Decreasing Nucleation Rate Research Articles

Short-range order patterns in Mg66Zn29Ca5 metallic glass: Phase transformations and crystallization kinetics

Thanks to outstanding mechanical and chemical properties, metallic glasses have been recently alluring for biomedical applications. However, inadequate knowledge of atomic rearrangements as well as kinetics and mechanisms of crystallization prevents further microstructure improvement controlling the properties. In this work, we reveal that hierarchical transformations happen during the crystallization of Mg66Zn29Ca5 metallic glass ribbons which were investigated using differential scanning calorimetry (DSC), electron microscopy (SEM and TEM), and X-ray diffraction (XRD). The activation energies corresponding to the crystallization of the glassy ribbons were evaluated by different thermodynamic models, i.e., Kissinger, and Augis and Bennett analysis. Our findings prove the formation of short-range order patterns in Mg66Zn29Ca5 metallic glass with a unit size of ∼ 0.247 nm. Furthermore, the applied heating rate affects the glass transformation only slightly but accelerates the growth process quickly. The phase transformations occurring in annealed samples were studied by XRD, proving that crystallization starts with the formation of α-Mg + metastable Mg51Zn20. The metastable Mg51Zn20 gradually transfers to Mg7Zn3 with increasing temperature, and also Ca2Mg5Zn13 precipitates. Close to the end of crystallization, Ca2Mg6Zn3 precipitates by consuming Ca2Mg5Zn13. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model was employed to obtain the Avrami exponent of the crystallization reaction. The average slope of the Avrami plots is close to 2, indicating that crystallization progresses with diffusional growth and decreasing nucleation rate.

Crystallization kinetics of Hf28Be18Ti17Zr17Cu7.5Ni12.5 high-entropy bulk metallic glass

Crystallization kinetics of Hf28Be18Ti17Zr17Cu7.5Ni12.5 high-entropy bulk metallic glass (HE-BMG) in the non-isothermal and isothermal conditions were investigated by differential scanning calorimetry (DSC). In non-isothermal crystallization condition, DSC traces exhibited three crystallization processes. The apparent activation energies of peak crystallization temperatures were calculated by Kissinger methods and found to be Ep1 > Ep2 > Ep3. The dependence of local activation energy on crystallized volume fraction was summarized by the Kissinger-Akahira-Sunose (KAS) method. The non-isothermal crystallization reaction was described mathematically using the Šesták-Berggren model. In the isothermal crystallization condition, with the increase of annealing temperature, the incubation time and crystallization time become shorter and the crystallization peak becomes higher. All Avrami indices were less than 2.5, indicating that crystallization of glassy alloy is dominated by a diffusion-controlled three-dimensional growth with a decreasing nucleation rate.

Crystallization behavior of a series of Zr-based metallic glasses

A series of Zr-Al-Ni-Cu metallic glasses were elaborately designed by combination of multiple optimization strategies and their nonisothermal and isothermal crystallization kinetics were systematically investigated. The results show that the compositions of these metallic glasses are off-eutectic. They have similar crystallization behaviors including glass transition temperature, crystallization temperature, single-step crystallization process, crystallization activation energy and Avrami exponent. The glass forming ability of these metallic glasses is almost equivalent, but slightly decreases with increasing Zr content. The crystalline phases are hp-Zr6Al2Ni and t-CuZr2 in both nonisothermal and isothermal cases for the partially and fully crystallized samples. The content of hp-Zr6Al2Ni decreases with increasing Zr content, while opposite for that of t-CuZr2. The crystallization mechanism is the growth of particles with decreasing nucleation rate for the studied Zr-based metallic glasses. This work would shed light on discovering the quantitative correlation between the microstructure and the crystalline phases for the metallic glasses.

Phase-transition kinetics of calcium-doped TiO2: A high-temperature XRD study

Controlling titanium dioxide crystallinity is crucial in view of the possible oxide applications. In this study, we examined the effects of calcium doping on the anatase-rutile phase transition via in-situ high-temperature X-ray diffraction. The phase transition temperature of calcium-doped TiO2 was approximately 900 °C, specifically 900 °C, 875 °C, and 900 °C for the 2%, 4%, and 6% calcium-doped TiO2, respectively. The larger radius of the calcium ion compared to that of the titanium ion hindered the calcium incorporation into the TiO2 lattice. Moreover, the hampered titanium-oxygen bond rotation during the phase transition led to an increase in the phase-transformation temperature. Furthermore, at higher calcium concentrations, the corresponding activation energies increased first and then decreased. This may have been controlled by the decreasing nucleation rate during the reaction.

Effect of Cu and Co addition on non-isothermal crystallization kinetics of rapidly quenched Fe-Sn-B based alloys

Crystallization kinetics of rapidly quenched Fe-Sn-B alloys under non-isothermal conditions were studied using differential scanning calorimetry. Formation of crystalline phases was analyzed by X-ray diffraction. Nominal chemical compositions were Fe81Sn7B12, (Fe3Co1)81Sn7B12 and (Fe81Sn7B12)99Cu1. Alloys were prepared by planar flow casting in the form of ribbons approximately 20 µm thick and 6 mm wide. Mechanism of crystallization was studied under framework of the Johnson-Mehl-Avrami-Kolmogorov model. Alloys exhibit two stages of crystallization. Results show decrease in activation energy of the first stage of crystallization with addition of Cu and increase with addition of Co. Crystallization mechanism of the first stage of crystallization for Fe81Sn7B12 and (Fe81Sn7B12)99Cu1 alloy starts as growth with increasing nucleation rate and continues as growth with decreasing nucleation rate. Addition of Co changes mechanism of crystallization. Which in case of (Fe3Co1)81Sn7B12 alloy starts as a growth with increasing nucleation rate. Then changes to growth with decreasing nucleation rate. After which nucleation rate decreases to zero. Rest of crystallization stage is governed by growth of pre-existing nuclei. In the first stage of crystallization α-Fe phase with bcc structure crystallizes from amorphous matrix. In the second stage of crystallization the remaining amorphous matrix crystalizes into tetragonal Fe2B phase and hexagonal FeSn phase. After the first stage of crystallization, 50 % to 55 % volume of studied alloys were crystalized. Addition of Cu decreases crystalline size of α-Fe crystallites by 60 % and decreases concentration of Sn in α-Fe phase by 0.8 at. %. Addition of Co doesn't affect the size of α-Fe crystallites and decreases the concentration of Sn in α-Fe phase by 1.7 at. %.

Crystallization Kinetics of the Fe68Nb6B23Mo3 Glassy Ribbons Studied by Differential Scanning Calorimetry

Fe-based metallic glass has wide industrial application due to its unique mechanical behavior and magnetic properties. In the present work, the non-isothermal crystallization kinetics in Fe68Nb6B23Mo3 glassy alloys were investigated by differential scanning calorimeter (DSC). The results indicate that both the glass transformation and crystallization process display an obvious kinetic effect. The activation energy is calculated using Kissinger’s method and Ozawar’s method. The activation energy for Tg (glass transition temperatures), Tx (crystallization initiation temperatures) and Tp (crystallization peak temperatures) calculated from Kissinger equation, is 308 ± 4, 342 ± 5 and 310 ± 7 kJ mol−1, respectively. The activation energy for Tg, Tx and Tp calculated from Ozawa equation is 322 ± 3, 356 ± 5 and 325 ± 7 kJ mol−1, respectively. With the increase of the crystallization volume fraction x, the Avrami exponent n(x) first decreases and then increases. At the preliminary step, 0 < x < 0.25, 2.5 < n(x) < 4.0 stands for the growth from a small size with an increasing nucleation rate. When 0.25 < x < 0.71, n(x) decreases from 2.5 to 1.5, indicating that this stage is controlled by the growth of small particles with a decreasing nucleation rate.

Crystallization behavior and thermal stability mechanism of plasma sprayed Al2O3-GAP amorphous ceramic coatings

A novel Al2O3-GdAlO3 (GAP) amorphous ceramic coating was in-situ deposited via atmospheric plasma spraying (APS). The crystallization behavior and thermal stability mechanism of Al2O3-GAP coating were studied via differential scanning calorimetry (DSC) under isochronal conditions. The nucleation process of Gd4Al2O9 (GAM) and α-Al2O3 phases were more sensitive than the growth process. On the contrary, the nucleation process of GAP phase was duller than the growth process. The activation energy of Al2O3-GAP coating was higher than those of almost all amorphous materials reported so far. GAM and α-Al2O3 phases had a semblable crystallization mechanism: the initial phase evolution was performed by diffusion-controlled growth with a decreasing nucleation rate. As the crystallization fraction increased, the nucleation rate continued to fall. In addition, it was demonstrated that Al2O3-GAP coating showed long failure time (te10%) of 3.0 × 107 min at 1000 K, which was conducive to the application of the coating under high temperature service conditions.

Bi-hierarchical porous Pt microspheres grown on Ti wire with TiO2nanotubes layer for selective alcohol sensing

ABSTRACTThis study focuses on the synthesis of bi-hierarchical porous Pt microspheres directly on titania nanotube arrays grown on a Ti wire for their application as a one-electrode selective alcohol sensor. We evaluate the synthesis conditions, morphology, structure of the obtained material using scanning, transmission electron microscopy and electron diffraction. The sensor performance is assessed in a one-electrode configuration, using thermocycling protocols both to heat and acquire a signal that we further process with a machine learning algorithm for selective determination of alcohols. We found that reduction of Pt precursor by formic acid facilitates the appearance of quasi-1D Pt structures without using any surfactant. High excess of formic acid yields the formation of quasi-dendritic Pt structures with the overall morphology of a sphere and channels whose diameter remains one of the TiO2 nanotubes. Our data suggest the growth of Pt spheres to be diffusion controlled with constant or decreasing nucleation rate that should include assembling of Pt nanorods. The fabricated sensors based on the synthesized structures show a chemiresistive response to methanol, ethanol and isopropanol vapors in the mixture with air, which we selectively determine using only one sensor.

Effect of Gd addition on non-isothermal and isothermal crystallisation of Cu–Zr–Al bulk metallic glass

Abstract The crystallisation kinetics of (Cu46Zr46Al8)100–xGdx (x = 0, 2 and 4 at.%) bulk metallic glasses in non-isothermal and isothermal conditions were studied by differential scanning calorimetry and X-ray diffraction. X-ray diffraction analysis shows that the crystallisation product Cu10Zr7 changes to Cu10Zr7 and Cu2Gd in the presence of Gd in non-isothermal and isothermal conditions. Crystallisation activation energy was calculated using the Kissinger and Ozawa methods in non-isothermal conditions and using the Arrhenius model in isothermal conditions. The results show that Gd addition triggers an increase in the energy barrier during crystallisation. The Johnson–Mehl–Avrami model was employed to analyse the crystallisation kinetics in the isothermal condition. The Avrami exponent, n, for Cu46Zr46Al8 is more than 2.5, which demonstrates that the crystallisation is mainly governed by diffusion-controlled three-dimensional growth with increasing nucleation rate. Comparably, n for (Cu46Zr46Al8)98Gd2 and (Cu46Zr46Al8)96Gd4 is 1.5 < n < 2.5, which suggests that the crystallisation is mainly determined by diffusion-controlled three-dimensional growth with decreasing nucleation rate.

Non-isothermal crystallization kinetics of the Zr50Cu34Al8Ag8 amorphous alloy

The crystallization kinetics of Zr50Cu34Al8Ag8 bulk metallic glass, under non-isothermal condition, is investigated using differential scanning calorimetry. The activation energies for crystallization of the alloy are calculated using the Kissinger and Ozawa equations. The crystallization mechanism of amorphous alloy under continuous heating is explored via the improved Johnson-Mehl-Avrami equation. The local avrami index is mainly in the range of 2 to 2.5, which indicates that the crystallization is mainly governed by diffusion-controlled three-dimensional growth with a decreasing nucleation rate.

Crystallization kinetics of Cu–Zr–Al bulk metallic glass with Nd addition

AbstractThe crystallization kinetics of (Cu48Zr48Al4)100−xNdx (x = 0, 2 at.%) bulk metallic glasses in nonisothermal and isothermal conditions were studied by differential scanning calorimetry and X‐ray diffraction (XRD). XRD analysis shows that the crystallization product Cu10Zr7 changes to Cu10Zr7 and Cu2Nd with Nd presence in nonisothermal and isothermal conditions. Crystallization activation energy was calculated in nonisothermal conditions using Kissinger and Ozawa methods and in isothermal conditions using Arrhenius model. The results show that the activation energy harbored by (Cu48Zr48Al4)98Nd2 expresses larger value compared with Cu48Zr48Al4, which indicate that the energy barrier for crystallization has enhanced along the Nd addition. The Johnson–Mehl–Avrami model was employed for analyzing the crystallization kinetics in isothermal condition. The Avrami exponent n for Cu48Zr48Al4 is more than 2.5, demonstrating that the crystallization is mainly governed by diffusion‐controlled three‐dimensional growth with increasing nucleation rate. Comparably, the Avrami exponent n for (Cu48Zr48Al4)98Nd2 is 1.5 < n < 2.5, suggesting that the crystallization is mainly determined by diffusion‐controlled three‐dimensional growth with decreasing nucleation rate. The evident enhancement in crystallization activation energy based on Nd addition is rationally made up of an influential factor commonly harbored on the enhanced glass forming ability for the rare‐earth comprising Cu–Zr–Al metallic glasses.

Non-isothermal crystallization kinetics of Ge-Cu-Te thin films for phase-change memory

We investigated the crystallization kinetics of Ge-Cu-Te ternary system phase-change thin films using non-isothermal differential scanning calorimetry and estimated the kinetic triplet (activation energy, pre-exponential factor, and reaction model) by the combined application of isoconversional and model-fitting approaches at different heating rates over a range of 10–40 K/min for GeCu2Te2 thin films. The results indicate a multi-step crystallization process where the activation energy of crystallization is not constant but decreases via three recognizable stages with crystallization fraction α. The local Avrami exponents suggest that two crystallization mechanisms containing three-dimensional growth with increasing nucleation rates in the range of crystallization fraction α ≤ 0.4 and three- and two-dimensional growths at a decreasing nucleation rate over the wide range of α > 0.4 are responsible for the whole crystallization process. Furthermore, the reaction model displays dependence on the heating rates for the GeCu2Te2 thin films, where heating rates of 10–30 K/min drive a second-order reaction while a three-halve order reaction occurs for a heating rate of 40 K/min, which is different from the second-order reaction for the GeCu2Te3 thin films. Also, both GeCu2Te2 and GeCu2Te3 thin films provide better data retention capability than the Ge2Sb2Te5 thin films without sacrificing the crystallization time. The results indicate that Ge-Cu-Te ternary system thin films have great potentiality in phase-change memory applications.

Effective role of minor silicon addition on crystallization kinetics of Cu50Zr43Al7 bulk metallic glass

The Si addition has a great impact on the glass forming ability of Cu50Zr43Al7 bulk metallic glass (BMG). To investigate the effective role of Si doping on the crystallization behavior of the BMGs, crystallization transformation kinetics of Cu50Zr43Al7 and (Cu50Zr43Al7)99Si1 BMGs were explored at non-isothermal and isothermal conditions by differential scanning calorimetry (DSC) experiments. The Si doping enhances the activation energy of crystallization transformation (resistance against crystallization) under both thermal circumstances. Avrami exponents were obtained by employing the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. In the isothermal regime, Avrami exponents in both BMGs were 2.2, which reveals that the dominant mechanism of crystallization is the three-dimensional growth with a decreasing nucleation rate of crystals. The time–temperature–transformation or TTT diagram displayed that minor alloying of Si displaced the crystallization transformation to elevated temperatures and longer incubation times.

Isothermal and non-isothermal crystallization kinetics of novel metalloid-bearing light rare-earth-based La64Al26Ga4C6 metallic glass with high thermal stability

The crystallization kinetics of a novel metalloid-bearing light rare-earth (RE)-based La64Al26Ga4C6 metallic glass (MG) under isothermal and non-isothermal conditions were systematically researched by Johnson-Mehl-Avrami (JMA), Arrhenius, Kissinger, Ozawa, Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall approaches using differential scanning calorimeter. The present MG possesses higher thermal stability than other light RE-based MGs without metalloid. As the isothermal crystallization temperature increases, the Avrami exponent estimated by JMA equation ranges from 2.36 to 2.94, implying that the isothermal crystallization mechanism varies from a three-dimensional diffusion-governed growth with a decreasing nucleation rate to a three-dimensional diffusion-governed growth with an increasing nucleation rate. Meanwhile, the apparent activation energy determined by Kissinger and Ozawa methods for characteristic temperatures indicates that glass transition is more difficult than crystallization process. Additionally, the local Avrami exponents show various trends, implying a transformable crystallization mechanism with crystallization proceeding. Furthermore, the crystallization process becomes easier due to the decrease of local activation energy.

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between Tg and Tx, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of Tg + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.

2 + 2] Photodimerization of Sulfonate Derivative of trans-Cinnamic Acid: Kinetics Study Using Solid State 13C NMR and Hybrid Material Inclusion

A sulfonic acid, namely p-sulfocinnamic acid (psca), has been synthesized and characterized spectroscopically as well as by single crystal X-ray diffraction. The crystal structure of psca reveals that it fulfils Schmidt’s topochemical rules for [2 + 2] photodimerization and the molecules to be dimerized are arranged in head-to-tail manner, confirming psca to be an α-type polymorph. The [2 + 2] photodimerization of psca has been monitored using solid state 13C CP-MAS NMR spectroscopy which certifies a 100% photoconversion to yield to 4,4′-disulfonate-truxillic acid (dsta). The Avrami constant, n = 1.8(1), derived from the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, indicates that [2 + 2] photodimerization proceeds with a decreasing nucleation rate and formation of dsta occurs through a one-dimensional heterogeneous linear growth. The crystal structure of dsta unveils it to be a centrosymmetric molecule. The solvent molecules present in the crystal lattice interconnect all truxillic acid molecules through a 3D hydrogen bonding network. Guest insertion of psca into a selected inorganic host-network, as a proof of concept for hybrid materials design, provided [Fe(H2O)6](psca)2·2H2O, which has been structurally characterized and investigated by 57Fe Mössbauer spectroscopy.

Study of the crystallization kinetics of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy

Abstract The non-isothermal crystallization kinetics with heating rates ranging from 10 K s-1to 80 K s-1and the isothermal crystallization kinetics during annealing from the glass transition temperature to the crystallization onset temperature of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy were studied in detail using X-ray diffraction and differential scanning calorimetry. During non-isothermal crystallization, it is more difficult to nucleate than to grow, and the crystallization resistance increases first and then decreases. During isothermal crystallization of the alloy from 713- 728 K, there are two exothermic peaks corresponding to a diffusion-controlled growth process with decreasing nucleation rate and increasing nucleation rate. From 733- 748 K, only one exothermic peak appears, and the growth process is controlled by the interface with decreasing nucleation rate. Isothermal crystallization is a process in which the crystallization resistance increases. The resistance of isothermal crystallization is less than that of non-isothermal crystallization.

Study of the crystallization kinetics of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy

Abstract The non-isothermal crystallization kinetics with heating rates ranging from 10 K s-1to 80 K s-1and the isothermal crystallization kinetics during annealing from the glass transition temperature to the crystallization onset temperature of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy were studied in detail using X-ray diffraction and differential scanning calorimetry. During non-isothermal crystallization, it is more difficult to nucleate than to grow, and the crystallization resistance increases first and then decreases. During isothermal crystallization of the alloy from 713- 728 K, there are two exothermic peaks corresponding to a diffusion-controlled growth process with decreasing nucleation rate and increasing nucleation rate. From 733- 748 K, only one exothermic peak appears, and the growth process is controlled by the interface with decreasing nucleation rate. Isothermal crystallization is a process in which the crystallization resistance increases. The resistance of isothermal crystallization is less than that of non-isothermal crystallization.

Enhanced Efficiency of Air-Stable CsPbBr3 Perovskite Solar Cells by Defect Dual Passivation and Grain Size Enlargement with a Multifunctional Additive.

The perovskite solar cells (PSCs) based on cesium lead bromide (CsPbBr3) with outstanding environmental stability and low preparation cost are regarded as one of the most promising photovoltaic devices for commercial applications. However, the performance of CsPbBr3 PSCs can be badly deteriorated by the intense charge recombination arising from the ionic defects at the grain boundaries of perovskite film. To cope with this issue, we adopt an amino acid of l-lysine with two amino and one carboxyl groups as a chemical additive to incorporate into perovskite film to simultaneously anchor the uncoordinated Pb2+ (Cs+) and halogen ion defects. Further, the grain size of CsPbBr3 perovskite is boosted from 688 to over 1000 nm after l-lysine incorporation as a result of the decreased nucleation rate and the sufficient growth of perovskite, which effectively reduce the grain boundaries for load defects. As expected, the optimized device achieves a best power conversion efficiency of 9.69% attributed to the remarkably reduced charge recombination and enhanced charge extraction arising from the efficient defects dual-passivation and enlarged grain size of perovskite film as well as the improved energy level alignment at the device interface after the introduction of l-lysine, which is elevated by 61.23% in comparison to 6.01% efficiency of the pristine one. Moreover, the unencapsulated device with l-lysine incorporation exhibits remarkable long-term stability in air with 80% RH at 25 °C and 0% RH at 80 °C as well as under continuous illumination conditions. This work provides an effective multifunctional additive for imperfection passivation and grain size enlargement of perovskite to build PSCs with high efficiency and stability.

The first- and second-order isothermal phase transitions in Fe3Ga-type compounds.

Structural features and kinetics of the transition between ordered metastable b.c.c.-derived D03 and equilibrium f.c.c.-derived L12 phases of Fe-xGa alloys (x= 27.2% and 28.0%) have been analyzed by in situ real-time neutron diffraction during isothermal annealing in the temperature range 405-470°C. It has been revealed that the transition proceeds with alternation of the first- and second-order phase transformations according to a D03 → A2 → A1 → L12 scheme, where A2 and A1 are disordered b.c.c. and f.c.c. structures. Deformations of the crystal lattice that arise due to these transitions are determined. The kinetics of the L12 phase nucleation and growth were analyzed in the frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model; however, only the early stage of the D03 → L12 transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L12 phase and decreasing nucleation rate in the Fe-27.2Ga alloy and indicates the presence of pre-existing nucleation centres of the L12 phase in the Fe-28.0Ga alloy.