Chemische Zusammensetzung und Mikrostruktur polymerabgeleiteter Gläser und Keramiken im System Si–C–O. Teil 2: Charakterisierung der Gefügeausbildung mittels hochauflösender Durchstrahlungselektronenmikroskopie und Feinbereichsbeugung

Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 2: Characterization of microstructure formation by means of high‐resolution transmission electron microscopy and selected area diffraction Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimensionally stable components can be cost‐effectively achieved by admixing reactive filler powders, chemical composition and microstructure development of the polymer‐derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550 °C are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si–C–O matrix into SiO₂, C and SiC was proved. The in‐situ crystallization could be considerably accelerated by adding fine‐grained powder of inert fillers, such as Al₂O₃ or SiC, which permits effective process control. In part 2, the microstructure is characterized by high‐resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β‐SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred °C. In this case, the polymer‐derived Si–C–O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO₂ was observed.     

Chemische Zusammensetzung und Mikrostruktur polymerabgeleiteter Gläser und Keramiken im System Si–C–O. Teil 1: Phasenidentifikation mittels Si‐L2,3‐Ionisationskanten‐Elektronenenergieverlustspektroskopie nach dem Fingerprint‐Verfahren und energiegefilterter Durchstrahlungselektronenmikroskopie

Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 1: Phase identification by means of Si‐L_2,3‐ionisation edge electron energy‐loss spectroscopy according to the fingerprinting technique and energy‐filtered transmission electron microscopy Polymer or precursor ceramics represent a novel class of high‐performance materials that are produced by controlled pyrolysis of organometal compounds. Shrinkage and porosity resulting from thermal decomposition can be compensated by adding chemically reactive filler powders. This technique permits manufacturing of dense ceramic bulk components true to size by cost‐effective near‐net‐shape forming of cross‐linked green compacts. For process control, definition of the course of the reaction of the active fillers and thus the exact knowledge of the chemical composition and the microstructure development of the solid residues during pyrolysis of the polymer precursor is required. Due to both the great technical importance of the final ceramic products and their economical availability, in the present work, thermal decomposition of a silicone resin (polymethylsiloxane) at temperatures between 525 and 1550 °C was characterized nanochemically and microstructurally. In part 1, the results of the quantitative phase identification by means of analytical electron microscopy are reported. The bonding state of silicon was determined by fine structure analysis (Si‐L_2,3‐ionisation edge) of the electron energy‐loss spectrum and has proved to be always mainly oxidic. By means of energy‐filtered transmission electron microscopy, elemental distributions in the nanometre range were recorded. The phase separation of the polymer‐derived Si–C–O matrix into SiO₂, C and SiC could be proved definitely. The characterization of structure formation by high‐resolution imaging and diffraction methods follows in part 2.     

Sol‐gel derived C‐SiC composites for re‐entry structures

Composites of carbon fibers, fabrics, or their precursors as reinforcement, and sol‐gel‐derived silicon carbide as matrix, have been developed, aiming at high‐temperature stable ceramics that can be utilized for re‐entry structures. These composites are produced via the sol‐gel process, starting with a sol‐gel reaction of a mixture of silane precursors. The sol‐gel‐derived resin is cast onto the reinforcement fibers/fabrics mat (carbon or its precursors) to produce a ‘green’ composite that is being cured. The ‘green’ composite is converted into a C‐SiC composite via a gradual heat‐pressure process under inert atmosphere, during which the organic substituents on the silicon atoms undergo internal oxidative pyrolysis via the schematic reaction: (SiRO_3/2)_n → SiC + CO₂ + H₂O The composition of the resultant silicon‐oxi‐carbide is tailorable via modifying the composition of the sol‐gel reactants. The reinforcement, when made of carbon precursors, is converted into carbon during the heat‐and‐pressure processing as well. The C‐SiC composites thus derived exhibit superior thermal stability and comparable thermal conductivity, combined with good mechanical strength features and failure resistance, which render them greatly applicable for re‐entry shielding, heat‐exchange pipes, and the like.     

Effect of Mn doping on the dielectric properties of BaTi<Subscript>0.9</Subscript>Sn<Subscript>0.1</Subscript>O<Subscript>3</Subscript> ceramics

Pure and Mn-doped BaTi_0.9Sn_0.1O₃ ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of BaTi_0.9Sn_0.1O₃ ceramics with 0, 0.2, 1 and 2 at.% Mn have been investigated. The results indicate that manganese ions enter the unit cell maintaining the perovskite structure of solid solution. Grain size of BaTi_0.9Sn_0.1O₃ ceramics sharply decreases after doping MnO₂. However, once MnO₂ content is more than 0.2 at.%, the average grain size of Mn-doped BaTi_0.9Sn_0.1O₃ ceramics increases with the increasing of MnO₂ content. The addition of manganese leads to the increase of the Curie temperature. The diffuseness of the phase transition of Mn-doped BTS ceramics decreases with the increase of Mn content, which may be due to grain size effect.     

Dielectric properties,microstructure and diffuse transition of Al-doped Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics

Pure and Al-doped BaZr_0.2Ti_0.8O₃ (short for BZT) ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of Al-doped BaZr_0.2Ti_0.8O₃ ceramics were investigated. These results indicate that aluminum ions enter the unit cell maintaining the perovskite structure of solid solution. The addition of aluminum leads to the change of the Curie temperature. The dielectric loss of the Al-doped BZT ceramics is higher than that of pure BZT ceramics, and increases as aluminum content increases. The diffuseness of the phase transition of Al-doped BZT ceramics weakens with the increasing of aluminum content. There is no obvious frequency dispersion around the dielectric constant peaks for Al-doped BZT ceramics. The coercive electric field (E _C) increases as Al content increases, and the remanent polarization (P _r) of Al-doped BZT ceramics is lower than that of pure BZT ceramics.     

Preparation of Pb(Zr,Ti)O<Subscript>3</Subscript>–Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> piezoelectric ceramics by dry–dry method

Ternary perovskite ceramics of Pb[(Zr_0.5Ti_0.5)_0.8−x (Mg_1/3Nb_2/3)_0.2+x]_0.98Nb_0.02O_3.01 (PZTMN, x = −0.075, −0.05, −0.025, 0, 0.025, 0.05, and 0.075 ), are synthesized via dry–dry method. B-site precursors of PZTMN ([(Zr_0.5Ti_0.5)_0.8−x (Mg_1/3Nb_2/3)_0.2+x ]_0.98Nb_0.02O_2.01, ZTMN) can be synthesized via a two-step solid state reaction method. The first calcination temperature is 1,300 °C, and the second is not higher than 1,360 °C. Incorporation of magnesium and niobium ions promotes the formation of the single phase solid solution with ZrTiO₄ structure. Single phase perovskite PZTMN is formed at 780 °C, much lower than that in conventional process. Dense ceramics can be sintered at about 1,260 °C with dielectric and piezoelectric properties comparable to that of wet–dry method and higher than that of conventional method. It seems that B-site precursor method is cost effective in preparation of ternary piezoelectric ceramics.     

Phase transitions of low-temperature sintering tungsten-doped ZnO-TiO<Subscript>2</Subscript> ceramics

WO₃-doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO₃ addition on the low-temperature sintering behavior, phase transition and dielectric properties of zinc titanate ceramics were investigated. The results show that the densification temperature of ZnTiO₃ ceramics can be reduced from 1150 to 900 °C with WO₃ addition and chemical processing. Small amount of WO₃ (<1.00wt %) accelerated the decomposition of hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ phase, while excessive addition (for example, 3.00wt %) restrained the decomposition. At the same time, the phase transition temperature from hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ is lowered by adding WO₃. WO₃ addition affects the dielectric properties of ceramics. The dielectric properties of WO₃-doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed the best dielectric properties for 1.0% WO₃-doped ceramics.     

Micromechanisms of crack extension in unmodified and modified epoxy resins

The failure behaviour of pure epoxy and modified epoxy resins has been studied in detail using fracture mechanics and scanning electron microscopy. The epoxy matrix was modified by a dispersion of rubbery and/or rigid fillers. The rubbery phase was a carboxyl-terminated butadiene-acrylonitrile rubber whilst the rigid fillers were either metastable zirconia (ZrO₂) particles or short alumina (Al₂O₃) fibres. These fillers were found to have a profound influence on the formation of stress-whitening zones and the concomitant slow crack growth behaviour. The geometry and the aspect ratio of fillers also appeared to dictate the extent of stress-whitening and stable crack growth.     

Microwave dielectric properties of Bi(Sc<Subscript>1/3</Subscript>Mo<Subscript>2/3</Subscript>)O<Subscript>4</Subscript> ceramics for LTCC applications

A novel microwave dielectric ceramics Bi(Sc_1/3Mo_2/3)O₄ with low firing temperature were prepared via the solid reaction method. The specimens have been characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and DC conductivity. The Bi(Sc_1/3Mo_2/3)O₄ ceramics showed B-site ordered Scheelite-type structure with space group C2/c. Raman analysis indicated that prominent bands were attributed to the normal modes of vibration of MoO₄ ^2? tetrahedra. The dielectric loss of Bi(Sc_1/3Mo_2/3)O₄ ceramics can be depended strongly the bulk conductivity by DC measurement. The superior microwave dielectric properties are achieved in the Bi(Sc_1/3Mo_2/3)O₄ ceramic sintered at 875 °C/4 h, with dielectric constant?~?25, Q?×?f ~?51,716 GHz at 6.4522 GHz and temperature coefficient of resonance frequency ~???70.4 ppm/°C. It is a promising microwave dielectric material for low-temperature co-fired ceramics technology.     

Studies on the radiopacity of experimental dental composite resins containing admixed SiO2−ZrO2 fillers

To produce radiopaque silica (SiO₂)-based fillers, zirconia (ZrO₂) powders were mechanically added to SiO₂ powders with ZrO₂ content up to 40 wt%. We evaluated the radiopacity of experimental composite resins consisting of (Bis-GMA + TEGDMA + CQ + DMAEMA) monomer mixture (25 wt%) and admixed SiO₂–ZrO₂ fillers (75 wt%), and compared their radiopacity with those of human dentin and enamel. It became confirmed that the radiopacity of experimental composite resins increased linearly with zirconium content, while the composite resin containing 80 wt% SiO₂-20 wt% ZrO₂ filler possessed radiopacity similar to that of human enamel. It was proved that the radiopacity of the composites could be precisely controlled by adjusting ZrO₂ content in SiO₂–ZrO₂ fillers.     

Dielectric properties and electrical conductivity of ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> ceramics

ZrO₂-CeO₂ (10, 18, and 23 mol % CeO₂) solid solutions have been synthesized via coprecipitation. The powders have been sintered at 1875 K, and the electrical conductivity and dielectric properties of the resultant ceramics have been studied. The dielectric relaxation observed in the ceramics can be understood in terms of ionic transport accompanied by the formation of dipoles relaxing in an ac electric field.     

Effect of Mn doping on the dielectric properties of BaZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> ceramics

Pure and Mn-doped BaZr_0.2Ti_0.8O₃ ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of Mn-doped BaZr_0.2Ti_0.8O₃ ceramics have been investigated. The results indicate that manganese ions enter the unit cell maintaining the perovskite structure of solid solution. The addition of manganese leads to the decrease of the Curie temperature. The dielectric loss of the Mn-doped BZT ceramics is lower than that of pure BZT ceramics, and decreases as Mn content increases. The diffuseness of the phase transition of Mn-doped BZT ceramics decreases with the increase of Mn content. There is no obvious frequency dispersion around the dielectric constant peaks for Mn-doped BZT ceramics. The coercive electric field and the remanent polarization decreases as Mn content increases.     

Influence of CuO and B<Subscript>2</Subscript>O<Subscript>3</Subscript> on sintering and dielectric properties of tungsten bronze type microwave ceramics: a case study in Ba<Subscript>4</Subscript>Nd<Subscript>9.3</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript>

The effect of CuO and B₂O₃ co-doping on the sintering behavior, microstructure and microwave dielectric properties of tungsten bronze type Ba₄Nd_9.3Ti₁₈O₅₄ (BNT) ceramics has been investigated by means of a traditional solid-state mixed oxide route. On the one hand, it was indicated that the mixture of CuO and B₂O₃ is an effective sintering aid for BNT matrix compositions owing to the existence of a low-temperature eutectic reaction. On the other hand, it was found that the addition of CuO and B₂O₃ has an obvious effect on microwave dielectric properties of BNT ceramics, depending on the amount of sintering aids, the sample density and microstructure. The liquid phases from sintering aids can promote densification, but simultaneously induce grain growth which tends to decrease the sintering driving force. BNT ceramics doped with 3 wt% CuO–B₂O₃ mixture can be well sintered at 950°C for 4 h and still exhibit relatively good microwave dielectric properties.     

Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3‐Based Lead‐Free Ceramics

High‐performance lead‐free piezoelectric materials are in great demand for next‐generation electronic devices to meet the requirement of environmentally sustainable society. Here, ultrahigh piezoelectric properties with piezoelectric coefficients (d₃₃ ≈700 pC N^?1, d₃₃* ≈980 pm V^?1) and planar electromechanical coupling factor (k_p ≈76%) are achieved in highly textured (K,Na)NbO₃ (KNN)‐based ceramics. The excellent piezoelectric properties can be explained by the strong anisotropic feature, optimized engineered domain configuration in the textured ceramics, and facilitated polarization rotation induced by the intermediate phase. In addition, the nanodomain structures with decreased domain wall energy and increased domain wall mobility also contribute to the ultrahigh piezoelectric properties. This work not only demonstrates the tremendous potential of KNN‐based ceramics to replace lead‐based piezoelectrics but also provides a good strategy to design high‐performance piezoelectrics by controlling appropriate phase and crystallographic orientation.     

Piezoelectricity Across a Strain‐Induced Isosymmetric Ferri‐to‐Ferroelectric Transition

We identify a first‐order, isosymmetric transition between a ferrielectric (FiE) and ferroelectric (FE) state in A‐site ordered LaScO₃/BiScO₃ and LaInO₃/BiInO₃ superlattices. Such a previously unreported ferroic transition is driven by the easy switching of cation displacements without changing the overall polarization direction or crystallographic symmetry. Epitaxial strains less than 2% are predicted to be sufficient to traverse the phase boundary, across which we capture a ≈5× increase in electric polarization. Unlike conventional Pb‐based perovskite ceramics with a morphotropic phase boundary (MPB) that show polarization rotation, we predict an electromechanical response up to 102 pC/N in the vicinity of the FiE‐FE phase boundary due to polarization switching without any change in symmetry. We propose this transition as an alternative ferroic transition to obtain a piezoelectric response, with the additional advantage of occurring in benign chemistries without chemical disorder.     

Nanocrystalline PbIn<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> ceramics and its properties

A ferroelectric relaxor PbIn_0.5Nb_0.5O₃ (PIN) ceramics has been obtained using a modified ceramic technology, with the sintering stage preceded by compression and shear straining of the synthesized charge in Bridgman anvils. The dimensions of ceramic grains after this pretreatment are spread over a range from 100 to 1250 nm. A comparative investigation of the properties of PIN ceramics obtained using the standard and modified technology showed that the proposed mechanical action at the charge preparation stage can be used for controlled modification of the properties of ferroelectric ceramics.     

Abrasive wear in filled elastomers

The abrasive wear of rubbers is strongly affected by the filler particles dispersed in the elastomer matrix. The fillers are incorporated usually for the purposes of mechanical reinforcement and improving the conductivity of the neat resins. It is found that the wear rates of the filled silicone rubbers increase slowly with filler concentration until a critical volume fraction,v _c, is reached, at which point they increase very rapidly with increasing filler concentration. This behaviour appeared to be universal in all the filled silicones we studied, regardless of the type of filler and silicone rubber used. However the magnitude of the critical filler fraction,v _c, can be changed significantly with the filler shape, resin cross-linking density and filler surface treatments. No reasonable relationship could be found between this wear behaviour and the mechanical properties measured in a macroscopic manner. Experimental evidence suggests that the incipient cracks that lead to wear losses may start within the thin layers of highly stressed material, the damage zones, surrounding the rigid particles. A simple model taking into account the stress concentration induced by the rigid fillers shows excellent correlation between the wear rate and the damage zones volume. With this new model, the observed wear behaviours can be explained satisfactorily.     

Effect of Ho-doping on structural,electrical and magnetic properties of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics prepared by Plasma-Activated Sintering

Different components of La_0.7?x Ho _x Sr_0.3MnO₃ (LHSMO, x = 0, 0.1, 0.2, 0.3) ceramics were fabricated by Plasma-Activated Sintering (PAS), so as to study the correlation between the contents of Ho³⁺ and the structural, electrical, magnetic properties. XRD and SEM confirmed that LHSMO ceramics prepared by PAS exhibited high-purity phase and dense microstructure. The measurement of electrical resistivity showed that the resistivity of LHSMO ceramics increased, and the metal–insulator transition temperature decreased with the increasing Ho-doping content. The resistivity data were then fitted using various empirical equations, and the conduction mechanism of LHSMO ceramics was found to be in accord with the electron–magnon scattering process in the low-temperature region and the small polaron hopping model in the high-temperature region. Lastly, we calculated the values of magnetoresistance of the LHSMO ceramics, which increased with increasing Ho-doping content, from 3.5% for x = 0 to 14.6% for x = 0.3. Therefore, the doping of Ho³⁺ into La_0.7Sr_0.3MnO₃ can effectively enhance the low-field magnetoresistance effect.     

Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Lead‐Free Antiferroelectric Silver Niobate Tantalate with High Energy Storage Performance

Antiferroelectric materials that display double ferroelectric hysteresis loops are receiving increasing attention for their superior energy storage density compared to their ferroelectric counterparts. Despite the good properties obtained in antiferroelectric La‐doped Pb(Zr,Ti)O₃‐based ceramics, lead‐free alternatives are highly desired due to the environmental concerns, and AgNbO₃ has been highlighted as a ferrielectric/antiferroelectric perovskite for energy storage applications. Enhanced energy storage performance, with recoverable energy density of 4.2 J cm^?3 and high thermal stability of the energy storage density (with minimal variation of ≤±5%) over 20–120 °C, can be achieved in Ta‐modified AgNbO₃ ceramics. It is revealed that the incorporation of Ta to the Nb site can enhance the antiferroelectricity because of the reduced polarizability of B‐site cations, which is confirmed by the polarization hysteresis, dielectric tunability, and selected‐area electron diffraction measurements. Additionally, Ta addition in AgNbO₃ leads to decreased grain size and increased bulk density, increasing the dielectric breakdown strength, up to 240 kV cm^?1 versus 175 kV cm^?1 for the pure counterpart, together with the enhanced antiferroelectricity, accounting for the high energy storage density.