Yulai Gao

Crystallization kinetics of an amorphous Zr–Cu–Ni alloy: calculation of the activation energy

Abstract The activation energies of amorphous Zr–Cu–Ni alloy under continuous heating conditions have been calculated using different methods based on differential scanning calorimetry (DSC) data. The activation energy for crystallization of amorphous Zr 70 Cu 20 Ni 10 alloy are determined as 313, 411, 293, 253 and 286 kJ/mol by means of the Kissinger, Ozawa, Cheng, Gao and Wang and Matusita equations, respectively. However, under isothermal crystallization the average value of activation energy of amorphous Zr 70 Cu 20 Ni 10 alloy is 355 kJ/mol. As for the amorphous Zr 70 Cu 20 Ni 10 alloy, the Ozawa equation gives the largest value of activation energy and the Gao and Wang equation shows the smallest one. The difference in the activation energy for crystallization of an amorphous alloy may be attributed to the different approximations in these models.

Crystallization behavior of ZrAlNiCu bulk metallic glass with wide supercooled liquid region

Abstract The crystallization processes of Zr55Al10Ni5Cu30 (at.%) bulk metallic glass from the amorphous state and supercooled liquid region during the continuous heating and isothermal annealing courses were investigated. The apparent activation energy derived from the Kissinger plots for glass transition Eg is higher than that for crystallization Ep and Ex calculated by the peak temperature Tp and onset crystallization temperature Tx during the continuous heating process. The isothermal activation energy obtained using Arrhenius equation shows that it increases accompanying the proceeding of the crystallization transformation. The incubation time of crystallization becomes longer and the crystallization process becomes slower as the annealing temperature is reducing during the isothermal process. The crystallization mechanism was studied using Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. The results indicate that the Avrami exponent n≈2 at the initial crystallization stage, and it alters gradually following the increasing crystallization volume fraction.

Primary crystallization in rapidly solidified Zr70Cu20Ni10 alloy from a supercooled liquid region

Abstract The primary crystallization of amorphous Zr70Cu20Ni10 alloy has been investigated by employing the differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It is interesting to note that the face-centered cubic Zr2Ni (fcc-Zr2Ni) phase precipitates first from the amorphous matrix when the amorphous Zr70Cu20Ni10 alloy is annealed at the supercooled liquid region. At the late crystallization stage, the Zr2Cu particles begins to precipitate. It is thought that at the initial stage the amorphous Zr70Cu20Ni10 alloy undergoes a phase decomposition process, accompanying with a compositional change, which is regarded as a preparation for the precipitation of the fcc-Zr2Ni particles. The whole crystallization process of amorphous Zr70Cu20Ni10 alloy can be divided into two parts, i.e., the interface-controlled transient nucleation and growth process to a critical size at the early stage, and the diffusion-controlled grain growth process at the late stage. This behavior has been analyzed from the viewpoints of thermodynamics and kinetics.

Nanocrystallization of Zr-Ti-Cu-Ni-Be bulk metallic glass

Abstract The nanocrystallization kinetics of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glass was investigated by differential scanning calorimetry (DSC) in the mode of continuous heating and isothermal annealing. In the case of continuous heating, three exothermic crystallization peaks can be observed, and the peak temperatures display a strong dependence on the heating rates, which can be fitted by a first order decay equation. The activation energies for crystallization are estimated to be E p1 =166.83±8.85 kJ/mol, E p2 =256.15±9.34 kJ/mol, E p3 =174.36±12.56 kJ/mol, which correspond to peak temperatures of T p1 , T p2 , and T p3 , respectively, indicating the formation of different crystallization phases at different stages. In the case of isothermal annealing, the crystallization products under isothermal annealing were observed and determined by transmission electron microscopy (TEM). The precipitation phases of the sample heated at higher temperatures were identified by X-ray diffraction (XRD), showing that the body-centered tetragonal (bct) Zr 2 Cu and hexagonal ZrBe 2 are the primary phases, in spite of the presence of other phases. These results are consistent with the complexity of the DSC curves obtained during the continuous heating and isothermal annealing.

Size and rate dependence of crystal nucleation in single tin drops by fast scanning calorimetry

The experimentally accessible degree of undercooling of single micron-sized liquid pure tin drops has been studied via differential fast scanning calorimetry. The cooling rates employed ranged from 100 to 14,000 K/s. The diameter of the investigated tin drops varied in the range from 7 to 40 μm. The influence of the drop shape on the solidification process could be eliminated due to the nearly spherical shape of the single drop upon heating and cooling and the resultant geometric stability. As a result it became possible to study the effect of both drop size and cooling rate in rapid solidification experimentally. A theoretical description of the experimental results is given by assuming the existence of two different heterogeneous nucleation mechanisms leading to crystal nucleation of the single tin drop. In agreement with the experiment these mechanisms yield a shelf-like dependence of crystal nucleation on undercooling. A dependence of crystal nucleation on the size of the tin drop was observed and is discussed in terms of the mentioned theoretical model, which can possibly also describe the nucleation for other related rapid solidification processes.

Dealloying behavior of rapidly solidified Al–Ag alloys to prepare nanoporous Ag in inorganic and organic acidic media

The dealloying processes of Al-Ag alloy ribbons consisting of two distinct phases of α-Al (Ag) and Ag2Al in the 5 wt.% H2SO4, 10 wt.% H3PO4, 10 wt.% C2H2O4 and 5 wt.% HCl solutions were investigated. The as-dealloyed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis coupled with SEM. It has been found that the resultant microstructure of the as-dealloyed samples is significantly influenced by the initial alloy composition and the acid kind. The four acid solutions are all able to leach out the Al from both the α-Al (Ag) and Ag2Al phases and the as-dealloyed samples exhibit the typical three dimensional (3D) bi-continuous nanoporous structure. The dealloying duration in four acid solutions ranges from 1.1 to 24 h, and the dealloying rates are influenced by acidity, acid concentration, and the diffusion coefficient (Ds) of Ag atoms which is affected by anions and interactions between anions and Al atoms. The specific surface area of the nanoporous silver (NPS) from the Al-15 at.% Ag alloy dealloyed in HCl was determined as 3.47 ± 0.08 m2 g−1 through N2 adsorption experiments and Brunauer–Emmett–Teller (BET) analysis. The Uv-vis absorption spectrum indicates that surface plasmon resonance (SPR) of silver nanostructure exists on the NPS samples, implying the potential application of NPS in surfaced enhanced Raman scattering (SERS)-active substrate, surface plasmon-based analytical devices, etc.

Size-dependent melting properties of Sn nanoparticles by chemical reduction synthesis

Tin nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH4 as reduction agent. The Sn nanoparticles smaller than 100 nm were less agglomerated and no obviously oxidized. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry. The melting temperatures of Sn nanoparticles in diameter of 81, 40, 36 and 34 nm were 226.1, 221.8, 221.1 and 219.5 ℃, respectively. The size-dependent melting temperature and size-dependent latent heat of fusion have been observed. The size-dependent melting properties of tin nanoparticles in this study were also comparatively analyzed by employing different size-dependent theoretical melting models and the differences between these models were discussed. The results show that the experimental data are in accordance with the LSM model and SPI model, and the LSM model gives the better understanding for the melting property of the Sn nanoparticles.

Dependence of crystal nucleation on prior liquid overheating by differential fast scanning calorimeter

The degree of overheating of a melt often plays an important role in the response of the melt to subsequent undercooling, it determines the nucleation and growth behavior and the properties of the final crystalline products. However, the dependence of accessible undercooling of different bulk melt samples on prior liquid overheating has been reported to exhibit a variety of specific features which could not be given a satisfactory explanation so far. In order to determine uniquely the dependence of accessible undercooling on prior overheating and the possible factors affecting it, the solidification of a pure Sn single micro-sized droplet was studied by differential fast scanning calorimeter with cooling rates in the range from 500 to 10,000 K/s. It is observed experimentally that (i) the degree of undercooling increases first gradually with increase of prior overheating; (ii) if the degree of prior superheating exceeds a certain limiting value, then the accessible undercooling increases always with increasing cooling rate; in the alternative case of moderate initial overheating, the degree of undercooling reaches an undercooling plateau; and (iii) in latter case, the accessible undercooling increases initially with increasing cooling rate. However, at a certain limiting value of the cooling rate this kind of response is qualitatively changed and the accessible undercooling decreases strongly with a further increase of cooling rate. The observed rate dependent behavior is consistent with a kinetic model involving cavity induced heterogeneous nucleation and cavity size dependent growth. This mechanism is believed to be relevant also for other similar rapid solidification nucleation processes.

Nickel oxide nanopetal-decorated 3D nickel network with enhanced pseudocapacitive properties

Metal oxides possess high theoretical specific capacitance, but their pseudocapacitive properties are restricted by the poor electronic conductivity. Here we present a strategy to synthesize a three-dimensional binder/conducting agent-free nickel oxide (NiO) electrode through the combination of anodization with calcination. The NiO electrode is composed of a 3D conductive nickel network decorated with nanopetal-like NiO arrays. The influence of calcination temperature has been investigated, with respect to the microstructure and pseudocapacitive properties of the NiO electrodes. The NiO electrode demonstrates great electrochemical properties, especially remarkable rate capability (82% retention of the highest value for the 25-fold enhanced current density) and cycling stability (good capacitance retention after 30000 cycles). Moreover, an asymmetric supercapacitor has been assembled using NiO as the positive electrode and activated carbon (AC) as the negative electrode. The NiO//AC supercapacitor presents excellent cycling stability (91.3% retention after 10000 cycles), and could power a mini fan as well as a commercial red LED for more than 270 min.

Dealloying strategy to fabricate ultrafine nanoporous gold-based alloys with high structural stability and tunable magnetic properties

In the present work, the dealloying of Al–Au-based precursors and formation of nanoporous Au-based alloys have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and energy dispersive X-ray (EDX) analysis. The results show that the addition of Ni and/or Co has no influence on phase constitution of rapidly solidified Al–Au–M (M = Ni, Co, or Ni/Co) alloys and a single-phase Al2(Au,M) intermetallic compound can be identified in these ternary and quarternary precursor alloys. The Al–Au-based precursors can be fully dealloyed in an alkaline solution under free corrosion conditions, and the dealloying results in the formation of novel ultrafine nanoporous Au-based alloys (Au(Ni), Au(Co) and Au(Ni,Co)) with ligaments/channels of ∼5 nm. The ultrafine nanoporous Au-based alloys possess extraordinarily high structural stability against thermal annealing. Moreover, due to the intrinsic magnetism of Ni and Co, the addition of Ni and/or Co leads to the formation of novel magnetic nanoporous alloys. The dealloying mechanism of these Al–Au-based precursors has been discussed based upon surface diffusion of Au adatoms and interaction between Au and additional elements. The present findings provide a new dealloying route to fabricate ultrafine nanoporous Au-based alloys with high stability and magnetic properties through alloy design of precursors.