Chen, I-Wei
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Publication Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: I, Y-SiAlON(2002-05-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunThe effect of seeding on the microstructure and mechanical properties of single-phase Y-α-SiAlON ceramics with elongated grains has been studied. Seeds of the intended α-SiAlON compositions but with different size, shape, and number of grains have been compared for their effects. The microstructure, resistance (R-curve) behavior, and Weibull modulus are strongly correlated to the number density of the seeds. The highest fracture toughness reached is ~12 MPa∙m1/2 and can be obtained with as little as 1% seeding. The thermodynamic stability of seeds has been examined and is attributed to their chemical composition.Publication The Effect of Powder Mixing Procedures on α-SiAlON(2005-10-03) Shuba, Roman; Chen, I-WeiVarious procedures of mixing starting powders for hot-pressing α-SiAlON ceramics were studied. They included different milling methods (attrition milling, ball milling, and sonication), liquid medium (water, isopropyl alcohol, and pyridine), and atmospheres (ambient air and nitrogen). These mixing procedures resulted in markedly different densification behavior and fired ceramics. As the powders experienced increasing oxidation because of mixing, the densification temperature decreased, the amount of residual glass increased, and α-SiAlON was destabilized and replaced by ß-SiAlON and AlN polytypes during hot pressing. These effects were mitigated when pyridine, nitrogen, and sonication were used. Several protocols that yielded nearly phase-pure, glass-free dense α-SiAlON were thus identified.Publication Nanoscale Engineering for Biomaterial Surfaces(2007-02-01) Lipski, Anna Marie; Jaquiery, Claude; Choi, Hoon; Eberli, Daniel; Stevens, Molly; Martin, Ivan; Chen, I-Wei; Shastri, V. PrasadPublication Formation of beta-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: II, Population Dynamics and Coarsening Kinetics(2003-09-01) Wang, Lingling; Tien, Tseng-Ying; Chen, I-WeiPrecipitation, growth, and coarsening of Si3N4 crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680oC has been studied. Contrary to the common observation in kinetics, coarsening rates of crystals in length and width are found to accelerate when the total volume of crystals remains little changed. This is attributed to the concomitant β-Si3N4 to β′-SiAlON transformation, which introduces an additional driving force for crystal dissolution and reprecipitation. As a result of the additional driving force, which has a nonmonotonic size dependence, the normalized size distribution is expected to evolve with time, initially broadening, then shifting skewing as the transformation passes the midpoint, and finally converging to a sharp distribution as the transformation completes. These evolutions have been observed in all the compositions studied.Publication A size-dependent nanoscale metal–insulator transition in random materials(2011-01-28) Chen, Albert B.K.; Wang, Yudi; Tung, Wei-Shao; Chen, I-Wei; Kim, Soo GilInsulators and conductors with periodic structures can be readily distinguished, because they have different band structures, but the differences between insulators and conductors with random structures are more subtle. In 1958, Anderson provided a straightforward criterion for distinguishing between random insulators and conductors, based on the 'diffusion' distance ζ for electrons at 0 K (ref. 3). Insulators have a finite ζ, but conductors have an infinite ζ. Aided by a scaling argument, this concept can explain many phenomena in disordered electronic systems, such as the fact that the electrical resistivity of 'dirty' metals always increases as the temperature approaches 0 K (refs 4–6). Further verification for this model has come from experiments that measure how the properties of macroscopic samples vary with changes in temperature, pressure, impurity concentration and applied magnetic field, but, surprisingly, there have been no attempts to engineer a metal–insulator transition by making the sample size less than or more thanζ. Here, we report such an engineered transition using six different thin-film systems: two are glasses that contain dispersed platinum atoms, and four are single crystals of perovskite that contain minor conducting components. With a sample size comparable to ζ, transitions can be triggered by using an electric field or ultraviolet radiation to tune ζ through the injection and extraction of electrons. It would seem possible to take advantage of this nanometallicity in applications.Publication Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution(2002-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-WeiThe effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa∙m1/2 and peak R-curve toughness of ~11 MPa∙m1/2 have been obtained for seeded Y-α-SiAlON ceramics using heating rates from 1oC/min to 25oC/min.Publication Machinable α-SiAlON/BN Composites(2006-02-19) Shuba, Roman; Chen, I-WeiDense machinable α-SiAlON/BN composites were fabricated by hot-pressing using turbostratic boron nitride (tBN) obtained from nitridation of melamine diborate. The tBN was added to the starting powders, or introduced as a coating that formed in situ on α-Si3N4 carrier powders during nitridation, and was subsequently converted to hexagonal boron nitride (hBN) during hot pressing by solution reprecipitation. These composites maintain high strength at 1000°C and their strength/hardness are much higher than similar composites prepared using commercial hBN powder, which yielded a coarser microstructure. Good machinability was achieved despite a flat R curve.Publication Formation of beta-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions(2003-09-01) Wang, Lingling; Tien, Tseng-Ying; Chen, I-WeiPrecipitation, growth, and coarsening of Si3N4 crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680oC has been studied. The initial nucleation of β-Si3N4 occurs mostly on α-Si3N44 because of the very high supersaturation of the liquid. After a brief period of growth, the crystals then undergo accelerated coarsening, decreasing the crystal concentration by almost 100 times with little change in the total crystal volume. Meanwhile, the crystals gradually transform from β-Si3N4, by substituting Si-N with Al-O, to β'-SiAlON of various compositions. The evolution of aspect ratio strongly depends on the Si/(Al,Mg,Y) ratio, which is rationalized by cation segregation to the interface driven by the acidity–basicity differential between the liquid and the crystal.Publication Synthesis of alpha-SiAlON Seed Crystals(2001-07-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-SunSingle-phase seed crystals of Ca- and Y-α-SiAlONs have been synthesized for tailoring microstructure of α-SiAlON ceramics. The influence of composition, sintering temperature, and nitrogen pressure on the size and morphology of seeds has been explored. Guidelines for α-SiAlON seed preparation and morphology control are provided.Publication Bulk dense fine-grain (1-x)BiScO3–xPbTiO3 ceramics with high piezoelectric coefficient(2008-11-13) Zou, Tingting; Wang, Xiaohui; Wang, Han; Zhong, Caifu; Li, Longtu; Chen, I-WeiHigh density fine grain (1−x)BiScO3–xPbTiO3 ceramics were successfully prepared by two-step sintering and their ferroelectric properties were investigated. Experimental evidence indicates the existence of a morphotropic phase boundary at the composition zx=0.635, which exhibits a piezoelectric coefficient d33 of 700 pC/N at room temperature, significantly higher than the reported values to date. Furthermore, a higher electromechanical coupling factor Kp=0.632 and a larger remnant polarization Pr=47.3 µC/cm2 were obtained. The paraelectric-to-ferroelectric phase transition occurs at 446 °C, slightly lower than in the coarse grain ceramics with a similar composition, suggesting a grain size effect. The local effective piezoelectric coefficient d33* was estimated to be 795 pC/N at 2.29 V, measured by scanning probe microscopy. Further atomic force microscope observation revealed the existence of 90° domains of about 60–70 nm in width, confirming the previous results that small domain structure enhances the piezoelectric properties.