Effect of Hafnium content on structural evolution of SiHfOC ceramics at high temperatures

SiHfOC ceramics with different Hf contents were prepared by the sol-gel method using polymethylsilsesquioxane and HfOCl₂·8H₂O as the raw materials. The high-temperature structural evolution of the SiHfOC ceramics with different Hf contents was investigated under an inert atmosphere. The results showed that Hf existed in the free state in the SiHfOC precursors with Hf:Si atomic ratios of 0.05 and 0.1. When the Hf:Si ratio was increased to 0.2, Hf was bridged in the SiOC network to form Si–O–Hf bonds, which changed the pyrolysis behavior of the ceramics and improved the ceramic yield. Further, the formation of Si–O–Hf bonds promoted the uniform dispersion of HfO₂ nanocrystals inside the ceramic, which further inhibited carbothermal reduction and increased the thermal stability of the SiHfOC ceramics.     

High‐Temperature Creep Behavior of SiOC Glass‐Ceramics: Influence of Network Carbon Versus Segregated Carbon

Three silicon oxycarbide samples with different carbon contents are analyzed in the present study with respect to their high‐temperature creep behavior. The tests were performed in compression at 1100°C, 1200°C, and 1300°C; in this temperature range the mechanism of creep relies on viscoelastic flow within the samples and has been modeled with the Jeffreys viscoelastic model. After the release of the applied mechanical stress, a viscoelastic recovery behavior was observed in all samples. The creep behavior of the investigated samples indicates two rheological contributions in SiOC: (i) a high viscous answer, coming from the silica‐rich network, and (ii) an elastic response from the segregated carbon phase within the samples. Furthermore, two distinct effects of the carbon phase on the HT creep behavior of SiOC were identified and are discussed in the present paper: the effect of the carbon presence within the SiOC network (the “carbidic” carbon), which induces a significant increase in the viscosity and a strong decrease in the activation energy for creep, as compared to vitreous silica; and the influence of the segregated carbon phase (the “free” carbon), which has been shown to affect the viscosity and the activation energy of creep and dominates the creep behavior in phase‐separated silicon oxycarbides.     

Polymer‐derived ceramic aerogels as sorbent materials for the removal of organic dyes from aqueous solutions

Polymer‐derived SiC and SiOC aerogels have been synthesized and characterized both from the microstructural point of view and as sorbent materials for removing organic dyes (Methylene Blue, MB, and Rhodamine B, RB) from water solutions. Their adsorbent behavior has been compared with a polymer‐derived SiC foam and a commercial mesoporous silica. The aerogels can efficiently remove MB and RB from water solution and their capacity is higher compared to the SiC foams due to the higher surface area. The SiOC aerogel remains monolithic after the water treatment (allowing for an easy removal without the need of a filtration step) and its maximum capacity for removing MB is 42.2 mg/g, which is higher compared to the studied mesoporous silica and many C‐based porous adsorbents reported in the literature. The reason for this high adsorption capacity has been related to the unique structure of the polymer‐derived SiOC, which consists of an amorphous silicon oxycarbide network and a free carbon phase.     

Cyclic Fatigue Life‐ and Crack‐Growth Behavior of Zirconia–Niobium Composites

A 3Y‐TZP/Nb composite fabricated by Hot‐pressing (HP) sintering with well‐distributed lamellar/flake–shaped metal particles (20 vol% fraction) has been studied under cyclic loading. Fatigue life was determined for the ceramic/metal composite as well as for monolithic zirconia to compare the sensitivities of both materials to cyclic stresses. In both cases, the fatigue test was performed according to ISO 6872. It was found that the 3Y‐TZP/Nb composite exhibits fatigue behavior which was compared with monolithic zirconia. The growth of fatigue cracks influences the bridging actions of the metallic grains and causes significant degradation in the mechanical properties of the composite material.     

Flexible and thermal-stable SiZrOC nanofiber membranes with low thermal conductivity at high-temperature

Ceramic nanofibers with excellent thermal stability and low thermal conductivity are highly desired for high-temperature thermal insulation applications. However, the incompatibility of thermal stability and low thermal conductivity at high-temperatures largely limit the practical use of conventional single-phase ceramic nanofibers. Here, we report the preparation of multi-phase SiZrOC nanofiber membranes (NFMs) composed of amorphous SiOC and ZrO₂ nanocrystals via electrospinning technique. The fabricated SiZrOC NFMs exhibited excellent high-temperature stability (∼1200 °C in Ar) and low thermal conductivity (∼0.1392 W m⁻¹ K⁻¹ at 1000 °C in N₂). The decreased thermal conductivity is achieved through a synergistic mechanism, that the multi-phase interfaces and the ZrO₂ nanocrystals create thermal transfer barriers to reduce the heat transfer, whilst the SiOC phase effectively suppresses radiative heat transfer. This unique combination of amorphous SiOC and ZrO₂ nanocrystals provides a novel strategy to prepare high-performance thermal insulation materials, and the obtained SiZrOC NFMs are promising high-temperature thermal insulation materials.     

Polyamide 6 nanocomposites incorporating cellulose nanocrystals prepared by In situ ring‐opening polymerization: Viscoelasticity,creep behavior,and melt rheological properties

The creep behavior and solid and melt linear viscoelasticity of novel polyamide 6 (PA6) nanocomposites reinforced with cellulose nanocrystals (CNCs) prepared via in situ anionic ring‐opening polymerization (ROP) were investigated to accelerate research efforts to develop new polymeric materials with improved properties for lightweight, load‐bearing applications. The obtained results showed that incorporation of relatively small amounts of ≤ 2wt% CNCs into the PA6 thermoplastic matrix gave nanocomposite samples with significantly enhanced creep and viscoelastic materials functions of the PA6 as indicated by lower creep strain, lower creep compliance, improved elastic recovery after removal of load, and reduced Arrhenius activation energies for time‐dependent viscoplastic flow. The obtained results were analyzed and interpreted by theoretical equations for predicting the viscoelasticity and creep behavior of polymeric systems. The melt rheological properties showed enhanced melt strength and elasticity. The formation of a percolated network structure of CNC was revealed by morphological observations that were consistent with the dynamic structure break‐up and reformation rheological experiments. The stiffness, rigidity of the CNCs along with their special ROP‐facilitated intrinsic strong chemical interactions with the PA6 matrix is believed to be responsible for the observed superior creep and viscoelastic materials functions even with very little CNC concentration. POLYM. ENG. SCI. 56:1045–1060, 2016. © 2016 Society of Plastics Engineers     

Structure and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites

The incorporation of SiOC polymer‐derived ceramics into porous carbon materials could provide tailored shapeable, mechanical, electrical, and oxidation‐resistant properties for high‐temperature applications. Understanding the thermodynamic and kinetic stability of such materials is crucial for their practical application. We report here the dependence of structures and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites (CBCFs) on the pyrolysis temperature using spectroscopic methods and high‐temperature oxide melt solution calorimetry. The results indicate that a SiOC ceramic pyrolyzed at 1200°C and 1600°C is energetically stable with respect to an isocompositional mixture of cristobalite, silicon carbide, and graphite by 4.9 and 10.3 kJ/mol, respectively, and more energetically stable than that pyrolyzed at 1450°C. Their thermodynamic stability is related to their structural evolution. SiOC‐modified CBCFs become energetically less stable with increasing preparation temperature and concomitant increase in excess carbon content.     

Limits to the Stability of the Amorphous Nature of Polymer‐Derived HfSiCNO Compounds

Substituting silicon by transition metals in polymer‐derived ceramics (PDCs) holds the potential for a new class of polymer‐derived ceramics for ultrahigh‐temperature structural applications. We present experiments that show that the solid solubility of HfO₂ extends to Hf/Si ratio of <0.22. The materials are synthesized from (miscible) organic precursors. Similar to silicon‐based materials they remain amorphous after pyrolysis at 1000°C. Small‐angle X‐ray scattering and Raman spectra remain essentially unaltered. It is postulated that Hf, like Si, forms mixed‐bond tetrahedra with C, O, and N. The difference in the enthalpy of Hf‐based, and Si‐centered tetrahedra is calculated using single‐bond energies, reinforcing the feasibility of substituting Si with Hf or with Zr atoms. Such polymer‐based HfSiCNO compounds made directly from liquid organics, by a simple manufacturing process, may also be relevant to nanoscale dielectrics with low leakage electric charge in microelectronics applications.     

Creep behavior of glass‐fiber‐reinforced nylon 6 products

The creep properties, that is, the velocity constant, activation energy, stress index, and time index, of a test piece (TP) cut from a glass‐fiber‐reinforced nylon 6 product were successfully determined by a compression creep test. In the determination of the creep properties, the experimental creep curves for the TP were fitted by finite element analysis (FEA). Fiber‐reinforced nylon 6 beams with different fiber orientations were also prepared, and their creep properties were successfully determined by a combination of the bending creep test and the corresponding analysis. The creep behavior of the press‐fit component composed of a metal collar and a fiber‐reinforced nylon 6 product was predicted by FEA with the determined creep properties of the TP. The predicted retention forces were in good agreement with the experimental ones. The effects of the fiber orientation on the long‐term reliability of the press‐fit component are also discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Creep and recovery behavior of novel organic‐inorganic polymer hybrids

A novel class of organic‐inorganic polymer hybrids were developed by meltblending up to 50 (v/v) % [about 83 (w/w) %] tin‐based polyphosphate glass (Pglass) and low‐density polyethylene (LDPE) in conventional plastics processing equipment. The creep and recovery behavior of these polymer hybrids at 30°C were studied to understand the effect of the Pglass on the creep resistance of the LDPE. The results suggest that the Pglass acts as a reinforcement and an increase in the Pglass loading leads to significantly lower creep strains. This creep resistance is further enhanced by pretreating the Pglass with coupling agents prior to incorporating them into the Pglass‐LDPE hybrids. The experimental creep compliance of these materials conformed excellently with empirical power‐law equation and a modified Burger's model, suggesting that the materials are linearly viscoelastic under the test conditions.     

Effect of mold temperature on the long‐term viscoelastic behavior of polybutylene terepthalate

The effect of mold temperature variation during injection molding on the long‐term viscoelastic behavior of polybutylene terepthalate (PBT) was studied by dynamic mechanical thermal analysis (DMTA) and flexural creep tests. The time–temperature superposition (TTS) principle was applied to the experimental data and the master curves were created to predict their long‐term behavior. The WLF and Arrhenius models were verified for the shift data in the investigating temperature range and the activation energies for the deformation process were calculated based on the Arrhenius equation. Further a four‐element Burger model was applied to the creep results to represent the creep behavior of the PBT processed at two different mold temperatures and to better understand the deformation mechanism. Differential scanning calorimetry (DSC) and density measurements were accomplished to characterize the process‐dependent microstructures. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Preparation and high‐temperature behavior of HfC–SiC nanocomposites derived from a non‐oxygen single‐source‐precursor

A single‐source precursor for the preparation of HfC‐SiC ceramics was synthesized via a Grignard reaction using bis(cyclopentadienyl)hafnium(IV) dichloride, trans‐1,4‐dibromo‐2‐butene, and (chloromethyl)trimethylsilane as raw materials. The composition, structure, pyrolysis process and high‐temperature behavior of the precursor were investigated. The results show that the precursor with a backbone comprising Hf–C, Si–C and CH=CH groups exhibits good solubility in common solvents, such as tetrahydrofuran, dimethylbenzene, and chloroform. Pyrolysis of the precursor at 1000°C yielded a microcrystalline HfC phase with a ceramic yield of 63.86 wt%. The pyrolytic products at 1600°C were HfC–SiC nanocomposite ceramics, which exhibited good thermal stability up to 2400°C. The formation of a (Hf,Si)C solid‐solution would be beneficial for densification during the sintering process. The non‐oxygen structure, high ceramic yield, homogeneous composition and excellent high‐temperature behavior of the pyrolytic products make the as‐prepared precursor a promising material for the preparation of high‐performance ultra‐high‐temperature ceramics.     

Impression creep of PMR‐15 resin at elevated temperatures

The polyimides formed from the polymerization of monomeric‐reactants (PMR) approach have been increasingly used as matrix materials in fiber‐reinforced composites on aerospace and space structures for high temperature applications. The performance of PMR‐based structures depends on the mechanical durability of PMR resins at elevated temperatures, including creep and stress relaxation. In this work, the creep behavior of PMR‐15 resin was studied using the impression technique in the temperature range of 563–613 K and the punching stress range of 76–381 MPa. It was found that there existed a steady state creep for the creep tests performed at temperatures of 563 K and higher, from which a constant impression velocity was calculated. The steady state impression velocity increased with temperature and punching stress with the stress exponent in the range of 1.5–2.2. The average of the apparent activation energy of the PMR‐15 was calculated as 122.7 ± 6.1 kJ/mol. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Thermodynamic Control of Phase Composition and Crystallization of Metal‐Modified Silicon Oxycarbides

Silicon oxycarbides modified with main group or transition metals (SiMOC) are usually synthesized via pyrolysis of sol‐gel precursors from suitable metal‐modified orthosilicates or polysiloxanes. In this study, the phase composition of different SiMOC systems (M = Sn, Fe, Mn, V, and Lu) was investigated. Depending on the metal, different ceramic phases formed. For M = Mn and Lu, MO_x/SiOC ceramic nanocomposites were formed, whereas other compositions revealed the formation of M/SiOC (M = Sn), MSi_x/SiOC (M = Fe) or MC_x/SiOC (M = V) upon pyrolysis. The different phase compositions of the SiMOC materials are rationalized by a simple thermodynamic approach which generally correctly predicts which type of ceramic nanocomposite is expected upon ceramization of the metal‐modified precursors. Calculations show that the thermodynamic stability of the MO_x phase with respect to that of the C–O system is the most important factor to predict phase formation in polymer‐derived SiMOC ceramic systems. A secondary factor is the relative stability of metal oxides, silicates, carbides, and silicides.     

Creep of Beryllia Single Crystals

The compressive creep behavior of single-crystal BeO was studied under inert atmosphere. The steady-state creep behavior was highly anisotropic because of easily activated basal slip. Crystals compressed along a [1101] axis deformed at temperatures as low as 650°C, whereas [1100]-aligned crystals showed comparable strain rates at temperatures 1650°C. The measured activation energies and stress exponents for creep were 371 kJ/mol and 4.65 for the [1101] alignment and 496 kJ/mol and 3.39 for the [1100] alignment, respectively. The precise mechanisms controlling creep behavior were not determined. Only one semiquantitative data point was obtained for the [0001] alignment, which indicates that the creep rate at 100 MPa and 1750°C is less than that for c -axis sapphire.     

High-Temperature Plastic Behavior of TZP–Ni Cermets

The creep behavior of tetragonal zirconia TZP–Ni cermets with metal contents below, close to, and above the percolation limit has been studied. Compressive creep tests were performed on as-received materials and samples in which the metal phase was chemically removed (ceramic skeletons). The stress exponent and the activation energy for plastic flow are independent of the nickel content and decrease continuously on increasing the stress and/or the temperature; skeleton structures display the same trend, suggesting that creep is controlled by the zirconia matrix. The steady-state constitutive equation for high-purity monolithic zirconia applies to the cermets when the stress is corrected with the porosity and volume fraction of percolated nickel.     

Crystallization behavior and foaming properties of polypropylene containing ultra‐high molecular weight polyethylene under supercritical carbondioxide

In this study, a systematic investigation on the nonisothermal crystallization kinetics of conversional polypropylene (PP) containing various amounts of ultra‐high molecular weight polyethylene (UHMWPE) was reported, and the effects of UHMWPE on crystallization behavior of these PP materials and their foaming properties were also presented. The kinetic studies revealed that the incorporation of UHMWPE into PP led to an increase in the crystallization temperature and temperature range during the crystallization process as well as the relative crystallinity. This behavior was attributed to a comprehensive effect of the nucleation and entanglement of the UHMWPE chains. The kinetic models based on Ozawa's and Mo's methods were used to analyze the nonisothermal crystallization behaviors. It was found that the latter succeeded in describing the nonisothermal crystallization behavior of the PP containing UHMWPE, while the former was not appropriate. The activation energy for the nonisothermal crystallization determined by Kissinger's method also indicated that the crystallization ability of PP was improved with the addition of UHMWPE. Owing to the modification of the crystallization kinetics of the PP materials by introduction of UHMWPE, the foaming properties (i.e., cell uniformity and expandability etc.) were improved significantly. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Tensile creep behavior of unidirectional glass‐fiber polymer composites

The tensile creep behavior of unidirectional glass‐fiber polymer composites was studied at three different temperatures, namely 298, 333, and 353 K. Testing was performed on the pure epoxy matrix, the 0° specimens as well as off‐axis at 15, 30, and 60 degrees in respect to the axis of tension. The creep strain rate was negligible at room temperature, while it was considerable at the higher temperatures examined. The materials exhibit nonlinear viscoelastic behavior, and the creep response of the composites was treated as a thermally activated rate process. The creep strain was considered to include an elastic, a viscoelastic and a viscoplastic part. The viscoplastic part was calculated through a functional form, developed in a previous work, assuming that viscoplastic response of polymer composites arises mainly from the matrix viscoplasticity. The model predictions in terms of creep compliances were found to be satisfactory, compared with the experimental results. POLYM. COMPOS. 26:287–292, 2005. © 2005 Society of Plastics Engineers.     

Creep Viscosity and Stress Relaxation of Gel-Derived Silicon Oxycarbide Glasses

The temperature dependence of the viscosity and stress-relaxation kinetics of sol–gel-derived SiOC glasses that contain up to 14 at.% carbon have been characterized in the temperature range of 1000°–1400°C. The viscosity, as determined from relaxation experiments, is in good agreement with the creep viscosity and is typically two orders of magnitude higher than the viscosity of vitreous silica. However, materials suffer from partial crystallization at >1150°C, and the precipitation of β-SiC nanocrystals induces a flow-hardening behavior and results in a dynamic increase in viscosity, especially at >1200°C and for glasses with a high carbon content.     

Crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 nanocomposites

This article presents the effects of nanoclay and supercritical nitrogen on the crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 (PA6) nanocomposites with 5 and 7.5 wt% nanoclay. Differential scanning calorimetry (DSC), X‐ray diffractometry (XRD), and polarized optical microscopy (POM) were used to characterize the thermal behavior and crystalline structure. The isothermal and nonisothermal crystallization kinetics of neat resin and its corresponding nanocomposite samples were analyzed using the Avrami and Ozawa equations, respectively. The activation energies determined using the Arrhenius equation for isothermal crystallization and the Kissinger equation for nonisothermal crystallization were comparable. The specimen thickness had a significant influence on the nonisothermal crystallization especially at high scanning rates. Nanocomposites with an optimal amount of nanoclay possessed the highest crystallization rate and a higher level of nucleation activity. The nanoclay increased the magnitude of the activation energy but decreased the overall crystallinity. The dissolved SCF did not alter the crystalline structure significantly. In contrast with conventionally injection‐molded solid counterparts, microcellular neat resin parts and microcellular nanocomposite parts were found to have lower crystallinity in the core and higher crystallinity near the skin. POLYM. ENG. SCI., 46:904–918, 2006. © 2006 Society of Plastics Engineers