M. Fontana

Crystallization processes of Ag-Ge-Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge25Se75)100−yAgy with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag8GeSe6 and the secondary phase GeSe2 was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag8GeSe6 phase in all the (Ge25Se75)100−yAgy samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.

Influence of Cu addition in the crystallization of the superionic glass (Ge25Se75)75Ag25

Abstract Amorphous samples of compositions (Ge 25 Se 75 ) 0.75 (Ag 100− x Cu x ) 0.25 with x =10, 20 and 30 at.% were obtained from the liquid state employing the melt-quenching technique. A decrease of the glass forming ability due to the addition of Cu is reported. The quenched samples and the crystallization products have been characterized by X-ray diffraction with CuKα radiation. The primary crystallization of the ternary phase Ag 8 GeSe 6 was observed. The solid solubility of Cu in this phase is supposed. The crystallization kinetics of the samples were studied under continuous heating calorimetric regime by means of differential scanning calorimetry. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The dependence of these properties on the concentration of Cu and the influence of Cu on the Ag 8 GeSe 6 crystallization kinetics is analyzed.

Crystallization kinetics driven by two simultaneous modes of crystal growth

Abstract In this work, we analyse the growth of nuclei precipitating from a supersaturated liquid. Two simultaneous habits of crystal growth are considered: interface-controlled three-dimensional growth with a growth rate u up to a threshold radio R c and then diffusion-controlled growth when steady growth is achieved at the initial stages owing to the large supersaturation in between the nucleus and the liquid neighbourhood. The threshold radio R c determines the growth habit of each grain. For isothermal processes, we discover that the temporal dependence of the crystalline fraction follows scaling laws. Two parameters summarize the master curves: firstly, a dimensionless magnitude named P, where P = (π/3)ID4 eff/u 5, where D eff is the effective diffusion coefficient and I the nucleation frequency; secondly, the time t c needed by the first created embryo to reach the threshold radio R c. At t c the growth switches from one mechanism to the other. These two quantities are also scaling factors for the temporal evolution of the density of grains and the mean grain size. We discuss the applicability of the model to real systems.

Quenched GaTeFe alloys near the Ga20Te80 composition

Rapidly quenched samples of Ga–Te and Ga–Te–Fe were obtained from the liquid, with a piston and anvil device, in a composition near the Ga6Te94 eutectic. X-ray diffraction with Cu (Kα) radiation and Mossbauer spectroscopy at room temperature with a 57Co (Rh) source were used to determine the structure of the samples. Either amorphous or partially amorphous structures were observed in the binary samples while partially amorphous structures with FeTe2 precipitates were observed in the ternary samples. The glass forming tendency is analyzed by comparing the amount of amorphous phase and crystalline phases present in samples with and without iron.

Crystallization kinetics and structural aspects of TeGaSn amorphous alloys

Cast and rapidly quenched samples of ternary system GaTeSn in the Te rich corner were studied by means of x-ray diffraction, 119Sn Mossbauer spectrometry at 77 K, and differential scanning calorimetry. The crystallization products, the activation energy, the crystallization enthalpy, and the crystallization and glass temperatures of the amorphous phases are reported. The addition of Sn to GaTe increases the glass forming range of the binary system and the thermal stability of the amorphous phase. In the crystalline samples the divalent Sn atoms are always surrounded by Te in octahedrally coordinated environments. It is observed that the addition of Sn, above a threshold concentration, stabilizes the high temperature phase Ga2Te5 at room temperature. The Mossbauer spectra obtained on amorphous samples indicate two kinds of environments at Sn sites: one ascribed to the octahedral coordination, the other one to Sn surrounded by Te with tetrahedral coordination. The Mossbauer results are related to the calori...

Crystallization process on amorphous Mg-Ga-Sn system

The crystallization processes on Mg81.09Ga18.91 and Mg80.50Ga17.76Sn1.67 amorphous samples were studied by means of X-ray diffraction and electrical resistivity isothermal measurements. A small amount of Sn added to the binary eutectic was found to modify the crystallization products and the evolution time. Crystallization activation energies were estimated.

Non-equilibrium and crystalline phases on the Mg-Ga-Sn system

Samples in the eutectic composition were obtained with the piston and anvil technique and the crystallization towards a metastable phase was observed upon annealing at room temperature. Small amounts of Sn were added to the binary MgGa in order to analyze its influence on the glass forming ability. Cast and rapid quenched ternary samples were studied employing X-ray diffraction, Mössbauer spectroscopy and electron probe microanalysis. The solubility of the third element was analyzed in the equilibrium phases Mg5Ga2, Mg2Ga and Mg2Sn. As far as we know, these are the first contributions to the knowledge of the MgGaSn system.

Characterization of Ag-Ge-Se bulk glasses by means of Mössbauer effect on 57Fe and 119Sn atomic nuclei

In this work, the structure of Fe and Sn doped Agx(Ge0.25Se0.75)100−x (x=0 to 25 at.%) intrinsically inhomogeneous glasses is analyzed employing 119mSn and 57Fe Mossbauer spectroscopy, X-ray diffractometry and scanning electron microscopy. 119mSn enters in the glass as a substitutional impurity for Ge whereas 57Fe enters as an interstitial impurity. Mossbauer spectra obtained with 119mSnO3Ca source, from samples containing about 1% 119Sn for Ge, reveal that the local order of Ge in both amorphous phases is basically the same whereas Mossbauer spectra obtained with 57Fe(Rh) source, from samples containing about 0.5% 57Fe, evidence the differences between both phases.

Structural Studies, Thermal Stability and Ionic Conductivity in the AgGeSe System

The solid state electrolytes used in microbatteries constitute an active technological investigation area. An interesting alternative is offered by the chalcogenide glasses which present fast ionic conduction [>10(Ωcm)]. There are three major features of these materials that must be characterized: the ionic change transport, and the thermal and chemical stability. The understanding and optimization of all these aspects require an appropriate knowledge of the composition-structureproperties relationship. In this work, bulk glasses of composition (Ge0.25 Se0.75)100-XAgX (10 at. ≤ x ≤ 25 at.) were studied. The glass forming ability, the thermal stability and crystallization kinetics were characterized by differential scanning calorimetry (DSC). Short and intermediate range order as a function of composition were obtained from the radial distribution function by X-ray diffraction (Mo(Kα) radiation). In addition, complementary structural studies were carried out using a Fe Mössbauer probe. Electrical properties were investigated by impedance spectroscopy in the frequencies range 5Hz 2MHz at different temperatures from room temperature to 363 K. A power law of the dependence of conductivity with silver content provides very good fits of experimental data, predicting a power law dependence of dc conductivity or a logarithmic dependence of activation energy on the modifier content.

Atomic and electronic structure of SnxPb(100−x)Te

Abstract Polycrystalline samples of Sn x Pb (100− x ) Te investigated at compositions x = 10, 25, 40, 62 and 80 atomic %. Their X-ray patterns were obtained, estimating the lattice constant in each case. The samples have also been studied by means of Mossbauer spectroscopy with a SnO 3 Ba source. The isomer shift measurements have been correlated with the free volume per atom.