Sharp Exothermic Peak Research Articles

Temperature-induced order-disorder structural phase transitions of two-dimensional isostructural hexamethylenetetramine co-crystals.

Hexamethylenetetramine-benzoic acid (1/2) (HBA) and hexamethylenetetramine-4-methylbenzoic acid (1/2) (HMBA) co-crystals undergo order-disorder structural phase transition from a low-temperature monoclinic crystal structure to a high-temperature orthorhombic crystal structure at the transition temperatures of 257.5 (5) K (Pn ↔ Fmm2) and 265.5 (5) K (P21/n ↔ Cmcm), respectively, using variable-temperature single-crystal X-ray diffraction analysis. The observed phase transitions were confirmed to be reversible first-order transitions as indicated by the sharp endothermic and exothermic peaks in the differential scanning calorimetry measurement. The three-molecule aggregate of HBA and HMBA consists of a hexamethylenetetramine molecule and two benzoic acid or two 4-methylbenzoic acid molecules, respectively. The acid molecules are ordered at the low-temperature phase and are equally disordered over two positions, which are related by a mirror symmetry, at the high-temperature phase. The two-dimensional supramolecular constructs common to both co-crystals are formed by three-molecule aggregates via weak intermolecular C-H...O and C-H...π interactions into molecular trilayers parallel to the ac plane with small XPac dissimilarity indices and parameters. The PIXEL interaction energies of all corresponding molecular contacts were calculated and the results are comparable between HBA and HMBA co-crystals, resulting in similar lattice energies and transition temperatures despite their two-dimensional isostructural relationship. The observed phase transitions of these two energetically similar co-crystals are triggered by similar mechanisms, i.e. the molecular rotator ordering and structural order-disorder transformation, which induced non-merohedral twinning with similar twin matrices in the low-temperature crystal form of both co-crystals.

Preparation, crystal structure, thermal behaviors and DFT studies of 4,4′,5,5′-tetraacetamine-3,3′-bi(1,2,4-triazole)

4,4′,5,5′-tetraacetamine-3,3′-bi(1,2,4-triazole) (TAcABTz) has been synthesized and characterized by X-ray diffraction, FT-IR and 1H and 13C NMR spectroscopy. TAcABTz Ethanol crystallizes in the monoclinic space group P21/n, and x-ray result shows that two triazole rings are almost coplanar except the methyl group. Thermal behaviors were also studied under the non-isothermal conditions by TG-DTA method, showing that two sharp exothermic peaks during thermal decomposition, and the kinetic parameters including activation energy (Ea) and pre–exponential factor (A) are 144.3 kJ mol−1 and 3.603 × 1012 s−1, respectively. In addition, geometry optimization, binding energy and HOMO–LUMO analysis of TAcABTz and TAcABTz Ethanol are conducted by density functional theory DFT/B3LYP calculation method with 6-311 + G** basis set. DFT calculations results show that the conformer A of TAcABTz Ethanol is more stable than conformer B.

Effects of ball-milling on fabrication of YAG ceramics by a phase transformation assisted spark plasma sintering

YAG ceramics were fabricated by spark plasma sintering assisted by the YAH-YAG phase transformation. These YAH powders were prepared by field-assisted rapid synthesis of the amorphous precursors using the planetary ball-milling. Moreover, the effects of ball-milling speed on the thermal behavior and morphology of the precursors and calcined YAH powders, as well as the subsequent sintering process were systematically studied. The results of thermal analysis of the YAH powders suggest that the exothermic peak for phase transformation shifts to the lower temperature and becomes sharper by increasing the ball-milling speed. The YAH powders synthesized from 225 rpm-milled precursors exhibit a relatively shallow and weak exothermic peak, indicating a slow phase transformation. Therefore, the samples sintered at 950 °C have the homogeneous microstructures from the center to edge. The in-line transmittances are 32% and 60% at 680 nm and 1000 nm, respectively. In the case of 250 rpm, however, the sharp and strong exothermic peak indicates a rapid phase transformation, resulting in an extremely rapid shrinkage between 890 °C and 900 °C during sintering. Microstructure evidence suggests that differential densification occurs during rapid phase transformation. The exterior of the sample is dense, while large pores trapped in the interior lead the sample to be opaque.

Crystalline-selected synthesis of ferrous oxalate dihydrate from spent pickle liquor

Ferrous oxalate dihydrate with different crystalline structures, polymorphs and sizes have been synthesized through a facile precipitation reaction from spent pickling liquors (SPL) by controlling synthesis conditions. The β-FeC2 O4 · 2H2 O in orthorhombic phase can be formed at room temperature, whereas α-FeC2 O4 · 2H2 O with a monoclinic unit cell can be obtained after aging at 80 °C. With the increasing aging temperature, the morphology of samples can be tailored from flower-like to prismatic-like and the particle-sizes are decreased. Innovatively, thermal decomposition behaviors of products have been investigated in details by X-ray diffraction pattern (XRD) and thermal gravimetric-differential scanning calorimetry (TG-DSC). The results show that both FeC2 O4 · 2H2 O samples can be transformed to α-Fe2 O3 in air atmosphere and Fe3 O4 in argon atmosphere at T ≧ 400 °C. In air atmosphere, there are two sharp exothermic peaks for β-FeC2 O4 · 2H2 O (250 °C) and α-FeC2 O4 · 2H2 O (239.5 °C) correspond to the decomposition reactions. In argon atmosphere, the decomposition end temperatures are 500 °C and 430 °C for β-FeC2O4 · 2H2O and α-FeC2O4 · 2H2O, respectively. This indicates that β-FeC2O4 · 2H2O has higher thermal stability in comparison with α-FeC2O4 · 2H2O.

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb2Si2O7 Coating

In the present work, Yb2Si2O7 powder was synthesized by solid-state reaction using Yb2O3 and SiO2 powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb2Si2O7 coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb2Si2O7 with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb2Si2O7 coating was about 0.68 W/(m K).

Improvement in the Thermal Safety of Rechargeable Lithium Batteries: Endothermic Behavior of Organic Positive-Electrode Materials

The current rechargeable lithium batteries have potential risks related to the thermal safety. These batteries sometimes suffers from an unexpected rapid temperature increase. This phenomenon, called thermal runaway, is triggered by some abuse conditions such as overcharge and internal short-circuits, which could lead to explosions or fire. Considering large scale battery applications including electric vehicles and stationary system, extreme caution for this safety control is particularly important. To ensure the battery safety, many approaches have been suggested. The use of a particular kind of separator, the combination with a solid electrolyte, and coating of electrode materials are common strategies. The introduction of an external battery management system is also applied. Furthermore, it is inevitable to find out a thermally safe electrode material. We have studied a series of organic active materials to reduce the amount of minor metal-based materials in the positive electrodes from the view point of the resource concern and have demonstrated that some low-molecular-weight quinone compounds show relatively large capacities based on their multi-electron transfer-type redox reaction [1‒3]. There are many other reports describing the charge/discharge performance of such organic active materials; however, the thermal behavior of these organic ones has been rarely discussed. Here, we compares the thermal behavior of a conventional lithium cobalt oxide (LiCoO2) and an organic active material using an example of 2,5-dimethoxy-1,4-benzoquinone (DMBQ) (Fig. 1a). This organic active material shows a large initial capacity of more than 300 mAh/g and relatively good cycle-life stability. Figure 1b shows the differential scanning calorimetry profiles of the charged (delithiated) LiCoO2-electrode and the DMBQ-electrode under the coexistence with an electrolyte solution. The LiCoO2-electrode showed a sharp exothermic peak at around 240oC. Similar exothermic behaviors are often observed for a lot of minor-metal oxides, which leads to the thermal runaway. As for the mechanism, it is commonly understood that charged minor-metal oxides often release oxygen at elevated temperatures, which then induces the combustion reaction of electrolyte solutions. On the other hand, the DMBQ-electrode showed an opposite behavior: some endothermic peaks were observed during the measurement. In general, many organic compounds have a melting point or decomposition point, and these reactions are endothermic. The endothermic behavior observed for the DMBQ-electrode in this study is also considered to reflect such melting and/or thermal decomposition of the DMBQ particles in the electrode. Since many organic materials do not contain any minor metals, the use of these materials can alleviate resource constraint issues. Furthermore, their endothermic behavior can be a promising characteristic, which would contribute to the development of a thermally safe battery system.

A Multi-Component Additive to Improve the Thermal Stability of Li(Ni1/3Co1/3Mn1/3)O2-Based Lithium Ion Batteries

To improve the safety of lithium ion batteries, a multi-component (MC) additive (consisting of vinylene carbonate (VC), 1,3-propylene sulfite (PS) and dimethylacetamide (DMAC)) is used in the baseline electrolyte (1.0 M LiPF6/ethylene carbonate (EC) + diethyl carbonate (DEC)). The electrolyte with the MC additive is named safety electrolyte. The thermal stabilities of fully charged Li(Ni1/3Co1/3Mn1/3)O2 (NCM) mixed with the baseline electrolyte and safety electrolyte, respectively, are investigated using a C80 micro-calorimeter. The electrochemical performances of the NCM/baseline electrolyte/Li and NCM/safety electrolyte/Li half cells are evaluated using galvanostatic charge/discharge, cyclic voltammetry and alternating current (AC) impedance. The experimental results demonstrate that the fully charged NCM-safety electrolyte system releases less heat and reduces the main sharp exothermic peak value to a great extent, with a reduction of 40.6%. Moreover, the electrochemical performances of NCM/safety electrolyte/Li half cells are not worse, and are almost as good as that of the NCM/baseline electrolyte/Li half cells.

Investigation of the thermal decomposition of a new titanium dioxide material

A new type of TiO2 material has been prepared from titanium(IV) oxysulfate via precipitation by cold ammonia solution and drying at room temperature. The structure remains unchanged during the preparation (controlled hydrolysis), and titania material with a similar rod-shaped structure is created. The decomposition process has been evaluated by TA/MS analysis as a three-step process. The first step was characterized by water and carbon dioxide loss, the second step is represented by the evolution of residual water and ammonia, and the third step is represented by a very sharp exothermic peak due to the phase transition (crystallization of amorphous phase to anatase) together with a small mass loss caused by evolution of oxygen and nitrous oxide. EPR spectra obtained in the temperature range 100–250 K confirmed the presence of Ti(III) ions and oxygen vacancies for the TiO2 precursor annealed up to 550 °C in a vacuum. Annealing to 500 °C resulted in the generation of paramagnetic Ti(III) centers and nitrogen species (·Nb and NO) in anatase lattice.

Crystal Structure and Properties of 7‐Imino‐3‐Nitroimino‐2,4,6,8‐Tetraazabicyclo[3.3.0]Octane Hydrochloride

AbstractThe chemical and physical properties of the energetic intermediate 7‐imino‐3‐nitroimino‐2,4,6,8‐tetraazabicyclo[3.3.0]octane hydrochloride (compound A) were investigated. The crystal structure and thermal behavior were measured by X‐ray diffraction and TG‐DTG‐DSC coupling system, respectively. The electronic structure was also studied by electronic density, nature bond orbital charges, frontier molecular orbital, density of state, and electrostatic potentials. Compound A crystallizes orthorhombic in space group Pna21. Thermodynamic properties indicate that it has moderate thermal stability with a sharp exothermic peak (298.5 °C, 10 K·min–1) and high critical temperature of thermal explosion (558.5 K). The positive electrostatic potentials of compound A were stronger than that of the negative electrostatic potentials, which can stabilize the molecule. Compound A can also be considered as an energetic intermediate, which may shine light on the exploring of other 7‐imino‐3‐nitroimino‐2,4,6,8‐tetraazabicyclo[3.3.0]octane hydrochloride based high density energetic materials.

Interaction of Resveratrol with Lipid Membranes

Background: Resveratrol is a phytoalexin synthesized by plants. It has antioxidant properties and is a popular nutritional supplement. Beneficial properties of resveratrol, such as, anticancer and anti-inflammatory properties have been reported. Resveratrol is a constituent of red wine and found in the skin of red grapes. In order to understand the interaction between resveratrol and biological membranes, we evaluated the effect of resveratrol on model lipid membranes using differential scanning calorimetry (DSC) and computational studies. Methods: Phospholipids such as, Dilauroylphosphatidylcholine (DLPC), Dimyristoyphosphatidylcholine (DMPC), Dipalmitoylphosphatidyl choline (DPPC), Distearoylphosphatidylcholine (DSPC), and 1-palmitoyl-2-oleyl phosphatidylcholine (POPC) were purchased from Avanti Polar Lipids (Alabaster, Alabama). Each phospholipid was mixed with resveratrol at different molar ratios, phospholipid: resveratrol, 10:1, 10:3 and 10:5. Computational simulations were also performed to evaluate the interactions of DSPC and resveratrol. Results: Resveratrol abolishes only the transition of the DLPC, which is the shortest phospholipid of those tested. It reduces the transition temperature for all the other phospholipids even at the lowest ratio tested, phospholipid: resveratrol, 10:1. Resveratrol reduces the transition temperature of DSPC from 55 °C to 51 °C. Furthermore, using DSC, we also observed another transition, a sharp exothermic peak above 275 °C in the interaction of resveratrol with DSPC. We performed computational simulations of the DSPC membrane at different temperatures with and without resveratrol. The simulation indicates that resveratrol affects the transition temperature of the DSPC, which is in agreement with our DSC data. In conclusion, our data indicate that resveratrol abolishes the transition of DLPC and acts as a plasticizer for phospholipids with longer fatty acyl chains.

Amorphous-i phase-big cube-amorphous cyclic phase transformation of mechanically alloyed Zr75Ni20Al5 system

Cyclic glassy-metastable-glassy phase transformations were investigated during mechanical alloying of Zr75Ni20Al5 powders, using high-energy ball milling technique. After 20 h of continuous milling, a single glassy phase was obtained. The synthesized glassy powder, which is homogeneous in structure and composition, showed a glass transition temperature lied at 510 °C and a large supercooled liquid region extended to 86.5 °C. This glassy phase, which crystallized through a single sharp exothermic peak at 596.5 °C, tended to transformed into icosahedral (i-phase) nano-quasicrystalline grains upon further milling to 30 h. This i-phase showed serious instability against the mechanical deformations generated by the milling tools, and hence transformed into big cube phase with increasing the milling time to 45 h. In a cyclic phase transformation the obtained big cube phase subsequently transformed into the same glassy phase after 60 h of milling. This observed cyclic phase transformation was observed twice upon further milling to 90 h without serious compositional changes. The iron contamination contents in the ball-milled powders had constant values of about 0.16 wt.% and did not change with changing the structure of the powders. The present work shows that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls), which leads to the cyclic transformations, depends on the milling time.

Physico-chemical investigations and thermal degradation behaviour of 4-ethylpiperazine-1-carbodithioic acid complexes of cobalt(II)

Some new cobalt(II) carbodithioates, viz. [Co(4-EtPipzcdtH)m]Xn (4-EtPipzcdtH = 4-ethylpiperazine-1-carbodithioic acid, X = Cl, Br, ClO4, NO3 and CH3COO when m = n = 2 and SO4, CO3 when m = 2 and n = 1), have been synthesized and characterized. Infrared and UV–Vis spectral data along with room temperature magnetic behaviour (2.50–2.85 B.M.) indicate chelating bidentate mode of coordination of ligand, 4-EtPipzcdtH with four coordination around Co(II). 1H NMR and ESMS of [Co(4-EtPipzcdtH)2]Cl2 further support its structure. Molar conductance study shows 1:2 electrolytic behaviour for complexes with Cl, Br, ClO4, NO3 and CH3COO as counter anions, while 1:1 electrolytic behaviour for CO3 and SO4 as counter anions. Ligand and its complexes have also been studied using non-isothermal techniques (TG and DSC). The anion has been found to play pronounced effect on the nature of thermal decomposition products. The DSC curves of the complexes displayed very sharp exothermic peaks.

Chemically Grown MoO3 Nanorods for Antibacterial Activity Study

In the present investigation, sea urchin like morphology of h-MoO3 nanorods are successfully synthesized by chemical bath deposition (CBD) technique. The thermal stability, structural details, morphology and compositional analysis of MoO3 was done using thermogravimetry (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS) techniques respectively. The thermal analysis reveals presence of sharp exothermic peak at 409oC indicating irreversible phase transition. X-ray diffraction pattern showed hexagonal to orthorhombic phase transition after annealing at 450oC. As synthesized h-MoO3 shows well oriented hexagonal rods with sea urchin like architecture while that of annealed MoO3 sample revealed 2D layer by layer growth. The SAED pattern confirms single crystalline nature of as synthesized h-MoO3 and polycrystalline nature of annealed α-MoO3. While XPS study of both confirms Mo+6 and O2- oxidation states of elements. Furthermore, characteristic antibacterial properties of h-MoO3 and α- MoO3 against gram positive Bacillus megaterium, Streptococcus aureus and gram negative Escherichia coli is noted.

Mg/Cu 超積層体における微細構造の遷移と水素吸蔵メカニズム

The micro/nano-structural transition and hydrogen absorption mechanism in Mg/Cu super-laminate composites (SLCs) were investigated. Differential scanning calorimetry (DSC) measurements were performed on Mg/Cu SLCs at several heating rates and till several repetition cycles up to twenty cycles, and micro/nano-structures of Mg/Cu SLCs were examined with stereomicroscope, digital-microscope, scanning electron microscope (SEM), and scanning transmission electron microscope (STEM). It is found that the micro/nano-structures of Mg/Cu SLCs change drastically at early cycles and reach a steady state after around the tenth cycle. The detailed examination of DSC profiles, and SEM and STEM observations of Mg/Cu SLCs suggest that the hydrogen absorption process consists of a fast and a slow reaction which shows a sharp exothermic peak around at 610 K and a broad exothermic peak around at 570 K connected with it in DSC profiles, respectively.

Synthesis and Characterization of Sodium 5‐Chlorotetrazolate Dihydrate by Chlorination of 1H‐Tetrazole

Abstract A convenient, simple work‐up procedure and low‐cost chlorination method was developed to prepare sodium 5‐chlorotetrazolate dihydrate by chlorination of self‐prepared 1H‐tetrazole with sodium hypochlorite solution. Several organic extracting solvents and factors (raw material ratio, reaction temperature, and reaction time) were investigated to optimize the operating conditions of the chlorination procedure. These products were characterized by 1H and 13C NMR spectroscopy, vibrational spectroscopy (IR), mass spectroscopy (MS), and single‐crystal X‐ray diffraction. In addition, the thermal behaviors of the resulting sodium 5‐chlorotetrazolate dihydrate and sodium tetrazolate monohydrate were investigated and compared by differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). The results indicated that the optimum chlorination conditions were a stoichiometric raw materials ratio of 1:12:12 (1H‐tetralzole : sodium hypochlorite solution : acetic acid solution), a reaction temperature of 55 °C, a reaction time of 6 h, affording a 92.76 % yield; moreover, acetone was determined to be the best extraction solvent in terms of yield and toxicity. Crystal data: sodium tetrazolate, orthorhombic, Pmc2(1), a = 5.847(4), b = 5.605(3), c = 6.387(4) Å, V = 209.3(2) Å3, Z = 2, ρ = 1.746 Mg·m–3; sodium 5‐chlorotetrazolate, orthorhombic, Pnma, a = 6.8611(19), b = 6.9243(19), c = 12.281(4) Å, V = 583.5(3) Å3, Z = 8, ρ = 1.850 Mg·m–3. Sodium tetrazolate has a melting point at 275.97 °C and an exothermic decomposition peak at 319.45 °C, whereas sodium 5‐chlorotetrazolate shows only a sharp exothermic peak at 237.33 °C due to violent decomposition without melting.

Preparation and magnetic properties of multiferroic CuMnO2 nanoparticles.

CuMnO2 nanoparticles with diameters of 64 nm were synthesized by a novel wet chemical method. An optimized two-step annealing method was developed through the analysis of thermogravimetric differential thermal analysis (TG-DTA) measurements in order to obtain single-phase CuMnO2. A sharp exothermic peak was observed in the DTA curve at approximately 500 K where structural changes of the copper oxides and manganese oxides in the precursor are expected to occur. It is believed that Cu+ ions were oxidized to Cu2+ ions and that Mn2+ ions were oxidized to Mn3+ ions in the Cu-Mn-O system. Deoxidization reactions were also found at approximately 1200 K. The optimized annealing temperature for the first step was determined to be 623 K in air. The optimized annealing temperature for the second step was 1173 K in an Ar atmosphere. Magnetization measurements suggested an antiferromagnetic spin ordering at approximately 50 K. It was expected that Mn3+ spin interactions induced magnetic phase transition affected by definite temperature.

Crystallization and magnetic properties of a 10Li2O–9MnO2–16Fe2O3–25CaO–5P2O5–35SiO2 glass

The crystallization behavior and magnetic properties of 10Li2O–9MnO2–16Fe2O3–25CaO–5P2O5–35SiO2 (10LFS) glass have been studied using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to observe the crystallization behavior and a superconducting quantum interference device (SQUID) for measurements of the magnetic properties. The DTA shows that the 10LFS glass has one broad exothermic peak at approximately 674 °C and one sharp (the highest) exothermic peak at 764 °C. When the 10LFS glass crystallized at 850 °C for 4 h, the crystalline phases identified by XRD were lithium silicate (Li2SiO3), β-wollastonite (β-CaSiO3), lithium orthophosphate (Li3PO4), magnetite (FeFe2O4) and triphylite (Li(Mn0.5Fe0.5)PO4). The SEM surface analysis revealed that the β-wollastonite and lithium silicate have a lath morphology. The TEM microstructure examination showed that the largest FeFe2O3 particles have a size of approximately 0.3 μm. When the 10LFS glass was heat treated at 850 °C for 16 h and a magnetic field of 1000 Oe was applied, a very small remnant magnetic induction of 0.01 emu g−1 and a coercive force of 50 Oe were obtained, which revealed an inverse spinel structure.

Spectral and Thermal Degradation of Melamine Cyanurate

Melamine cyanurate, an organic crystalline complex was, synthesized by evaporation of an aqueous solution containing equimolar quantities of melamine and cyanuric acid. The synthesized compound has been subjected to various characterizations like Powder XRD, FT-IR, TG-DTG, SEM, and SHG. The presence of sharp diffraction peaks in the XRD confirms that the products are highly crystalline. The average particle size was calculated using the Debye-Scherrer formula, and it was found to be 3.067 μm. Thermal behavior of the grown crystal has been studied by TG-DTG analysis. From TG-DTG, it is found that the title crystal possesses good thermal stability. The activation energy was calculated using the Broido, Coats-Redfern, and Horowitz-Metzger methods. A sharp peak exothermic peak at 405.40°C was assigned as the melting point of the title material. SEM reveals the morphology of the synthesized salt. No detectable signal was observed during the Kurtz-Perry technique.

MA법에 의한 V-Co계 비정질합금의 제조 및 구조분석

본 연구에서는 출발원료로 <TEX>${\sigma}$</TEX>-VCo 금속간화합물과 <TEX>$V_{50}Co_{50}$</TEX> 혼합분말을 각각 사용하여 기계적 합금화에 따른 비정질화 가능성을 조사하였다. X선 회절에 의해 얻어진 전구조인자 S(Q) 및 동경분포함수 RDF(r)의 결과로부터 볼밀링이 진행됨에 따라 비정질상의 구조적 특징이 분명히 관찰되었다. 120시간 MA 처리에 의하여 두 경우 모두에서 비정질상이 생성됨을 알 수 있었다. 60시간 동안 MA 처리한 <TEX>$V_{50}Co_{50}$</TEX> 분말시료의 열분석 에서는 약 <TEX>$600^{\circ}C$</TEX>에 비정질상의 결정화에 의한 발열 peak가 관찰되었다. X선 회절법에 의해 얻어진 전구조인자 및 동경분포함수의 분석으로부터 MA 시간에 따라 출발 결정상은 비정질상의 특징적인 원자구조로 서서히 변화함을 알 수 있었다. In the present study, we investigated the effect of mechanical alloying (MA) on the formation of amorphous VCo system through solid state reaction during ball milling. Two types of powder samples, <TEX>${\sigma}$</TEX>-VCo intermetallic compound and <TEX>$V_{50}Co_{50}$</TEX> powder mixture, were applied as a starting materials. With increasing milling time, a structural characteristics into the amorphous state is distinctly observed from the structural factor and radial distribution by X-ray diffraction. Amorphization has been observed in all two types of samples after the milling for 120 hrs. DSC spectrum of <TEX>$V_{50}Co_{50}$</TEX> powder sample milled for 60 hrs indicates a sharp exothermic peak from the crystallization at <TEX>$600^{\circ}C$</TEX>. The structure factor, S(Q) and radial distribution function, RDF(r), observed by X-ray diffraction gradually change into a structure characteristic of an amorphous state with increasing MA time.

Effect of Mechanical Milling and Cold Pressing on Co Powder

Cold pressing (CP) of the amorphous-like Co powder suppressed most of the XRD peaks, in particular the peak along (100) plane. The DSC curve of unmilled CP Co powder has shown a distinct sharp exothermic peak at 615C°. Upon annealing at 700C°, only the FCC phase with lattice parameter of 3.51 Å was detected by XRD. Our results implied that the exotherm at 615C° corresponds to compaction-pressure-assisted HCP to FCC first-order phase transition. The XRD analysis of 30 h milled powder revealed for the first time the FCC phase with a=3.80 Å. However, due to presence of (100) and (210) peaks, this phase is thought to be FCT with lattice parameters a=b=3.80 and c=3.07 Å. Consequently, the high-energy milling carried out in the current work induced for the first time HCP to FCT transition in Co. Upon CP of milled powder, the lattice parameter a shrunk from 3.80 to 3.75 Å. However, during annealing of the CP milled Co powder at 750C°, the FCT to FCC transition occurred, yielding the FCC phase with a=3.51 Å.