Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

Hydrolysis of polyurea under high pressure of carbon dioxide

A chemical decomposition of polyurea (PUA) by hydrolysis under high pressure of carbon dioxide (CO₂) was proposed. The hydrolysis of PUA was carried out at 190 °C for 2 h under 7.0 MPa of CO₂ in the presence of water. The hydrolysis reaction gave white residual solid and water soluble compound. The white residual solid was characterized to be degraded PUA by FT-IR spectrum and elemental analysis. ¹H-NMR spectra of the water soluble compound revealed that the hydrolysis of PUA produced diamine only from repeating unit of PUA, which was a component of PUA. This hydrolysis gave the corresponding diamine for quantitative yields.     

Reversible capture and release of carbon dioxide by binary system of polyamidine and polyethylene glycol

CO₂ absorbents were prepared from polyethylene glycol and the polyamidines having N,N′-disubstituted amidine structure in the main chain synthesized through acid-catalyzed melt polycondensation of orthoesters and α,ω-diamines. The homogeneous binary mixtures with the polyamidines captured CO₂ much more efficiently under CO₂ flow than the one with polyethyleneimine. Furthermore, we investigated the CO₂ capture and release by the binary mixtures in terms of effects of the volatility and the structure of polyamidines, temperature, and polyethylene glycol. Taking into consideration the results thus obtained, we conducted CO₂ capture/release cycles with the CO₂ capture step at 40 °C and with the CO₂ releasing step at 80 °C in an alternating manner, thereby demonstrating the repeatability of CO₂ capture and release by the binary system of the polyamidine and polyethylene glycol.     

Fabrication of Functionally Graded SiO2‐Mo Material by Centrifugation and Floc‐Casting of Highly Concentrated Slurry

To fabricate functionally graded materials, a highly concentrated slurry of SiO₂‐Mo system was prepared and centrifugal force was applied in an attempt to achieve a graded composition. Subsequently, we formed a homogeneous green body with compositional gradation by floc‐casting at 80°C, which was then fired at 1750°C for 10 min in Ar. The sintered body had compositional ratios of SiO₂ and Mo as well as electrical conductivities that changed gradually along the direction of centrifugal force. The results demonstrate that centrifugation and control of slurry characteristics such as flocculation are effective in fabricating functionally graded SiO₂‐Mo materials.     

ADDUCT FORMATION OF β-DIKETONATO LANTHANOIDS(III) WITH CARBOXYLIC ACIDS HAVING HYDROXYL AND METHOXYL GROUPS

Adduct formation of 2-thenoyltrifluoroacetonato(tta) lantha-num(III), europium(III), and ytterbium(I II) with α-pheny 1 1 actic, α-methoxy phenyl acetic, salicylic, and o-methoxy benzoic acids in chloroform has been studied by solvent extraction technique. It has been found that 1anthanoid(111) chelates form more stable adducts with these carboxylic acids in the sequence: Yb(tta)₃ < Eu(tta) ₃ < La(tta) ₃. The adduct formation constants as well as the separation factors obtained as the ratio of the constants between a pair of metals are the largest for α-phenyl lactic acid among these acids. The characteristics with α-phenyl 1actic acid would be caused by the action as bidentate unlike benzoic acid derivatives which work as monodentates owing to the internal complexa-tion through hydrogen bonding.     

Chemical modification of polynorbornene: transformation of ester moiety containing polynorbornene to carboxylic acid structure

Polynorbornene (PNB) having methyl ester in side chain was partially hydrolyzed under alkaline condition in mixed organic solvents containing tetramethylammonium hydroxide as a base. Through hydrolysis of methyl ester group in PNB under basic condition, PNB having carboxylic acid can be obtained and degree of hydrolysis (DH) were evaluated by ¹H NMR measurement from 0.1 to 0.8, depending upon reaction conditions such as reaction time, solvent, and amount of base. The resulting hydrolyzed PNB were characterized by differential scanning calorimetry, Fourier transform infrared spectroscopy, and size exclusion chromatography. No remarkable decreasing of molecular weight was observed and chain scission does not occur during the hydrolysis of PNB in the reaction condition examined here. The glass transition temperature of hydrolyzed PNB increased with increase in DH which can be attributed to the existence of intermacromolecular association of carboxylic acid in hydrolyzed PNB. Esterification of PNB having carboxylic acid was also carried out by using alkyl halide and DBU.     

High thermal stable thermoplastic-thermosetting polyimide film by use of asymmetric dianhydride (a-BPDA)

In order to obtain thermoplastic (before curing) and thermosetting (after curing) polyimides with high T_g for adhesive film, we prepared novel polyimides having phenylethynyl group in the side chain (44% of concentration of curing group) from asymmetric 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 3,4′-oxydianiline (3,4′-ODA) or 1,3-bis(4-aminophenoxy)benzene (1,3,4-APB) or 1,3-bis(3-aminophenoxy)benzene (1,3,3-APB), and 2,4-diamino-1-(4-phenylethynylphenoxy)benzene (mPDAp). Among three kinds of polymer, uncured polyimide of a-BPDA/1,3,4-APB; mPDAp had rather high T_g (265 °C, DMA) and thermoplasticity (E′ drop>10³ at T_g). After curing reaction of phenylethynyl group, the T_g of the polyimide was increased dramatically (364 °C, DMA). The polyimide derived from 1,3,4-APB having less concentration of curing group (20%) was also prepared to improve further film flexibility and toughness.     

High-Temperature Oxidation at 1500° and 1600°C of SiC/Graphite Coated with Sol–Gel-Derived HfO2

Oxidation of SiC compositionally graded (SCGed) graphite coated with HfO₂ derived from HfCl₄ by a sol–gel process was performed at 1500° and 1600°C in a flowing gas mixture of Ar and O₂ (80/20 kPa). SCGed graphite was produced by reaction of graphite with either molten Si or SiO gas at 1450°C. The sol–gel-derived HfO₂ precursor was deposited on SCGed graphite by a dip-coating method. Isothermal and cyclic oxidation of uncoated- and HfO₂-coated SCGed graphite was studied by monitoring overall weight change using an electro-microbalance. Scanning electron microscopy with energy-dispersive X-ray analysis was used to observe the surfaces and cross-sections of the oxidized HfO₂-coated SCGed graphite. The formation of HfSiO₄ was confirmed on the outer layer of the oxidized sample, beneath which a thin silica layer was formed. The improved oxidation resistance of SCGed graphite by coating with HfO₂ is discussed on the basis of the formation of these two layers.     

Sintering Kinetics at Constant Rates of Heating: Effect of Al2O3 on the Initial Sintering Stage of Fine Zirconia Powder

The sinterabilities of fine zirconia powders including 5 mass% Y₂O₃ were investigated, with emphasis on the effect of Al₂O₃ at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al₂O₃ increased the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact including Al₂O₃ was lower than that of a powder compact without Al₂O₃. The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al₂O₃ included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al₂O₃ enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage.