Retarded Phase Transformation Research Articles

Heterogeneous and retarded phase transformation of ferrihydrite on montmorillonite surface: The important role of surface interactions

Abstract The formation of heteroaggregates is critical to controlling the stabilization and transformation of nanominerals and mineral nanoparticles (NMMNs) in nature, but the underlying mechanisms remain to be deciphered. In this work, we study the effect of surface interactions between ferrihydrite (Fh) and montmorillonite (Mnt) within their heteroaggregates on the transformation behaviors of Fh. A series of heteroaggregates composed of Fh and Mnt were synthesized by modulating their mass ratios and synthesis methods, i.e., directly complexing Fh with Mnt (Fh-Mnt) or in situ growing Fh on Mnt (Fh/Mnt). Structural characterization using XRD, TG-DSC, TEM, and FTIR indicated that Fh particles coated more evenly on the Mnt surface within the heteroaggregates synthesized by in situ growing Fh on Mnt and with lower Fh to Mnt ratio, and accordingly these heteroaggregates showed stronger surface interactions between Fh and Mnt. The phase transformation of Fh to hematite (Hem) on the heteroaggregates can be significantly affected during the heating treatment. Compared with that of pure Fh, the transformation of Fh on all of the heteroaggregates was retarded (e.g., slower transformation rate and smaller produced Hem particles), particularly for the samples with stronger surface interactions (e.g., Fh/Mnt with lower Fh to Mnt ratio). Noticeably, the heated heteroaggregates may simultaneously contain pristine Fh, intermediate maghemite, and transformed Hem, showing a heterogeneous transformation behavior of Fh. The strong interactions between Fh and Mnt will enhance the dispersion of Fh and restrict the structural rearrangement of Fh (particularly those at the interface) during the phase transformation process, resulting in retarded and heterogenous transformation of Fh on these heteroaggregates. These findings not only enrich our knowledge of the phase transformation characteristics of Fh but also advance our understanding of the important role of mineral surface interactions in stabilizing NMMNs in nature.

Growth evolution and formation mechanism of η′-Cu6Sn5 whiskers on η-Cu6Sn5 intermetallics during room-temperature ageing

The phase-transformation-induced damage of Cu6Sn5 is an emerging reliability issue in the manufacturing of 3D ICs. Although the retarded phase transformation from η-Cu6Sn5 to η′-Cu6Sn5 at room temperature can produce a large expansion in volume, how the transformation stress threatens the joint reliability during usage is poorly understood. In this paper, the evolution characteristics of quenched η-Cu6Sn5 bumps were observed during ageing at 25 °C for 1–40 d Due to the retarded phase transformation, η′-Cu6Sn5 whiskers spontaneously nucleated and grew on the surfaces of η-Cu6Sn5 bumps. The orientation relationship between the two phases favourable for whisker growth was confirmed, and two necessary conditions for whisker formation were discussed. In addition, the potential harmfulness of whisker growth was analysed. The study will help expose the phase-transformation-induced damage of Cu6Sn5 during room-temperature ageing and may reduce the failure risk of entire Cu6Sn5 intermetallic joints in future large-scale applications of 3D ICs.

Formation of diverse B2+O structure and hardness of Mo-modified Ti-22Al-25Nb alloys upon cooling

Mo-modified Ti-22Al-25Nb alloys are sintered at 900 °C, 1000 °C, 1070 °C, and 1150 °C, followed by quenching and furnace cooling, respectively. We made comparisons of phase composition and microstructure between quenched and furnace-cooled alloys to interpret the phase transformation and microstructure evolution during cooling. The phase transformation was B2→O for the alloy cooled from B2+O region, and was B2→B2+O for those cooled from α2+B2+O, α2+B2, and B2 phase regions. Moreover, the morphology was B2+O colonies for those cooled from B2+O and α2+B2+O phase regions, and was Widmanstätten structure for those cooled from α2+B2, and B2 phase regions. Kinetics analysis indicated that the activation energy for the phase transformation of α2→B2 was 170 kJ mol−1 for Ti-22Al-25Nb, while it was 330 kJ mol−1 for Mo-modified Ti-22Al-25Nb. The retarded phase transformation in Mo-modified Ti-22Al-25Nb further led to curvy Widmanstätten structure when the alloy was furnace-cooled from single B2 phase region. The hardness of the Mo-modified Ti-22Al-25Nb alloys is up to 488 HV, while that of the Ti-22Al-25Nb alloys without Mo addition is less than 400 HV.

Understanding aggregation hindered Li-ion transport in transition metal oxide at mesoscale

Conversion based transition-metal oxides as a promising class of anode materials, require proper nanostructuring for enhanced Li-ion storage capabilities. However, aggregation is found to be a common issue in nanomaterial systems, and can have detrimental effects on transport properties in composite electrodes. By employing a model transition-metal oxide anode with unique two-dimensional holey nanostructures, we investigated underlying reasons for the limited electrochemical kinetics induced by mesoscale aggregation. Through combined electrochemical and in situ characterization techniques, we demonstrate that aggregation leads to hindered interfacial charge transfer and retarded phase transformation, with the influence on kinetics escalating with more aggregation. These results shed light on more dedicated structural design for effective battery electrodes across multiple length scales.

Synthesis and anisotropic properties of alumina-silica aerogels constructed by silica sols infiltrated into unidirectional frozen alumina templates

Abstract Alumina-silica aerogels (ASAs) were firstly prepared by using alumina aerogels as template via ice-template method and then silica sols infiltrated via vacuum impregnation. Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the microstructures characteristics and phase composites. Compared with the alumina templates (ATs), ASAs presented the porous characteristic of nano-level aerogels. The infiltrated silica can retard phase transformation of alumina. The anisotropic mechanical and thermal conduction properties of ATs in parallel and perpendicular to ice-crystal growth directions were demonstrated, which was related to the anisotropic porous structures of ATs due to the difference of main and secondary skeleton structures forming in unidirectional freeze. After infiltrated silica sols, the mechanical property was increased but the thermal conductivity was decreased. The anisotropic characteristics of ASAs still were maintained. Using ice template as a support, shape-stabilized aerogel composites can be constructed.

Enhanced thermal stability and mechanical properties of nitrogen deficient titanium aluminum nitride (Ti0.54Al0.46Ny) thin films by tuning the applied negative bias voltage

Aspects on the phase stability and mechanical properties of nitrogen deficient (Ti0.54Al0.46)Ny alloys were investigated. Solid solution alloys of (Ti,Al)N were grown by cathodic arc deposition. The kinetic energy of the impinging ions was altered by varying the substrate bias voltage from −30 V to −80 V. Films deposited with a high bias value of −80 V showed larger lattice parameter, finer columnar structure, and higher compressive residual stress resulting in higher hardness than films biased at −30 V when comparing their as-deposited states. At elevated temperatures, the presence of nitrogen vacancies and point defects (anti-sites and self-interstitials generated by the ion-bombardment during coating deposition) in (Ti0.54Al0.46)N0.87 influence the driving force for phase separation. Highly biased nitrogen deficient films have point defects with higher stability during annealing, which cause a delay of the release of the stored lattice strain energy and then accelerates the decomposition tendencies to thermodynamically stable c-TiN and w-AlN. Low biased nitrogen deficient films have retarded phase transformation to w-AlN, which results in the prolongment of age hardening effect up to 1100 °C, i.e., the highest reported temperature for Ti-Al-N material system. Our study points out the role of vacancies and point defects in engineering thin films with enhanced thermal stability and mechanical properties for high temperature hard coating applications.

Phase transformation behavior of alumina grown on FeAl alloys with reactive element dopants at 1273 K

The short-term isothermal oxidation behavior of B2 FeAl alloys doped with different reactive elements (REs) at 1273 K was investigated. All the RE-doped alloys showed retarded phase transformation from θ-to α-Al2O3 as compared to the RE-free alloy. The Y- and La-doping even led to much lower transformation rate than the Hf-, Zr- and Ce-doping. The effect mechanism associated with RE ion size and valence on the phase transformation of alumina was discussed. The additions of RE ions which have large size and matched valence with Al ions could be anticipated to inhibit the transformation more effectively and the combined effects of retarding θ-α phase transformation, promoting α phase nucleation and suppressing Al outward diffusion produced by RE doping might provide FeAl with improved oxidation property.

Study on Li-Rich Layered Cathode Material for Li-Ion Batteries

The high-energy-density Li-rich layered materials are promising cathode materials for the next-generation high-performance lithium-ion batteries[ 1 ]. They have attracted a lot of attentions due mainly to their high reversible capacity of more than 250 mAh·g-1 at low charge-discharge current. However several drawbacks still hinder their applications, such as voltage decay caused by an undesired phase transformation during cycling and poor rate capability[ 2 ]. To conquer these issues, the authors applied F doping and surface modification on the pristine Li1.2Mn0.54Ni0.13Co0.13O2. To conquer the poor cycling stability, fluorine doping was conducted on Li1.2Mn0.54Ni0.13Co0.13O2. Except for the enhanced capacity retention, F doping shows remarkable effect to stabilize the layered structure by retarding the undesired layered-to-spinel phase transition. As shown in Figure 1, Due to the severe layered-to-spinel phase transformation in the pristine material[ 3 ], the cathodic peak for Mn reduction decays from 3.38V to 2.78V in 100 cycles, accompanied by severe capacity decrease. However, the F doped materials exhibit more stable discharge voltage in 100 cycles. Surprisingly, the Li1.12Mn0.54Ni0.13Co0.13O1.92F0.08 material does not show any new cathodic peak before 50 cycles and even in the 100thcycle, only a small hump below 3V could be identified, regarded as the emergence of spinel phase. This retarded phase transformation could be validated by the Ex situ TEM analysis of the four electrodes after 100 cycles in Figure 2. By retarding the undesired phase transformation, the lattice distortion and amorphourization of the original layered structure is successfully suppressed as well. The stabilized structure benefits the cycling stability of F doped materials. To improve the rate capability of the pristine Li-rich layered cathode material, surface modification was conducted on Li1.2Mn0.54Ni0.13Co0.13O2 using NH4F by thermal annealing at low temperature. Material characterizations reveal that the modification process triggers fluorine doping and phase transition from layered phase to spinel phase at the particle surface, as shown in TEM images and FFT results (Figure 3). Figure 4 shows the rate performances of the pristine Li1.2Mn0.54Ni0.13Co0.13O2 and the modified materials. The pristine material could deliver a high reversible capacity of about 250 mAh·g-1 at 0.1C (25 mA·g-1). However, its discharge capacity is about 109 mAh·g-1 at 1C, which is only 43% of the capacity at 0.1C. Compared with the pristine material, the materials modified by NH4F could exhibit greatly improved rate capability. Particularly, the material modified by 20 wt.% NH4F has a discharge capacity as high as 172 mAh·g-1 at 1C, which is over 87% of its capacity at 0.1C. Moreover at even higher rate like 5C, the discharge capacity of the material modified by 20 wt.% NH4F still can reach 126 mAh·g-1 while the discharge capacity of the pristine one is only 41 mAh·g-1. Generally, the NH4F modified Li1.2Mn0.54Ni0.13Co0.13O2 exhibits greatly improved rate performance and satisfactory cycling stability compared to the pristine material, which can be attributed to the modified particle surface. Firstly, the spinel shell of the particle provides three-dimensional Li+ ion diffusion paths[ 4 ], which creates fully opened surface, enabling fast Li+ ion transfer at the electrode/electrolyte interface. Secondly, the formation of spinel shell prevents the Ni segregation at the surface, thus suppressing its negative effect on Li+ion diffusion. Finally, the fluorine doped spinel surface improves the surface stability during wide-voltage-range charge-discharge process, resulting in improved cycling stability. In both works, the enhancement in the electrochemical properties of modified materials are comprehensively investigated using Power X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical impedance spectroscopy and electrochemical tests.

The effect of fetal bovine serum on high phosphate-induced calcium phosphate formation, aggregation and crystal phase transformation

In this study, we investigated the influence of fetal bovine serum (FBS) on calcium phosphate formation, aggregation and phase transformation by adding different concentration of inorganic phosphate ions (Pi) into Dulbecco's Modified Eagle's Medium (DMEM). Using particle sizes analyzer, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and sodium dodecyl sulfate-polyacrylamide gelelectrophoresis (SDS-PAGE), it was revealed that in the media without FBS, high Pi induced crystalline calcium phosphate formation and aggregation. Whereas in the media containing FBS, some serum proteins such as albumin and fetuin-A were not only incorporated into the calcium phosphate nanoparticles, but also decreased supersaturation of the solutions, reduced zeta potential of the particles and induced short-range order bending lattice formation. The results showed that FBS prevented high Pi-induced calcium phosphate formation and aggregation and retarded phase transformation from amorphous to crystalline calcium phosphate in biological media.

Abiotic reduction of 2-line ferrihydrite: effects on adsorbed arsenate, molybdate, and nickel

The abiotic reduction of X-ferrihydrite (X-FH, where X = 0, As, Mo, or Ni at various Fe/X molar ratios) was investigated by reacting Fe(II)(aq) at solution concentrations of 0.5 mM or 10 mM and at target pH values of 8 or 10 (using lime water as a base) for 7 days. Under all reaction conditions tested, the measured pH was always lower than the target; this difference was greatest for As-FH (at up to 5 pH units). The control FH sample behaved as expected and transformed to lepidocrocite (LP) and goethite (GT) phases. For As-FH, the sample containing less As (Fe/As = 32.9) transformed to LP–GT phases but phase transformation in the sample with more As (Fe/As = 4.47) was inhibited. Solution concentrations of As were below the detection limit for the Fe/As 32.9 sample but As release was evident for the Fe/As 4.47 sample. For Mo-FH, phase transformation to LP–GT phases was observed at lower target pH (8) conditions under both reacting Fe(II)(aq) concentrations. At the higher target pH (10) and using 0.5 mM Fe(II)(aq), phase transformation inhibition was observed for Mo-FH varieties that contained both high (Fe/Mo 12.5) and low (Fe/Mo 31.5) concentrations of Mo. This is the first time an element forming an outer-sphere complex on FH (e.g., Mo) has been shown to retard phase transformation; such phenomena are usually observed for metalloids that form inner-sphere complexes with FH (e.g., As). Under all conditions, Mo was released into solution (up to 340 ppm) and under some conditions was then readsorbed by the solid phase. Finally, all Ni-FH samples exhibited phase transformation under the reaction conditions tested; however, magnetite (MG) and a green rust-like phase were observed in addition to the LP–GT phases. Under all reaction conditions, the largest amount of Ni was released into solution on the first day of reaction, after which the amount in solution decreased with time due to its readsorption into the solid phase.

Fabrication and characterization of neat and aluminium-doped titanium (IV) oxide fibers prepared by combined sol–gel and electrospinning techniques

Sol–gel and electrospinning techniques were incorporated to produce polyvinylpyrrolidone (PVP)/titanium (IV) oxide composite fibers from solutions containing PVP and titanium tetraisopropoxide, with or without aluminium nitrate as the source of aluminium dopant. Upon the calcination of the as-spun fibers, the neat and the aluminium-doped titania fibers were obtained. Increasing the calcination temperature resulted in the decrease in the fraction of anatase phase within the fibers, as well as the increase in titania crystallite sizes. The presence of aluminium dopant, however, was found to greatly affect both physical and chemical properties of the synthesized titania fibers. Aluminium nitrate accelerated condensation of titanium oxide species during the sol–gel process, which resulted in increased viscosity of the spinning solution and consequently affected the diameters of the as-spun fibers. Aluminium dopant also played the major roles in both regulating the nucleation rate during crystallization of titania and controlling the growth mechanism of titania crystallites. As a result, the aluminium dopant caused the crystallite size of titania to decrease and retarded phase transformation from anatase to rutile.

Hierarchical anatase TiO2 porous nanopillars with high crystallinity and controlled length: an effective candidate for dye-sensitized solar-cells

Hierarchical anatase TiO(2) porous nanopillars with high crystallinity and stability were fabricated by the following four steps: (i) synthesis of a titanium glycolate nanopillar precursor; (ii) transformation into stable nanopillars consisting of amorphous TiO(2) primary nanoparticles at low temperature; (iii) treatment with ethylenediamine aqueous solution to protect the nanopillar structure, restrain nanoparticle growth and retard phase transformation; (iv) calcination at temperatures as high as 700 degrees C to form hierarchical anatase TiO(2) porous nanopillars. The results of SEM, TEM and XRD reveal that these TiO(2) nanopillars have a porous structure, and that their length can be easily controlled by the volume ratio of ethylene glycol to tetrabutyl titanate. In addition, dye-sensitized solar-cells (DSSCs) were assembled with sandwich-structure photoanodes, which are composed of a layer of TiO(2) nanopillars sandwiched between two layers of TiO(2) nanoparticles. The photoelectrical measurement results show that the power conversion efficiency of DSSCs enhances with the increase of nanopillar length and crystallinity. This is attributed to the effective transfer of photogenerated electrons in TiO(2) nanopillars and their porous microstructures in favor of dye adsorption, as demonstrated by surface photovoltage spectroscopy (SPS) and electrochemical impedance spectroscopy (EIS).

Characterization of Visible Light Response N-F Codoped TiO 2 Photocatalyst Prepared by Acid Catalyzed Hydrolysis

N-F codoped TiO 2 (TONF) photocatalysts were prepared using acid catalyzed hydrolysis method from mixed aqueous solution of TiCl 4 and NH 4F. The photocatalytic activity of the TONF was evaluated through the degradation of phenol under both visible and UV light irradiation. X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscope (SEM), and N 2 adsorption isotherm were used to characterize the obtained powders. The results showed that N-F codoped TiO 2 exhibited significant improvement of visible light catalytic activity. N-F codoping could improve dispersion of TiO 2, inhibit particle size agglomeration, and retard phase transformation. Doped N could extend the light response of TiO 2 to visible light region. In addition, narrower band gap formed by F-doping was beneficial to the high visible light photocatalytic activity.

Visible-light-driven boron/ferrum/cerium/titania photocatalyst

A boron/ferrum/cerium/titania photocatalyst with visible-light-induced performance was prepared by the conventional sol–gel method, in which tetrabutyl titanate (Ti(O- nC 4H 9) 4) was used as the precursor and boric acid (H 3BO 3), ferric nitrate enneahydrate (Fe(NO 3) 3·9H 2O), and cerium nitrate hexahydrate (Ce(NO 3) 3·6H 2O) were the sources of boron, ferrum, and cerium, respectively. The microstructure and optical property of the photocatalyst were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N 2 adsorption–desorption isotherm, and UV–vis diffusive reflectance spectroscopy (DRS). It was found that the as-prepared photocatalyst composed of anatase and that the presence of impurities could retard phase transformation of TiO 2 from anatase to rutile at elevated temperatures and could prohibit growth of polycrystalline, which favored forming photocatalyst with large surface area. Degradation of 2,4-dichlorophenol (DCP) under visible light illumination was used to evaluate the photocatalytic activity. Comparative investigations showed that photocatalytic performance of the boron/ferrum/cerium/titania photocatalyst was the highest among the all test samples. It was testified that boron doping led to the band gap narrow and the response to visible light, and that ferrum and cerium impurities, which presented in the forms of FeO/Fe 2O 3 and Ce 2O 3/CeO 2, dispersed on the surface of TiO 2, suppressed the electron and hole recombination, and resulted in the enhancement of catalytic performance. The results exhibited that the synergistic effects of boron, ferrum and cerium played an important role in the band gap narrow and the increase of photoactivity.

The influence of age-hardening on turning and milling performance of Ti–Al–N coated inserts

Abstract Ti–Al–N coatings are well known for their excellent properties and age-hardening abilities. Here we show that the life-time of coated inserts during turning of stainless steel can be increased to 200% by post-deposition vacuum annealing at 900 °C combined with a ~ 1 K/min vacuum furnace cooling. During milling of 42CrMo steel an increase in tool life-time to 140% is only obtained if the cooling condition after annealing at 900 °C contains a fast segment with 50 K/min from 800 to 700 °C. Thereby, the Co-binder in cemented carbide exhibits a retarded phase transformation from cubic to hexagonal. Consequently, the fracture toughness of the cemented carbide is reduced only from ~ 10.8 to 10.4 MPa√m while the coating still has an adhesive strength of ~ 65 N. Our results indicate that best machining performances of coated inserts are obtained after annealing at 900 °C where the supersaturated Ti 0.34 Al 0.66 N coating undergoes spinodal decomposition to form nm-sized cubic TiN and AlN domains resulting in a hardness increase from 34.5 to 38.7 GPa. Additionally, we demonstrate that careful attention needs to be paid on the influence of annealing conditions on adhesive strength and fracture toughness of coated inserts.

Formation of nanoporous and nanocrystalline anatase films by pyrolysis of PEO–TiO 2 hybrid films

Poly(ethylene oxide)–titania (PEO–TiO 2) organic–inorganic hybrid films were synthesized through a sol–gel process and spin coating. The mixed precursor sols were aged for 6 and 12 h, respectively, prior to the spin coating. Hybrid films were crystallized to the anatase phase first and then further heat treated at elevated temperatures for different time periods to analyze the anatase-to-rutile phase transformation. Quantitative X-ray diffraction (Q-XRD) results were employed for Johnson–Mehl–Avrami (JMA) kinetic analyses and the Avrami exponent ( n) was determined for each film. The Arrhenius plots were created based on the JMA plots and the activation energy ( Q) values for the phase transformation were determined for each film. The difference in the phase transformation kinetics in the films was discussed based upon the difference in the activation energy values and thus the molecular structures of each hybrid film. Nanoporous and nanocrystalline anatase films were successfully achieved through the retarded phase transformation during the pyrolysis of the organic–inorganic hybrid films. Owing to the maximized surface area these nano-structured anatase films are highly expected to show the enhanced photocatalytic efficiency compared to common TiO 2 films as well as sintered bulk forms.

Effects of acidic and basic hydrolysis catalysts on the photocatalytic activity and microstructures of bimodal mesoporous titania

Bimodal mesoporous titania powders with high photocatalytic activity were prepared by hydrolysis of titanium tetraisopropoxide in the presence of HNO 3 or NH 4OH under ultrasonic irradiation. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface areas, and X-ray photoelectron spectroscopy (XPS). Photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The effects of catalysts added during hydrolysis on the microstructure and photocatalytic activity of the TiO 2 powders were investigated. The results showed that HNO 3 enhanced the growth of brookite, while NH 4OH not only retarded phase transformation of the TiO 2 powders from amorphous to anatase and anatase to rutile but also suppressed the growth of brookite. All TiO 2 powders calcined from 400 to 600 °C showed bimodal pore-size distributions in the mesoporous region: one was intraaggregated pores with maximum pore diameters of ca. 4–9 nm and the other interaggregated ones with maximum pore diameters of ca. 35–50 nm. At 700 °C, the pore-size distributions of all samples exhibited monomodal distribution of the interaggregated pores due to the collapse of the intraaggregated pores. The photocatalytic activity of the TiO 2 powders prepared by this method and calcined at 400 °C exceeded that of Degussa P-25 when the molar ratio of HNO 3 or NH 4OH to H 2O was less than 0.05.

Microstructural Analysis of Liquid‐Phase‐Sintered β‐Silicon Carbide

The microstructures of fine‐grained β‐SiC materials with α‐SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high‐resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β‐SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α‐SiC. Energy‐dispersive X‐ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.

Effects of the Process Variable on Sputtered TiSix Polycide Gate Electrodes for sub-0.15 μm Memory Device Application

We examine effects of the process variables of transistor gate structures on material and electrical properties of films and patterned polycide gate lines at sub-0.15 μm regime. The process variables include molar ratio of Si/Ti in composite targets 2.2, 2.3, and 2.4), sputter deposition temperature, film thickness, rapid thermal annealing (RTA) process condition, and postannealing in a furnace. At high Si precipitate formation is highly pronounced at the film surface as RTA temperature increases and contributes to the increase in surface roughness of the films. Moreover, processes of higher x targets show a greater number of add-on particles and more retarded phase transformation. The RTA process after film deposition completely transforms films to phase even at very fine lines when annealed at 800°C or higher for 20 s. The maximum thermal budget allowed for postannealing is 800°C for 60 min, where no appreciable thermal degradation is shown in bar resistance regardless of target composition. At this postannealing condition, excellent bar resistance (∼4.4 Ω/□) and thermal stability are shown at 0.12 μm linewidth when films were sputtered at 200°C. films sputtered at 500°C show a stronger (040) C54 preferred orientation and greater degraded thermal stability than the films deposited at 200°C, and exhibit a bar resistance of ∼5 Ω/□ at 0.12 μm linewidth. © 2001 The Electrochemical Society. All rights reserved.

Effect of Annealing Conditions on Microstructural Development and Phase Transformation in Silicon Carbide

The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine‐grained β‐SiC ceramics containing α‐SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.