Allotropic Transition Research Articles

Identification and characterization of promising phase change materials for solar cooling applications

Solar cooling technology is an attractive way to use solar thermal energy to produce cooling for buildings. The employment of taphase change materials (PCMs) as heat storage medium, to increase the range of utilization of solar thermal energy, thus improving the overall system performance, is considered very attractive. Nevertheless, in order to allow the development of latent heat storage prototypes for such an application, it is mandatory to verify the thermo-physical performance as well as the long-term stability of the available materials. To this aim, in the present paper, the most attractive commercial PCM as well as neat chemical compounds operating in the temperature range between 80°C and 100°C, perfectly suitable for non-concentrating solar cooling systems, have been identified and completely characterized. In particular, several cycles have been performed on each material, to verify possible instabilities in their behavior. Most of the neat materials have confirmed to be promising for this application, thanks to their really high melting enthalpy, up to 255J/g as showed for Aluminum Ammonium Sulfate Dodecahydrate. Nevertheless, all these materials are still not stable, showing high supercooling, allotropic phase transition, incongruent melting and even absence of re-crystallization, which makes necessary an intense work to bring them to a reliability level sufficient for real application. On the contrary, the commercial PCMs, even if mostly characterized by lower melting enthalpy, ranging between 120 and 150J/g, confirmed their stability, which makes them ready for practical applications.

An evolved gas analysis method for the characterization of sulfur vapor

In this study, thermal analysis coupled with soft ionization mass spectrometry was used for the speciation of sulfur vapor. Its investigation is still a challenge, since it exhibits a complex molecular composition. A thermal analysis–mass spectrometry system was equipped with a single-photon ionization time-of-flight mass spectrometer. Single-photon ionization is a soft ionization method which produces less or no fragments. Ionization is performed with vacuum ultraviolet photons generated by an electron beam-pumped excimer lamp and a deuterium lamp. The applicability of the developed system was demonstrated for the determination of sulfur vapor composition. Several allotropic modifications were detected in the gas phase depending on temperature. Since the ionization energies of the species are between 8.6 and 9.7 eV, soft ionization is crucial for this reason. Thermogravimetric and DSC data deliver additional information to the mass spectrometric data and enable the detection of allotropic transitions in solid and liquid phase.

Correlation Between Magnetic Properties and Allotropic Phase Transition of Fe–Co–V Alloy

In this study, the allotropic phase transition and its effect on the magnetic behavior of FeCo–7 wt%V alloy were investigated. It was found that γ phase is observed in the microstructure in the as-cast condition, and it diminishes after severe cold rolling (90% reduction). After annealing at temperatures higher than 500 up to 750 °C, the γ phase is observed in the structure, again. But, this phase is disappeared by annealing at temperatures above 750 °C due to the formation of vanadium-rich precipitates. Thermocalc software was used in order to elucidate the influence of vanadium percent on the stability of γ phase in Fe–Co alloys. Also, magnetic studies showed that the saturation induction is reduced by annealing at temperatures from 500 up to 750 °C, which is related to the formation of residual non-magnetic γ phase.

Bulk metastable cobalt in fcc crystal structure

Bulk metastable pure cobalt was fabricated by a simple sintering route with micrometer-sized Co particles as raw materials. The as-prepared bulk Co is composed of dominant metastable face-centered cubic (fcc) phase and a trace of hexagonal close-packed (hcp) phase at room temperature. The crystal structures and magnetic properties of the metastable bulk are very sensitive to external stresses. Obvious fcc-to-hcp phase transformation occurred under tensile loading, which was accompanied by significant increase of the coercivity and remanence. It is believed that the interface energy plays a key role in hindering the fcc-to-hcp allotropic transition, leading to the existence of metastable bulk Co with the high-temperature fcc structure at room temperature.

Reduction of the allotropic transition temperature in nanocrystalline zirconium: Predicted by modified equation of state (MEOS) method and molecular dynamics simulation

The temperature at which polymorphic phase transformation occurs in nanocrystalline (NC) materials is different from that of coarse-grained specimens. This anomaly has been related to the role of grain boundary component in these materials and can be predicted by a dilated crystal model. In this study, based on this model, a modified equation of state (MEOS) method (instead of equation of state, EOS, method) is used to calculate the total Gibbs free energy of each phase (β-Zr or α-Zr) in NC Zr. Thereupon, the change in the total Gibbs free energy for β-Zr to α-Zr phase transformation (ΔGβ→α) via the grain size is calculated by this method. Similar to polymorphic transformation in other NC materials (Fe, Nb, Co, TiO2, Al2O3 and ZnS), it is found that the estimated transformation temperature in NC Zr (β→α) is reduced with decreasing grain size. Finally, a molecular dynamics (MD) simulation is employed to confirm the theoretical results.

Nature of high hardness in the Co70Mo30 alloy

The Co70Mo30 alloy structure formed at different temperatures was studied by means of transmission electron microscopy and photoelectron spectroscopy. It has been found that at temperatures below the solidus temperature the alloy has a tendency toward phase separation which leads to formation of a quasi-cellular microstructure in the form of clusters strongly enriched in Mo and Co, respectively, but existing within a common fcc lattice of the solvent. At low temperatures, an allotropic phase transition α-Co → ε-Co occurs in the alloy and the strain contrast, arising at such transitions is superimposed on the quasi-cellular structure. The Co70Mo30 alloy acquires high hardness due to the fact that the crystal lattice in the clusters strongly enriched in Mo remains face-centered.

The metastability and martensitic transformation of Co particles

Abstract Co particles with different sizes and different crystal structures were prepared by annealing of loose particles as well as sintering of compact Co particles in vacuum. The allotropic transition characteristics of Co particles were investigated to determine the effect of particle size and surface/interface energy on the metastability of Co particles. It was found that there existed a critical size below which the high-temperature phase (fcc) could be achieved at room temperature. The dimension of the critical size of particles was dependent on the surface/interface energy, i.e., lower surface/interface energy gave rise to smaller critical size. In the view point of the suppression of martensitic transformation by extra pressure resulting from the surface/interface energy, the metastability of Co particles was discussed.

Nanomechanical responses of intermetallic phase at the solder joint interface – Crystal orientation and metallurgical effects

Abstract In this study, the relationships between crystal structures, metallurgical effects, and mechanical properties of the most common intermetallic compound formed at the interface of solder joints, Cu6Sn5, were investigated using nanoindentation. Experimental results show that the ( 11 2 ¯ 0 ) oriented hexagonal Cu6Sn5 exhibited anisotropic mechanical behavior compared to those with random growth directions. The closest atomic packing density of the ( 11 2 ¯ 0 ) plane in hexagonal Cu6Sn5 resulted in higher hardness and notably, greater stiffness. Subjected to long time aging at 150 °C, hexagonal Cu6Sn5 was transformed into the equilibrium monoclinic structure, resulting in a reduced modulus and thus inferior ability for plasticity. Alloying of Ni, Mn and rare earth elements (La and Ce) had various contributions to the allotropic transition and thus nanoindentation responses. It was found that the differences in atomic radius between the solute elements and Cu affected the kinetics of the allotropic transformation and also the mechanical performance of Cu6Sn5. There exists a critical value for the modulus/hardness ratio (E/H) of about 17.3–17.5, below which the indent morphology showed a brittle characteristic.

Further Observations on Tin Pest Formation in Solder Alloys

The most recent observations of the response of bulk samples of several commercial solder alloys, exposed to temperatures below the allotropic transition for tin (13C) for extended periods, are reported. Damage associated with tin pest development has been arbitrarily graded into six levels, and the formation of visible a-phase warts used for comparative purposes. Since the previous examination, some 2 years ago, tin pest has been observed for the first time in the traditional Sn-37Pb solder alloy after exposure at 18C and 40C, and actual warts were apparent in as-cast Sn-0.5Cu stored at 40C and in as-cast Sn-3.5Ag after exposure at 18C. No tin pest was detected in Sn-Zn-3Bi after exposure for periods up to 6 years. Tin pest continued to develop in those lead-free alloys in which it had previously been observed, indicating the probability that all would eventually disintegrate in time. In general, prior thermal or mechanical treatment accentuated tin pest formation. The influence of exposure temperature was unclear, since some alloys (Sn-0.5Cu and Sn-3.8Ag-0.7Cu) experienced more damage at 18C, but others (Sn-37Pb and Sn-3.5Ag) were more susceptible at 40C. From a consideration of the findings and other published information, it is contended that impurity levels below 0.1 mass% (1000 ppm) play a vital role in determining whether tin pest develops in realistic timescales. A major factor in the absence of tin pest, to date, on actual joints may be simply the mismatch between the timescales experienced in service and those in long-duration laboratory tests.

Size Dependent Allotropic Transition of Co Fine Particles

In order to investigate the size dependent allotropic transition of Co in wide size range, nanometer scaled (9 nm-350 nm) and micrometer sized (1 microm-18 microm) Co particles were prepared by laser induced pyrolysis, reduction as well as high temperature annealing treatment. In wide size range, from 9 nm to 10 microm for example, the crystal structure of the Co particles is dependent on their thermal history in which the particle involved in preparation or in annealing treatment. Only when the particle size is very large, much greater than 10 microm for example, the fcc-hcp transformation could occur during the cooling from high temperautre. According to the effect of surface energy on the martensitic transformation, the size dependent allotropic transformation of Co was discussed.

Kinetics and microstructural aspects of the allotropic transition in tin

The allotropic phase transition in tin has attracted rather heterogeneous interests for the practical consequences it has in wide and different ranges of applications, spanning from the degradation of organ pipes to the failure of microelectronic soldering alloys. One of the main aspects that any experimental work on this issue had to face is the extremely low kinetics of the transition from the high temperature body centered tetragonal polymorph, β-phase, to the low temperature diamond cubic, α-phase. In an attempt to speed up the transition and get further insight into its kinetic aspects, we used an approach based on high-energy ball-milling. Indeed, the heavy plastic deformation and the associated high density of microstructural defects are expected to be effective in reducing most of the kinetic constraints to the transition. Moreover, in view of the claimed heterogeneous character of the nucleation of the transition, we investigated this aspect by preparing ball-milled powder mixtures in which 1 wt.% of InSb was added to tin, together with 2 wt.% graphite as process control agent, in order to achieve a fine dispersion of this nucleant (InSb) substance. Cooling and heating experiments, conducted on the ball-milled powder mixtures: Sn–2C and Sn–1InSb–2C, demonstrated that, under the adopted experimental conditions, heterogeneous nucleation in the presence of InSb was the only mechanism capable to induce the β → α phase transition. The transition could be followed by differential scanning calorimetry, although the limited scanning times were not sufficient for a full accomplishment of the transition. The results obtained so far indicate that a further speed up of the transition kinetics may come from a more prolonged milling, capable to drive the average crystallite size below the value achieved in the present investigation.

Recent Observations on Tin Pest Formation in Solder Alloys

The most recent observations of the response of bulk samples of several lead-free solder alloys, exposed to temperatures below the allotropic transition for tin for extended periods, are reported. Tin pest has been observed in Sn-0.5Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-3.0Ag-0.5Cu alloys at both −18°C and −40°C. The process is slow and inconsistent, usually requiring several years, but may eventually result in complete disintegration of the sample. No tin pest was detected in Sn-Zn-3Bi or in the traditional Sn-37Pb solder alloy after exposure for up to 4 and 10 years, respectively. It is suggested that nucleation is affected by local composition and that extremely small amounts of either intentional solute or impurity are influential. Growth of tin pest is accompanied by a large volume change, and it is likely that stress relaxation ahead of the expanding grey tin front is a controlling factor. A stronger matrix would be more resistant in this case, and at the temperatures of exposure Sn-37Pb is stronger than either Sn-3.5Ag or Sn-0.5Cu. The absence of tin pest, to date, on actual joints is attributed to their restricted free surface area and the greater strength associated with very small samples.

Kinetics of allotropic phase transformation in cobalt powders undergoing mechanical processing

The allotropic transition of hexagonal close-packed cobalt powders to the face-centered cubic phase under mechanical treatment conditions has been studied. Processing parameters have been systematically changed to investigate the phase transformation kinetics, which has been related to the number and the average energy of collisions. A phenomenological interpretation of transformation rate constants is also proposed. A minimum threshold energy at impact exists below which no phase transformation is detected.

Microstructural study of titanium carbonitride produced by combustion synthesis

The self-propagating high-temperature synthesis (S.H.S.) process, which is promising for the fabrication of ceramic materials, was chosen to elaborate titanium carbonitride materials. The influence of parameters such as nitrogen gas pressure and carbon ratio on the microstructure was studied. A single phase product of Ti(C,N) is obtained for a carbon ratio under 15 at.% and a nitrogen pressure of 36 MPa. The increase of the carbon ratio corresponds to a decrease of the maximum temperature reached during the synthesis. Time resolved X-ray diffraction measurements (TRXRD) with the synchrotron radiation were used to determine the reaction mechanisms. We could observe that the synthesis of Ti(C,N) is preceded by the formation of titanium nitride. This reaction is initiated by the allotropic transition of α-Ti phase into β-Ti. In the final material the presence of sub-stoichiometric phases such as Ti 2N and α-Ti was observed.

Tin pest issues in lead-free electronic solders

Tin pest is the product of the β → α allotropic transition at 13.2°C in pure tin. It is a brittle crumbly material, often responsible for the total disintegration of the sample. The transformation involves nucleation and growth, with an incubation period requiring months or years for completion. Experimental observations reveal a substantial inconsistency and an incomplete understanding of the process. Some alloy additions promote tin pest by reducing the incubation time, whereas others retard or inhibit its formation. Traditional solder alloys have generally been immune to tin pest in service due to the presence of lead, and bismuth and antimony as common impurities. However, the new generation of lead-free solders are more dilute—closely resembling tin. A much debated question is the susceptibility of these alloys to tin pest. Bulk samples of tin-0.5 copper solder undergo the transition at −18°C although not at −40°C after five years exposure. Other lead-free alloys (Sn–3.5Ag, Sn–3.8Cu–0.7Cu and Sn–Zn–Bi) are immune from tin pest after a similar period. Large scale model joints exhibit tin pest but it appears that actual joints may be resistant due to the limited free solder surface available and the constraint of intermetallic compounds and components. It seems likely that impurities are essential protection against tin pest, but for long term applications there is no certainty that tin pest and joint deterioration will never occur.

Mechanisms of the Allotropic Transition of Sn

The mechanisms of α-Sn nucleation on foreign seeds, including metastable ones, are considered. Based on earlier and new experimental data for induction times of α-Sn nucleation from 102 to 1011 s, it is suggested that spontaneous α-Sn nucleation is impossible because of kinetic hindrances in the solid phase.

Synergistic effects in selective oxidation catalysis: does phase cooperation result in site isolation?

Considerations about the possible relations between phase cooperation inducing synergistic effects and site isolation providing for selectivity in mild oxidation of hydrocarbons are proposed. Synergistic effects between several phases arise because of the coherence of their interfaces due to a low mismatch between their structural patterns. As outlined in the paper, the enhanced reactivity is encountered not only in catalytic reactions but also in non-catalytic transformations like allotropic transitions, sublimation, etc. The situations providing for site isolation are given: defective structures, doping and supporting effects on bulk and monolayers, encapsulation of active sites in zeolites are reviewed.

Face-centered-cubic to Hexagonal-close-packed Transformation in Nanocrystalline Ni(Si) by Mechanical Alloying

An allotropic transition from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) Ni(Si) solid solution in Ni95Si5 and Ni90Si10 during nanocrystallization by mechanical alloying is reported. The transformation was identified as a defect-induced melting accompanied by a volume expansion of 8.6% and was observed when fcc Ni(Si) reached a critical crystallite size of 10 nm. Calculation based on equation of state showed that a 37% reduction in tetragonal shear modulus and a negative pressure of about 8.7 GPa were generated at the onset of transformation.

A study of the allotropic transition of hemoglobin in a magnetic field

1. The increase in the capacity of blood caused by allotropic transition of hemoglobin is greater if venous blood is magnetized.

The Hf–Au system

The Hf–Au system has been reinvestigated by means of differential thermal analysis, X-ray diffraction and electron probe microanalysis. The existence of the seven intermetallic phases, previously reported, Hf2Au, HfAu, Hf7Au10, HfAu2, HfAu3, HfAu4 and HfAu5 was confirmed. We specify here their type of formation. For the equiatomic compound, we have shown the presence of a martensitic transformation and of allotropic transitions.