahigh-Temperature Ceramic Nanocomposites (UHTC-NCs)

Amorphous SiHfBCN ceramics were prepared from a commercial polysilazane (HTT 1800, AZ-EM), which was modified upon reactions with Hf(NEt2)4 and BH3·SMe2, and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase composition, by means of spectroscopy techniques (Fourier transform infrared (FTIR), Raman, magic-angle spinning nuclear magnetic resonance (MAS NMR)), as well as X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments of the SiHfBCN samples in an inert gas atmosphere (Ar, N2) at temperatures in the range of 1300-1700 °C showed the conversion of the amorphous materials into nanostructured UHTC-NCs. Depending on the annealing atmosphere, HfC/HfB2/SiC (annealing in argon) and HfN/Si3N4/SiBCN (annealing in nitrogen) nanocomposites were obtained. The results emphasize that the conversion of the single-phase SiHfBCN into UHTC-NCs is thermodynamically controlled, thus allowing for a knowledge-based preparative path toward nanostructured ultrahigh-temperature stable materials with adjusted compositions.

Strong Influence of Polymer Architecture on the Microstructural Evolution of Hafnium‐Alkoxide‐Modified Silazanes upon Ceramization

The present study focuses on the synthesis and ceramization of novel hafnium-alkoxide-modified silazanes as well as on their microstructure evolution at high temperatures. The synthesis of hafnia-modified polymer-derived SiCN ceramic nanocomposites is performed via chemical modification of a polysilazane and of a cyclotrisilazane, followed by cross-linking and pyrolysis in argon atmosphere. Spectroscopic investigation (i.e., NMR, FTIR, and Raman) shows that the hafnium alkoxide reacts with the N-H groups of the cyclotrisilazane; in the case of polysilazane, reactions of N-H as well as Si-H groups with the alkoxide are observed. Consequently, scanning and transmission electron microscopy studies reveal that the ceramic nanocomposites obtained from cyclotrisilazane and polysilazane exhibited markedly different microstructures, which is a result of the different reaction pathways of the hafnium alkoxide with cyclotrisilazane and with polysilazane. Furthermore, the two prepared ceramic nanocomposites are unexpectedly found to exhibit extremely different high-temperature behavior with respect to decomposition and crystallization; this essential difference is found to be related to the different distribution of hafnium throughout the ceramic network in the two samples. Thus, the homogeneous distribution of hafnium observed in the polysilazane-derived ceramic leads to an enhanced thermal stability with respect to decomposition, whereas the local enrichment of hafnium within the matrix of the cyclotrisilazane-based sample induces a pronounced decomposition upon annealing at high temperatures. The results indicate that the chemistry and architecture of the precursor has a crucial effect on the microstructure of the resulting ceramic material and consequently on its high-temperature behavior.

Crystallization Behavior and Controlling Mechanism of Iron-Containing Si−C−N Ceramics

The crystallization behavior and controlling mechanism of the Si-Fe-C-N system based on polymer-derived SiCN ceramic filled with iron metal powder has been studied. The composite preparation conditions allow the formation of a random distribution of metallic particles in the polymer matrix volume for the Si-C-N system. Pyrolysis of the composite material at 1100 degrees C indicates the presence of one crystalline phase Fe(3)Si. While the sample pyrolyzed at 1200 degrees C reveals the formation of both Fe(3)Si and Fe(5)Si(3) phases, a crystallization of beta-SiC is additionally observed by increasing the temperature up to 1300 degrees C. The propensity for the formation of SiC is due to the presence of Fe(5)Si(3), where a solid-liquid-solid (SLS) growth mechanism was suggested to occur. X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermal gravimetric analysis with mass spectroscopic detection (TGA-MS) were employed to investigate the crystallization behavior of the Si-Fe-C-N system.

Precursor-Derived Ceramics

The condensation of organometallic compounds into merely inorganic materials by a proper thermal treatment (referred to as thermolysis or pyrolysis) under controlled atmosphere is a unique and fairly simple process of producing new types of ceramics, which are—due to their origin—referred to as precursor- or polymer-derived ceramics (PDCs).

Corrosion behavior of silicon oxycarbide-based ceramic nanocomposites under hydrothermal conditions

Abstract Silicon oxycarbide-based ceramic nanocomposites (SiOC, SiZrOC and SiHfOC) were prepared by means of hot pressing techniques and their behavior upon hydrothermal corrosion at moderate temperatures (up to 250°C) was investigated. The results indicated linear corrosion behavior for all samples. The corrosion rates of the SiOC ceramic materials were found to be remarkably lower than those of silicon carbide and comparable to values reported for silicon nitride. Furthermore, SiZrOC and SiHfOC were found to show improved resistance with respect to the non-modified SiOC, due to a unique synergistic effect: whereas zirconia/hafnia act as “reinforcing” phases with respect to hydrothermal corrosion (due to their extremely low solubility in water under the testing conditions), the silicon oxycarbide matrix protects the MO2 phase from a corrosion-induced t-MO2 → m-MO2 phase transformation. Consequently, the prepared silicon oxycarbide-based materials exhibit high potential for applications which require high resistance in corrosive media at moderate temperatures.

Single-Source Magnetic Nanorattles By Using Convenient Emulsion Polymerization Protocols

A novel strategy to achieve easily scalable magneto-responsive nanoceramics with core/shell and nanorattle-type or yolk/shell architectures based on a ferrocene-containing polymer precursor is described. Monodisperse nanorattle-type magnetic particles are obtained by using convenient semicontinuous emulsion polymerization and Stöber process protocols followed by thermal treatment. The particles are characterized by TGA, TEM, WAXS, DLS, XPS, and Raman spectroscopy. Herein, established synthetic protocols widen opportunities for the convenient bottom-up strategies of various ferrocene-precursor-based spherical architectures for advanced ceramics with potential applications within fields of sensing and stimuli-responsive nanophotonics.

Can we predict the formability of perovskite oxynitrides from tolerance and octahedral factors

Perovskite oxynitrides AB(O,N)3 represent an emerging class of materials suitable for applications in the fields of clean energy and environmental protection. Nitrogen substitution for oxygen allows for a significant enrichment of possible perovskite structures for combinations of cations that are not achievable in perovskite oxides. A model that utilizes the tolerance and octahedral factors is developed for assessing the formability of the perovskite structure in oxynitrides and for predicting new perovskite oxynitrides that have not been synthesized so far. Our model considers the alteration of the interatomic distances and cationic radii in oxynitrides when compared to those in oxides and nitrides. In the first step we identify the stability field of the perovskite structure in oxynitrides from the crystal structure data for perovskite oxynitrides synthesized so far. In the next step we address the formability of the perovskite structure for compositions not studied yet. For instance, we predict that among potentially piezoelectric oxynitrides, YSiO2N and YGeO2N are not stable in the perovskite-type structure; YZrO2N and YSnO2N are in turn formable, whereas for possible candidate of photocatalytic oxynitrides according to DFT calculations MgTaO2N is not formable in perovskite structure; YTiO2N, CdTaO2N and CdNbO2N appear to be feasible. Moreover, we predict the formability of perovskite structures for Zn2+, Cd2+, Y3+, Hf4+, Fe4+and Sn4+, as well as Pr3+, Nd3+, and Sm3+ oxynitrides. As none of these compounds has been yet synthesized, our model can be applied for designing and guiding the synthesis of novel perovskite structures in oxynitrides.

Carbon substitution for oxygen in silicates in planetary interiors

Significance Carbon is an important element in the Earth and other planets, but its concentration, chemical and structural form, and dynamics throughout the crust, mantle, and core are incompletely known. Based on the thermodynamic stability of a class of synthetic materials, the polymer-derived ceramics containing Si, C, and O, and on new NMR data for such systems containing a network-modifying metal (Li), this paper suggests that the substitution of C for O (rather than C for Si) in molten, amorphous, and crystalline silicate structures may provide a hitherto hidden reservoir of carbon in planetary interiors. Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon- and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiOxC4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO2, C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiOxC4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10–100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.

One for all: cobalt-containing polymethacrylates for magnetic ceramics, block copolymerization, unexpected electrochemistry, and stimuli-responsiveness

Novel cobalt-containing homo- and diblock copolymers with poly(methyl methacrylate) (PMMA) are synthesized by atom transfer radical polymerization (ATRP) of a neutral cobalt-complex methacrylate. An efficient route for a single-step synthesis of the cobalt precursor based on easily-available starting materials followed by esterification with methacrylic acid is presented. The cobalt-methacrylate monomer is furthermore polymerized by thermal, free radical and statistical copolymerization with MMA and investigated with respect to (absolute) molar masses, polymer composition, and thermal properties. ATRP affords block copolymers as evidenced by 1H NMR spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). The cobalt-containing homopolymers are investigated and tailored with respect to their thermal conversion into magnetic cobalt oxides and elemental cobalt which is evidenced by X-ray diffraction (XRD), Raman spectroscopy, and superconducting quantum interference device (SQUID) magnetometry measurements. The (reversible) electrochemistry of the cobalt-containing polymethacrylates and block copolymers thereof are thoroughly addressed by cyclic voltammetry (CV) studies. Interestingly, the prepared metalloblock copolymers exhibit redox-responsiveness (both reduction and oxidation) and thus structure formation in the presence of a reduction or oxidation reagent are demonstrated by transmission electron microscopy (TEM).