Chemistry of the Group VB Pentoxides. VI. The Polymorphism of Nb<sub>2</sub>O<sub>5</sub>

Holtzberg, F.; Reisman, Arnold; Berry, Margaret; Berkenblit, M.

doi:10.1021/ja01566a004

Cited by 158 publications

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“…The socalled high temperature, monoclinic form [9] was the only stable modification e ncountered in the present work. This finding agrees with that reported in studies of other binary phase diagrams [9,10]. Nonequilibrium.…”

Section: Resultssupporting

confidence: 93%

Phase equilibria in the system niobium pentoxide-germanium dioxide

Levin¹

1966

J. RES. NATL. BUR. STAN. SECT. A.

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T he phase equilibrium diagram for the system Nb,Os-GeO, has been dete rmined experimentally, using the quenc hing tec hnique and examining the samples by optical microscopy and x·ray powder diffractometry. The system contains one compound, 9Nb,Os' GeO" wh ic h melts Inco ngruently at 1420 0c. A eutectic be tween this compou nd and GeO, is located at about 97 mol pe rcent GeO, and 1090 °C . The system doe s not show liquid immiscibility, and it is concluded that the ionic fi eld strength limit for two·liquid separation in the series of glass form ers occurs with the Nb+ s cation .Key Word s: Germanium dioxid e, immiscibility, niobium pe ntoxid e, phase equilibri a.

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Section: Resultssupporting

confidence: 93%

Phase equilibria in the system niobium pentoxide-germanium dioxide

Levin¹

1966

J. RES. NATL. BUR. STAN. SECT. A.

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“…Holzberg [10] has agreed that the b value of 3.825 A is probably more correct. Table 1 shows the indexed X -ray pattern of high-temperature Nb20 5, as found in the present study, compared with the indexed values shown by Holzberg et al [8] and Magneli and Lagergren [9]. The parameters found for the material used in the present study are a = 21.08 A, b= 3.823 A, c= 19 .33 A, and {J = 119° 48'.…”

supporting

confidence: 65%

“…The H-form or high-temperature form was reported as stable at temperatures above 1,200 0 to 1,250 0 C. In this study wherever free Nb20 5 was found, the high-temperature form was the one observed in all heated specimens, although the low-temperature form was used as the starting material. Holzberg, Reisman, Berry, and Berkenblit [8] concluded that Nbz0 5 has only two crystalline polymorphs, the high-and the low-temperature forms , with an irreversible phase transformation , 'Y to a (low to high) at 830 0 C. They gave the parameters I of the high-temperature form as, monoclinic, a = 21.34 A, b= 3.816 A, c= 19.47 A, and {J = 120 0 20' . Magneli and Lagergren [9] have used the parameters a= 21.50 A, b= 3.825 A, c= 20 .60 A and {3 = 121 ° 45'.…”

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confidence: 99%

Phase equilibrium relations in the binary system lead oxide-niobium pentoxide

Roth¹

1959

J. RES. NATL. BUR. STAN.

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The phase equilibrium diagr am for the bin ary system lead oxide-niobium pent oxide has been construct.ed from observations of fu sion characteri s tics and X-ray diffraction d ata. The system contains six binary compounds with PbO : Nb20 , ratios of 3: 1, 5: 2, 2:1,3:2, 1:1, and 1: 2. The compound PbO·NbzO, was found to melt congruen tly at 1,343° C and have a stable, reversible phase t ra nsformation temperature from t he lowtemperature rhombohedral form to t he high-temperature tetragonal form at 1,150° C. , The 5: 2, 2: 1, and 1: 2 compounds melt congruently at 1,220°, 1,233°, a nd 1, 337° C, respect ively; the 3: 1 and 3: 2 compo unds melt incon gr uently at 985° and 1,233° C, resp ec tivcly.

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“…A11 electronic, automatic control device was used which records the times to traverse intervals of potential (about 4 seconds for 10 volts a t the current density used) and terminates formation a t a preset potential with respect to a reference electrode. At [11][12] CAN.ADI.AN JOURKAL OF CHEMISTRY. VOL.…”

Section: E X P E R I M E N T a L Methodsmentioning

confidence: 99%

Anodic Oxide Films on Niobium: Thickness, Dielectric Constant, Dispersion, Reflection Minima, Formation Field Strength, and Surface Area

Young¹

1960

Can. J. Chem.

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The wavelengths of minimum specular reflectivity (at 11' incidence) due to interference were determined using a spectrophotometer for a series of films formed on chemically polished niobium. With a value of the refractive index n = 2 . 4 6 f 1% a t 4358 A wavelength by the Abelhs method (reported elsewhere), the spectrophotometric data give the refractive index as a function of the wavelength X, 11 = 2.26+0.398/(X/103 A-2.56)l.~. T o analyze the spectrophotometric results, an auxiliary measure of thickness was required (though, with the chart given below, the thickness of a film may be determined directly from spectrophoto~netric measurements alone). A combination of measurements of the a-c. capacity and of the charge required to form the films gives a suitable measure of thickness (in terms of p /~, where r = dielectric constant and p = density) which is not dependent on a knowledge of the true surface area. The spectrophotometric data provide a calibration of this measure of thickness and thus give E /~. With p = 4.36 g cm+ (reported for the bullc amorphous oxide) this gives e about 41 (compared with about 27.6 for TazOs). The effective surface area of the chemically polished metal was then found to be about 7% greater than the apparent area. At the ionic current density used to form the films (10 ma cm-=), the field strength in the oxide was estimated as 4.96X106 v cm-1 within a few per cent uncertainty. Because the field to produce a given ionic current is l o t k than with Ta20s films, the capacity of films formed to a given voltage a t a given current density and temperature is not so much greater for niobium than for tantalum as the dielectric constants might lead one to expect. I t is suggested that there may be a correlatio~l between dielectric constant and ionic conductivity. The NbzOa films recrystallize like Taros films under an applied field but more readily, a t least with the purity of metal now available. INTRODUCTIONAs might be expected from their close chemical similarity, niobium and tantalum behave very similarly when anodically polarized. The present investigation of some of the properties of the film material followed the same method as was applied recently to tantalum (1). The aim was to obtain further data towards the coillparative study of the various inetals (Al, Nb, T a , Ti, Zr, W, etc.) which show similar anodic film forming properties. Niobium is of particular interest because of its possible future use as an electrolytic capacitor anode. The kinetics of forn~ation and solne other topics have been discussed elsewhere (2, 3, 1). E X P E R I M E N T A L METHODSThe electrodes (2XO.5 till with a tab) were cut (using a die to obtain a reproducible size) fro111 Fansteel h~Ietallurgica1 Corporation 0.005-in. sheet of greater than 99.9% purity. They were stopped off and chemically polished as previously described (2). T o obtain uniformity in the surface finish they were prepared in batches. After being stored dry, they were dipped in a dichromate-sulphuric acid cleaning mixture bef...

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Chemistry of the Group VB Pentoxides. VI. The Polymorphism of Nb₂O₅

Cited by 158 publications

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Phase equilibria in the system niobium pentoxide-germanium dioxide

Phase equilibria in the system niobium pentoxide-germanium dioxide

Phase equilibrium relations in the binary system lead oxide-niobium pentoxide

Anodic Oxide Films on Niobium: Thickness, Dielectric Constant, Dispersion, Reflection Minima, Formation Field Strength, and Surface Area

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Chemistry of the Group VB Pentoxides. VI. The Polymorphism of Nb2O5

Cited by 158 publications

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Phase equilibria in the system niobium pentoxide-germanium dioxide

Phase equilibria in the system niobium pentoxide-germanium dioxide

Phase equilibrium relations in the binary system lead oxide-niobium pentoxide

Anodic Oxide Films on Niobium: Thickness, Dielectric Constant, Dispersion, Reflection Minima, Formation Field Strength, and Surface Area

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Chemistry of the Group VB Pentoxides. VI. The Polymorphism of Nb₂O₅