Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Enhancement of Mechanical and Self‐Healing Performance in Multiwall Carbon Nanotube/Rubber Composites via Diels–Alder Bonding

Mechanically robust and self‐healing rubbers are highly desired to satisfy the increasing demand of high‐performance smart tires and related materials. Herein, a self‐healing rubber nanocomposite with enhanced mechanical and self‐healing performance based on Diels–Alder chemistry has been investigated. The furfuryl grafted styrene‐butadiene rubber and furfuryl terminated MWCNT (MWCNT‐FA) are reacted with bifunctional maleimide to form a covalently bonded and reversibly cross‐linked rubber composite. Obvious reinforcing effect is obtained at high cross‐linking density. Over 200–300% increase in the Young's modulus and toughness can be achieved in the rubber nanocomposites with 5 wt% MWCNT‐FA. Meanwhile, the healing efficiency increased with MWCNT‐FA content. MWCNT‐FA plays dual roles of effective reinforcer and a kind of healant.

     

Al2O3 Nanoparticulate LZS Glass–Ceramic Matrix Composites for Production of Multilayered Materials

This work reports the processing steps of Al₂O₃ (1–5 vol%) nanoparticulate (d_V.50 = 13 nm) LZS glass–ceramic matrix (19.58Li₂O·11.10ZrO₂·69.32SiO₂, mol%, d_v.50 = 3.5 μm) composites for production of multilayered materials with thermal expansion gradients obtained by tape casting. Suspensions were prepared in water to solids contents ranging from 40 to 47 vol% using ammonium polyacrylate as a deflocculant, and an acrylic copolymer and polyvinyl alcohol as binders. Optimum performance was achieved by sonication and controlling the rheological properties for every step of the process. To prepare the composites, different concentrations (1, 2.5 and 5 vol%) of nanoalumina were added to fresh, as‐prepared LZS suspensions, by changing the solid contents as required to maintain similar viscosities. Green tapes with high uniformity, without macroscopic defects and easy to handle were sintered to relative densities between 89% and 94%. Dense and homogeneous laminates with gradual composition with increasing concentrations of alumina were obtained.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Reaction‐Bonded Boron Carbide/Magnesium–Silicon Composites

Reaction‐bonded boron carbide was manufactured by infiltrating porous boron carbide preforms at 1273 K with a Mg‐Si eutectic alloy. The resulting composite material consists, in addition to the original B₄C, of SiC, Mg₂Si, and a Mg‐rich complex boride/carbide Mg_x(Al,Si)_y(B,C)_z phase. The composites display high hardness (1700 HV), Young's modulus (356 MPa) and a moderate bending strength (230 MPa). The ballistic efficiency (of about 6.7), as determined by the depth of penetration method, is much higher than that of alumina and similar to that of silicon‐infiltrated reaction‐bonded composites.     

Preparation and Dielectric Characterization of Lead‐Free Niobate Glass‐Ceramic Composites Added with Lu2O3

Lead‐free niobate glass‐ceramic composites added with Lu₂O₃ were synthesized through melt‐quenching followed by controlled crystallization techniques. X‐ray diffraction and scanning electron microscopy showed that a minor phase determined as LuNbO₄ was formed as the Lu₂O₃ addition exceeded 1 mol% in the basic glass‐ceramic. The formation of LuNbO₄ was due to the saturation of Lu³⁺ doping in the major crystalline phases. Analyses of the dielectric properties and breakdown strength (BDS) indicated that they were closely related to the formation of this new minor phase. Although the dielectric constant was not increased continuously, the dielectric loss tangent was well controlled below 0.017, and the BDS increased from 40.2 to 47.2 kV/mm for the glass‐ceramic composites with increasing Lu₂O₃ addition. This study provided the Lu₂O₃ added lead‐free niobate glass‐ceramic as an attractive candidate for making high‐energy density capacitors.     

Joining of Porous Alumina with a CaO–Al2O3–SiO2 Glass‐Ceramic

A CaO–Al₂O₃–SiO₂ (CAS)‐based glass interlayer was developed for joining of porous alumina membrane tubes with dense alumina in this work. The results indicated that the interfacial microstructure of the joint was highly sensitive to the quench rate from the joining temperature, which rendered crystallization of CaTiSiO₅ at a fast quench rate but CaAl₂Si₂O₈ at a slow quench rate due to the interfacial reaction between the CAS glass interlayer and the substrate. An extra crystallization treatment during quench, i.e., dwelling at 800°C–900°C for 2 h, produced a multiphase interlayer consisting of LiAlSi₂O₆, CaTiSiO₅, and CaAl₂Si₂O₈. All joints were evaluated by the thermal shock test. The results showed that the LiAlSi₂O₆‐containing joint interlayer had much lower thermal shock resistance than those without LiAlSi₂O₆.     

Adaptive Polymeric Coatings with Self‐Reporting and Self‐Healing Dual Functions from Porous Core–Shell Nanostructures

In biological system, early detection and treatment at the same moment is highly required. For synthetic materials, it is demanding to develop materials that possess self‐reporting of early damage and self‐healing simultaneously. This dual function is achieved in this work by introducing an intelligent pH‐responsive coatings based on poly(divinylbenzene)‐graft‐poly(divinylbenzene‐co‐methacrylic acid) (PDVB‐graft‐P(DVB‐co‐AA)) core–shell microspheres as smart components of the polymer coatings for corrosion protection. The key component, synthesized PDVB‐graft‐P(DVB‐co‐AA) core–shell microspheres are porous and pH responsive. The porosity allows for encapsulation of the corrosion inhibitor of benzotriazole and the fluorescent probe, coumarin. Both loading capacities can be up to about 15 wt%. The polymeric coatings doped with the synthesized microspheres can adapt immediately to the varied variation in pH value from the electrochemical corrosion reaction and release active molecules on demand onto the damaged cracks of the coatings on metal surfaces. It leads simultaneously to the dual functions of self‐healing and self‐reporting. The corrosion area can be self‐reported in 6 h, while the substrate can be protected at least for 1 month in 3.5 wt% NaCl solution. These pH‐responsive materials with self‐reporting and self‐healing dual functions are highly expected to have a bright future due to their smart, long‐lasting, recyclable, and multifunctional properties.     

Crack Healing in Ti2Al0.5Sn0.5C–Al2O3 Composites

Oxidation induced crack healing of Al₂O₃ composites loaded with a MAX phase based repair filler (Ti₂Al_0.5Sn_0.5C) was examined. The fracture strength of 20 vol% repair filler loaded composites containing artificial indent cracks recovered fully to the level of the virgin material upon isothermal annealing in air atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM‐EBSD analysis of crack microstructure indicates two different oxidation reaction regimes to govern the crack filling: near the surface SnO₂, TiO₂, and Al₂O₃ were formed whereas deeply inside the cracks Al₂O₃ and TiO₂ and metallic Sn were detected. The presence of elemental Sn was attributed to partial oxidation of aluminum and titanium which lowered the local oxygen concentration below a threshold value required for Sn oxidation to SnO₂. Thus, Ti₂Al_0.5Sn_0.5C may represent an efficient repair filler system to trigger oxidation induced crack healing in ceramic composites at temperatures below 1000°C.     

High‐temperature Bending Strength of Self‐Healing Ni/Al2O3 Nanocomposites

Bending strength of 5 vol.% Ni/Al₂O₃ composites as a function of testing temperature is investigated at temperatures ranging from room temperature to 1200°C. Self‐healing performance at high temperatures of the composites is evaluated by conducting high‐temperature bending tests for as‐sintered, as‐cracked, and as‐healed specimens. Bending strength of as‐sintered specimens dramatically decreases from 995 MPa at room temperature to 205 MPa at 1200°C. Additionally, the plastic deformation of the as‐sintered specimens occurs when the testing temperature reaches to 1200°C. The values of high‐temperature bending strength of as‐healed specimens are comparable with those of as‐sintered specimens. Similar to that of as‐sintered specimens, bending strength of as‐healed specimens degrades when the testing temperature increases. Results of the present study indicate that the recovery of bending strength by the self‐healing function is able to achieve at temperatures as high as 1200°C. Unlike the mechanical behaviors at high temperatures of as‐sintered and as‐healed specimens, the bending strength of as‐cracked specimens slightly increases with the increase of testing temperature. This phenomenon is attributed to the effect of the self‐healing mechanism during high‐temperature bending tests.     

Self‐Healing and Shape‐Memory Superconducting Devices

Intelligent materials possess the function of self‐judgment and self‐optimization while sensing external stimuli such as stress, temperature, moisture, pH, electric or magnetic fields, or light. Besides, they often require self‐healing—the ability to repair damage spontaneously—or shape‐memory—the ability to return from a deformed state to their original shape induced by an external stimulus. Introducing such intelligence into superconducting (SC) devices is highly desirable to meet the critical requirement of maintenance‐free performance. Here, self‐healing and shape‐memory functions are realized in liquid metal based SC devices using smart packaging polymers. Without deteriorating their superconductivity, the SC devices can repair themselves by simply raising the temperature, without any other treatment. Beyond the specific functions achieved here, this work sheds new light on future SC devices with advanced functions such as self‐diagnosis, self‐adjusting, and sensing.     

2D‐ and 3D‐Photoluminescence Imaging of Eu(III) Embedded in CeO2 Nanomatrix

2D‐ and 3D‐photoluminescence characteristics of Eu(III) doped in CeO₂ nanoparticles were fully imaged for the first time. Their fundamental natures were also examined by scanning electron microscopy (SEM), X‐ray diffraction (XRD) crystallography, and UV–visible absorption spectroscopy. The magnetic dipole ⁵D₀ → 7F₁ transition was dominated by an indirect transition associated with a O^2?–Ce⁴⁺ charge‐transfer band of CeO₂. The electric dipole ⁵D₀ → ⁷F₂ transition was dominated by a direct transition of Eu(III), indicating that Eu(III) replaces Ce(IV) at octahedral sites (O_h and O) with and without an inversion center. Upon annealing, the photoluminescence intensity caused by direct transition was dramatically decreased, whereas that induced by indirect transition was greatly enhanced. These findings indicate that charge transfer to the Eu(III) at the octahedral (O_h) site with the inversion center is more efficient than that to the Eu(III) site without an inversion center. The absolute quantum yield for the 10 mol% Eu(III)–CeO₂ was found to be ? = 0.007 at an excitation wavelength of 350 nm. The photoluminescence of Tb‐doped CeO₂ was briefly discussed for comparison.     

Self‐Detecting and Self‐Healing Reinforce Elastomer Doped with Aggregation‐Induced Emission Molecules

Most of elastomers for fabrication of comfortable epidermal devices and smart actuators produce responsive signals by the stimuli‐induced deformation. Herein, a dynamic visualization of external stimuli rather than the deformation through synthesis of a self‐healing poly(dimethylsiloxane) (PDMS)‐based elastomer doped with aggregation‐induced emission (AIE) molecules is reported. The self‐healing PDMS‐based elastomer is designed and synthesized through molecule integration of reversible multi‐strength H‐bonds and permanent covalent crosslink sites. The adjustment of the weight ratio of elastic cross‐linker offers tunable mechanical properties of the resultant elastomer. After doping such an elastomer with AIE molecules of 1,1,2,2‐tetrakis(4‐nitrophenyl)ethane, the elastomer composite displays strong on–off fluorescence depending upon mechanical damage and temperatures, which can be used to detect the breaking and self‐healing performances, as well as the temperature change. The strategy described here provides another way to develop smart polymeric elastomers for practical applications.     

Frequency and Temperature‐Independent Electrical Transport Properties of 2BaO–0.5Na2O–2.5Nb2O5–4.5B2O3 Glass‐Ceramics

The temperature (300–973 K) and frequency (100 Hz–10 MHz) response of the dielectric and impedance characteristics of 2BaO‐0.5Na₂O–2.5Nb₂O₅–4.5B₂O₃ glasses and glass nanocrystal composites were studied. The dielectric constant of the glass was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). The temperature coefficient of dielectric constant was 8 ± 3 ppm/K in the 300–600 K temperature range. The relaxation and conduction phenomena were rationalized using modulus formalism and universal AC conductivity exponential power law, respectively. The observed relaxation behavior was found to be thermally activated. The complex impedance data were fitted using the least square method. Dispersion of Barium Sodium Niobate (BNN) phase at nanoscale in a glass matrix resulted in the formation of space charge around crystal‐glass interface, leading to a high value of effective dielectric constant especially for the samples heat‐treated at higher temperatures. The fabricated glass nanocrystal composites exhibited P versus E hysteresis loops at room temperature and the remnant polarization (P_r) increased with the increase in crystallite size.     

Microwave Processing for Improved Ionic Conductivity in Li2O–Al2O3–TiO2–P2O5 Glass‐Ceramics

Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) was synthesized using a glass‐ceramics approach through crystallization in a conventional box furnace and a modified microwave furnace. The microstructure of samples that were microwave processed at 1000°C showed a larger average grain size (0.87 μm) when compared with the grain size of conventionally processed samples (0.30 μm) at the same temperature. Microwave processing led to significant enhancement of the conductivity when compared with conventional processing for all crystallization temperatures investigated. The highest total conductivity achieved was of glass microwave processed at 1000°C, with a conductivity of 5.33 × 10^?4 S/cm. This conductivity was five times higher than that of LATP crystallized conventionally at the same temperature.     

Highly Transparent and Colorless Self‐Healing Polyacrylate Coatings Based on Diels–Alder Chemistry

The efficient integration of reversible polymer networks into acrylate‐based polymeric materials is of peculiar interest for the development of coatings that combine high transparency with self‐healing ability. In this work, reversible networks are obtained by reacting a series of linear copolymers of furfuryl methacrylate with aliphatic bismaleimides through Diels–Alder (DA) reaction between furan and maleimide moieties. Owing to dynamic crosslinking, the obtained coatings exhibit thermal reversibility, as determined by differential scanning calorimetry and dissolution experiments. Furthermore, upon heating over the retro‐DA temperature, an excellent recovery of mechanically induced surface damages proves successful thermal remendability. Compared to previous reports on DA‐based acrylate networks, the presented thermally responsive coatings exhibit outstanding transparency and absence of color, as a result of an accurate choice of suitable monomeric precursors. In addition, a pronounced hydrophobic behavior and excellent adhesive properties make the proposed material particularly suitable for optical applications.     

Influence of Crystallization on the Conversion of Sm3+→Sm2+ in SrO–Bi2O3–K2O–B2O3 Glass‐Ceramics

Sm³⁺‐doped glass 13SrO–2Bi₂O₃–5K₂O–80B₂O₃ was fabricated by the conventional melt‐quenching technique. The glass‐ceramics were obtained by heating the as‐prepared glasses in air atmosphere at selected temperatures 550°C, 600°C, 615°C, and 650°C, respectively. The luminescence spectra of both Sm³⁺ and Sm²⁺ were detected in the ceramic heated at 650°C where crystalline phase is formed. The as‐prepared glass and the ceramics heated at 550°C, 600°C, and 615°C show only the emission due to Sm³⁺. In the sample heated at 650°C in air atmosphere, however, part of Sm³⁺ ions was converted to Sm²⁺, giving rise to sharp emission lines which are characteristic of Sm²⁺ in crystalline state. It is suggested that Sm²⁺ ions are located at Sr²⁺ site in the ceramic while Sm³⁺ ions are located at Bi³⁺ sites. The Sm²⁺‐doped glass‐ceramic has a high optical stability because the fluorescence intensity decreases by only about 8% of its initial value upon excitation at 488 nm Ar⁺ laser.     

Glass‐Ceramics in the System BaO–SrO–ZnO–SiO2 with Adjustable Coefficients of Thermal Expansion

Glasses from the system BaO–SrO–ZnO–SiO₂ with different Ba/Sr ratios were characterized regarding crystallization behavior as well as the thermal expansion of almost fully crystallized glasses. Depending on the SrO concentration, different crystalline phases precipitate from the glasses. Those with low SrO concentrations precipitate crystals with the structure of low‐temperature BaZn₂Si₂O₇ as one of the major phases. Higher SrO concentrations cause the formation of Ba_1?xSr_xZn₂Si₂O₇ solid solutions with the structure of high‐temperature BaZn₂Si₂O₇. Both, the low‐ as well as the high‐temperature phase exhibit very different thermal expansion behaviors ranging from a very high coefficient of thermal expansion in the case of the low‐temperature phase to a very low coefficient of thermal expansion in the case of the high‐temperature phase. The glass‐ceramics with the highest and that with the lowest coefficient of thermal expansion measured between 100°C and 800°C show a difference of 7.9 × 10^?6 K^?1, which is caused solely by a substitution of BaO with SrO. In contrast, the maximum variation in the thermal expansion of the glasses was only 1.5 × 10^?6 K^?1. The microstructure of sintered and afterward crystallized glass powders was analyzed via scanning electron microscopy and showed crack‐free samples with low porosity.     

Self‐Healing Behavior of Barium–Lanthanum–Borosilicate Glass and Its Reactivity with Different Electrolytes for SOFC Applications

The diffusion couples of lanthanum‐based barium borosilicate glass with high‐ and low‐temperature electrolytes have been heat‐treated at 850°C and 800°C, respectively, for 5, 100 and 750 h. These prepared diffusion couples have been characterized using various techniques like X‐ray diffraction (XRD), scanning electron microscopy (SEM), X‐ray dot mapping, and electron probe microanalysis (EPMA). The thermodynamic parameters like frequency factor, crystallization constants, free volume, and bulk thermal expansion coefficients have been calculated to understand the behavior of glass. Interestingly, glass revealed self‐healing tendency with heat treatment duration.