m-Bromophenolic epoxy resins for electronic packaging

Meta-halogenated phenols are generally known to be more chemically and thermally stable than their ortho- or para-halogenated counterparts. A reactive intermediate, produced by the bromination of 2,4,6-trimethylphenol is being used as an alkylating agent to incorporate this stable m-bromophenol moiety into varieties of organic compounds and polymers. In electronic encapsulation applications, epoxy derivatives of novolacs containing m-bromophenol have exhibited superior hydrolytic and thermal stability as compared with the conventional tetrabromo bisphenol-A epoxies which are ortho-brominated phenolics. The m-bromophenol moiety contributes to the extended device reliability while meeting flame retardency requirements as well.     

Electrical Conduction Behavior of La1+xSr1−xGa3O7–δ Melilite-Type Ceramics

Ceramics of the melilite-type compound La_{1+ x} Sr_{1− x} Ga₃O_7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La_{1+ x} Sr_{1− x} Ga₃O_7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10⁻¹ S·cm⁻¹ at 950°C.     

Characterization of Precursor-Derived Silicon-Carbon-Nitrogen Ceramics after Creep Testing

Precursor-derived Si-C-N ceramics after creep testing in air were characterized using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). XRD analysis showed that the crept Si-C-N ceramics were covered by an α-cristobalite layer. TEM observations revealed that the precipitated nanocrystallites in the crept Si-C-N ceramics were β-SiC. Between α-cristobalite and crept Si-C-N ceramic, there was an intermediate zone in which Si₂N₂O nanocrystallites were distributed homogeneously. Moreover, Si₂N₂O nanocrystallites were often found covering the surface of nanosized gas channels in the crept Si-C-N ceramics, where no α-cristobalite phase was detected. Based on these observations, a two-step oxidation mechanism of Si-C-N ceramics during creep testing in air was proposed.     

Phase Equilibrium Investigations on Na–K–(β+β")-Alumina

The activities of Na₂O and K₂O dissolved in mixed-alkali Na–K–(β+β")-Al₂O₃ (NKBA) have been determined by using yttria stabilized zirconia (YSZ) as a solid electrolyte in the following galvanic cells: The approach enables to verify in situ the establishment and maintenance of the β/β"-equilibrium, and to characterize it as a function of the phase composition of NKBA. The results can be expressed as follows:      

Electric Fatigue in Antiferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

The fatigue behavior of lead zirconate stannate titanate (PZST) ceramics prepared by spark plasma sintering (SPS) was investigated. Polarization and strain hysteresis loops were monitored. The material shows a high resistance to fatigue because of bipolar electric cycling. Both maximum strain and switchable polarization first show a fatigue stage 0 to 10⁵ cycles and then a fatigue-free period up to 10⁸ cycles. The maximum losses of maximum strain and switchable polarization are 18% and 10% of their initial values, respectively. The dominant fatigue mechanism is assigned to the pinning of domain walls by charged defects.     

Synthesis of Ultrahigh-Temperature Si–B–C–N Ceramic from Polymeric Waste Gas

A "green" route to ultrahigh-temperature Si–B–C–N ceramic from vacuum-degassing waste gas of polyborosilazane {B[C₂H₄Si(CH₃)NH]₃} _n (T2-1) has been developed. After gas-to-gel transformation, an amorphous ceramic Si_5.3B_1.0C₁₉N_3.7 was derived from the gel by dehydrocoupling and polymer-to-ceramic transformation. The ceramic started to form a nanostructure at 1700°C and resisted thermal degradation up to 2200°C in argon. This suggests that vacuum-degassing waste gases of polymer precursors may be perfect raw materials for various advanced ceramics.     

Modelling of long-term diffusion–reaction in a bentonite barrier for radioactive waste confinement

Bentonites have been proposed as buffer material for barriers in geological disposal facilities for radioactive waste. This material is expected to fill up by swelling the void between the canisters containing the waste and the surrounding ground. However, the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term.The purpose of the present study was two-fold. Firstly, it was hoped to simulate the chemical transformations (geochemical and cation exchange reactions) coupled with diffusion of chemical-elements into the engineered barrier under repository conditions by applying a thermokinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass transport).Secondly, it was hoped to apply a simplified method to estimate the swelling capacity evolution by a volume balance in the fluid-saturated engineered barrier, considering that the decay of swelling capacity is directly proportional on the volume of transformed montmorillonite and, taking into account that it may be partially compensated by the volume of neo-formed swelling clays.The system modelled herein was considered to consist of 1-m thick zone of water-saturated engineered barrier. This non-equilibrated system was placed in contact with a geological fluid on one side, which was then allowed to diffuse into the barrier, while the other side was kept in contact with a source of metallic iron. Reducing initial conditions(P_O2 0; Eh = − 200 mV) and a constant reaction temperature (100 °C) were considered.The results showed that the EB in contact with the geological fluid was highly transformed after 10,000 years, whereas the most significant chemical processes were illitization, cation exchange and saponization, extending up to 20 cm into the EB. Chemical transformations of minor importance in the EB were identified as well, such as a neo-formation of silicates (quartz, cristobalite), anhydrite, laumontite, magnetite and chlorite in the system.A simplified method based on volume balance showed that the swelling capacity of the bentonite barrier is slightly affected after 10,000 years of diffusion–reaction (D close to 1) because the volume of neo-formed swelling-clays is almost directly proportional to the volume of transformed Na/Ca-montmorillonite, except for a strong illitization and/or neo-formation of non-swelling clays. In the present study, this simple approach predicted that the decay of swelling capacity of the engineered barrier is drastically affected close to the geological barrier-engineered barrier interface. Out this zone the swelling capacity decay lies between 5% and 11%.     

Crystallization of Polymer-Derived Silicon Carbonitride at 1873 K under Nitrogen Overpressure

The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si₃N₄·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at p _N2 < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ∼10 bar, and is crystalline above ∼10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si₃N₄ are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ∼10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si₃N₄ crystals is calculated to be 30 kJ/mol.     

Thermal Stability, Phase Evolution, and Crystallization in Si-B-C-N Ceramics Derived from a Polyborosilazane Precursor

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazane, {B[C₂H₄Si(H)NH]₃} _n , were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X-ray diffraction (XRD). The phase evolution was additionally followed by ¹¹B and ²⁹Si MAS NMR as well as by FT-IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.