Metallurgically grown Schottky junction in directionally solidified eutectic Ag–Si alloys

Metallurgically grown M(metal)–S(semiconductor) phase boundaries in directionally solidified Ag Si alloys were investigated relative to their behaviour as Schottky junctions. By the aid of point contacts I/V characteristics have been measured which correspond to these ones of Schottky diodes. The Si crystallites were proved to be n-doped. Reverse breakdown voltages of 8 … 24 V are not incompatible with the purity of the used materials. In forward direction the value of diode factor n = 1.1 … 5 shows the existence of several current flow mechanisms. In order to be able to interpret the characteristics it is necessary to have exact information on the specific resistivity of the Si crystallites and on the content of grain boundaries and dislocations in silicon. Beyond that the complicated shape of Si crystallites prevents the necessary determination of the interface size.     

Porous crystal structures obtained from directionally solidified eutectic precursors

Interconnecting cage-like porous structures of several halide compounds were prepared by the selective leaching of one eutectic phase method. The binary eutectic precursors were prepared by directional solidification using the Bridgman crystal growth technique. Porous NaMgF₃ (40% pore volume), CaF₂ (57% pore volume) and BaF₂ (43% pore volume) crystals were obtained after water leaching the NaF component of the directionally solidified NaF/NaMgF₃, NaF/CaF₂ and NaF/BaF₂ eutectics with the appropriate entangled microstructure. The growth conditions for eutectic-coupled growth and the morphology of the eutectics have been determined. In the coupled growth regime, the size of the eutectic phases “λ” is fairly uniform and varies with the eutectic growth rate “v” as λ²v=constant, which allows us to control the pore size within the 0.5–10 μm range. The simplicity and versatility of the eutectic growth also allows us to fabricate highly aligned porous structures at relatively high production rates.     

The preferential orientation of the directionally solidified Si–TaSi2 eutectic in situ composite

The Si–TaSi₂ eutectic in situ composite is a favorable field emission material due to relatively low work function, good electron conductivity, and three-dimensional array of Schottky junctions grown in the composite spontaneously. The preferential orientation during directional solidification is determined by the growth anisotropy. In order to obtain the preferential direction of the steady-state crystal growth, the transmission electron microscopy (TEM) is used for analysis. It is found that the preferential orientation of the Si-TaSi₂ eutectic in situ composite prepared by Czochralski (CZ) technique is [3 3¯ 2¯] Si∥[0 0 0 1] TaSi₂, (2 2 0)Si∥(2 2¯ 0 0) TaSi₂. Whereas the preferential orientation of the Si–TaSi₂ eutectic in situ composite prepared by electron beam floating zone melting (EBFZM) technique is [0 1¯ 1¯ ]] Si∥[0 0 0 1] TaSi₂,(0 1¯ 1) Si∥(0 1¯ 1 1)TaSi₂. The preferential directions of the Si-TaSi₂ eutectic in situ composites prepared by two kinds of crystal growth techniques are distinctly different from each other, which results from different solid–liquid interface temperatures on account of the different crystal growth conditions, e.g. different solidification rate, different temperature gradient, different solid–liquid interface curvature and different kinetic undercooling.     

Microstructure selection map for rapidly solidified Al-rich Al–Ce alloys

The effect of rapid solidification on the microstructure of Al–8 Ce, Al–20 Ce and Al–36.6 Ce (wt%) alloys has been investigated using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. A microstructure selection map has been established for the rapidly solidified Al-rich Al–Ce alloys. Rapid solidification has no effect on the phase constitution but has a marked effect on the microstructure of the Al–8 Ce and Al–20 Ce alloys. For the Al–36.6 Ce alloy, however, rapid solidification has a significant effect on both the microstructure and the phase constitution.     

Interfacial stability and grain diameter of the metal phase of directionally solidified Al-Si-type eutectics

The morphology of the solid-liquid interface and the metal phase grain diameter of Al—Si-type eutectics (Al—Si, Ag—Si, Ag—Ge, Zn—Ge) were investigated in the range of a growth rate R = 0.2 … 20 mm/h and of a temperature gradient at the solid-liquid interface G = 2 … 25 K/mm. Three types of interfacial morphologies depending on the G/R ratios were found out. The G/R ratio of the transition from a planar to a nonplanar solid-liquid interface corresponds to the critical G/R|_c ratio, which can be calculated by the criterion of the constitutional undercooling. The grain diameter of the metal phase depends on growth parameters as follows: d_K ∼ R^—rG^—g with r = 0.33 … 0.43 and g = −0.37 … 0.03.     

Solidification behaviour of binary organic eutectic alloys

The phase diagrams of binary organic faceted-faceted systems of p-dibromobenzene with acenaphthene, p-chloronitrobenzene and acetanilide show the formation of a simple eutectic with 0.53, 0.61, and 0.64 mole fraction of p-dibromobenzene, respectively. Crystallization data, determined by the capillary method, suggest that the eutectics of p-dibromobenzene with acenaphthene and p-chloronitrobenzene solidify with alternate nucleation mechanism and the crystallization of the eutectic of p-dibromobenzene-acetanilide system takes place by the side-by-side growth of the phases involved. With a view to throw light on the thermodynamic behaviour of eutectic alloys of non-metals, heats of fusion of their transparent organic analogues and their eutectics were determined, and using these values different thermodynamic parameters were calculated. The microstructures of the eutectics show their typical characteristic features.     

Formation of atomic solution in liquid eutectic alloys

The influence of Pb, In, Bi and Ge on structure of tin within 0–12.5 at.% concentration range has been studied by means of X-ray diffraction method. Samples were prepared by melting in vacuum furnace filled with pure argon from ingots of Sn(99.999%), Pb(99.99), In(99.999), Bi(99.99), Ge(99.999) and checked by means of X-ray microanalysis The structure factors and pair correlation functions were analyzed. Concentration dependencies of structure parameters for liquid Sn-enriched melts in binaries with eutectic point are described. It is shown that atomic arrangement in tin-based molten alloy deviates from random atomic distribution within concentration range where these alloys can be considered as diluted solutions. Limited solubility in solid state in eutectic Sn-based systems is a precursor to deviation from random atomic distribution in liquid state.     

Generation of etch pits on sectional planes of unidirectionally solidified cd-zn eutectic samples

A series of experiments were carried out for generating defined etch pits on various sectional planes of unidirectionally solidified Cd-Zn eutectic samples. This was done as preliminaries of real structure investigations. For that purpose chemical, electrolytic, and ion beam procedures were tested.     

On the mechanism of lamellar spacing adjustment in eutectic alloys

Well controlled undirectional solidification experiments with continous increased rate were performed for eutectic alloys in the systems Ag-Cu, Al-Zn, Al-Cu and Al-Ni. In the Ag-Cu and Al-Zn alloys it was observed that discontinous changes in the lamellar spacing occured at low solidification rates. At high solidification rates such discontinuities could not be observed. Anyway the lamellar spacing always larger than observed at constant rate experiments. In Al-Cu alloys discontinous changes in the lamellar spacing with increasing solidification rate were also observed. However, after each change the lamellar spacing increased gradually up to a lamellar distance of 20% lower than before the abrupt decrease. All-Ni eutectic alloys gives a rod like eutectic structure. In this alloy no discontinous changes could be observed. A theoretical analyses of the observation based on well known theoretical models are discussed.     

Density studies of liquid alloys SnAg and SnZn with near eutectic compositions

The density values for SnAg and SnZn alloys with near eutectic compositions were determined by the gamma-absorption method both in the liquid and solid states. Sn-rich melts of the AgSn system were found to be microheterogeneous, whereas SnZn melts demonstrate the behavior typical for true solutions.     

Solidification behaviour of eutectic and monotectic alloys: Succinonitrile - acenaphthene system

The phase diagram of succinonitrile-acenaphthene system, determined by the thaw-melt method, shows the formation of a eutectic at 0.99 mole fraction of succinonitrile and a monotectic at 0.35 mole fraction of succinonitrile with a large miscibility gap. The linear velocity of crystallization (v) of the pure components, eutectic and monotectic, studied at different undercooling (ΔT) by measuring the rate of movement of the growth front in a capillary, suggests that crystallization data obey the equation: v = u(ΔT)ⁿ where u and n are constants depending on the nature of solidification of the compounds. While the microstructural investigation of eutectic shows broken lamellar and eutectic dendrite-type morphologies that of monotectic exhibits fibrous and broken fibrous-type microstructures depending on the rate of solidification.     

Metastable microheterogeneity of melts in eutectic and monotectic systems and its influence on the properties of the solidified alloy

Experimental results that are the basis for the existence of metastable microheterogeneities in melts of eutectic and monotectic systems are summarized. Among these, recent results of small angle neutron scattering experiments are discussed in some detail. It is shown that the influence of a homogenizing heat treatment of a melt on the structure of crystalline castings and amorphous ribbons obtained after its solidification is very important.     

Structural properties of liquid Au–Si and Au–Ge alloys with deep eutectic region

Au–Si and Au–Ge alloys have a deep eutectic region around 19 at.% Si and 28 at.% Ge, respectively. The metallic glass was prepared by quenching around 25 at.% Si near the eutectic composition for the Au–Si alloys [2] which has stimulated many researchers to study on the structural properties of such liquid alloys. However their results were different from each other, which seems due to the experimental difficulties from heavy absorption of both X-ray and neutron beams by Au atom in diffraction experiments. X-ray diffraction measurements have been carried out for liquid Au–Si and Au–Ge alloys around the eutectic region by the transmission method using high-energy X-rays to investigate the atomic arrangements in the liquid state. From the temperature dependence of the observed structure factors, the partial pair correlation and the detailed atomic arrangements will be discussed by Reverse Monte Carlo analysis. The produced atomic arrangements around the eutectic region suggest a substitutional structure and also an increase in liquid density with decreasing temperature.     

The effect of manganese additions on the structure changes proceeding in rapidly solidified Al-Cu alloys

The effect of Mn additions (0… 2 wt%) on the decomposition of rapidly solidified Al-4.0 wt% Cu alloys (cooling rate 10³ to 10⁴ K/s; LQ treatment) were studied during ageing between RT and 450 °C by hardness, X-ray methods and electron microscopy. The results were compared with alloys homogenized in the region of the solid solution (SQ treatment). (i) The LQ treatment results in a quite better homogeneous distribution of the alloyed elements than the SQ one, that is less particles of intermetallic phases are present in the ascast state. (ii) At T < 250 °C Mn additions affect the decomposition kinetics by trapping of vacancies (retardation) and the diminution of the solubility of Cu atoms (acceleration). The first effect dominates in the stage of G.P. zone formation, the second one during precipitation of intermediate phases. (iii) At T ≧ 300 °C the intermetallic compound Cu₂Mn₃Al₂₀ forms associated with a significant increase of the hardness.     

Influence of various conditions of convective mixing of melt on the structure and magnetoresistance of eutectic InSb-NiSb alloys obtained by directional crystallization

The microstructures are analyzed of the eutectic alloys solidified in the quasi-binary InSb-NiSb system under different conditions of convective mixing of melts grown in an ampoule by the Czochralski and directional crystallization methods under earth and microgravity conditions. It is established that growth of the phases in directional solidification of the InSb-NiSb eutectics is determined not only by the diffusion mechanism of the component redistribution at the crystallization front but also by the diffusion-convection mechanism. The comparative analysis of the influence of the crystal microstructure on the value of the magnetoresistance, R _M , is performed for the first time. It is shown that the R _M value of directionally crystallized InSb-NiSb eutectics depends on the density of the NiSb needles, disorder of the eutectic structure, and electrophysical parameters of the matrix.     

Modification of eutectic morphology

In recent years, considerable interest had developed in the potential use of the eutectic reaction to produce “in situ composites.” For this purpose, close control of the microstructure produced by eutectic freezing is required. The most widely known example of structure control in a eutectic is the “modification” of the morphology of the Al-Si eutectic by the addition of a small quantity of Na or by a suitable change in freezing conditions. Many other cases of structure modification have been observed, notably fibrous-lamellar transitions. The factors governing such modification of eutectic morphology will be reviewed and the prospects for improved structure control in eutectic systems will be discussed.     

Physical chemistry of binary organic eutectic and monotectic alloys; 1,2,4,5-tetrachlorobenzene−β-naphthol and 1,2,4,5-tetramethylbenzene-succinonitrile systems

Phase diagrams of 1,2,4,5-tetrachlorobenzene–β-naphthol and 1,2,4,5-tetramethylbenzene–succinonitrile systems which are organic analogues of a nonmetal–nonmetal and a nonmetal–metal system, respectively, show the formation of a simple eutectic (melting point 103.7°C) with 0.71 mole fraction of β-naphthol in the former case and a monotectic (melting point 76.0°C) with 0.07 mole fraction of succinonitrile and a eutectic (melting point 52.5°C) with 0.97 mole fraction of succinonitrile in the latter case. The growth behaviour of the pure components, the eutectics and the monotectic studied by measuring the rate of movement of the solid–liquid interface in a capillary, suggests that the data obey the Hillig–Turnbull equation, v=u(ΔT)ⁿ, where v is the growth velocity, ΔT is the undercooling and u and n are constants depending on the nature of the materials involved. From the values of enthalpy of fusion determined by the DSC method using Mettler DSC-4000 system, entropy of fusion, interfacial energy, enthalpy of mixing and excess thermodynamic functions were calculated. The optical microphotographs of pure components and polyphase materials show their characteristic features.     

Structural characterization of anthracene-picric acid eutectic system

The eutectic composition has been derived from phasediagram studies and the nature of eutectics has been established by thermal and spectral studies. Diffraction studies reveal the establishment of preferential ordering and the crystalline character of eutectics.