K. Shahi

Preparation, conductivity, complex permittivity and electric modulus in AgIAg2OSeO3MoO3 glasses

Abstract A new quaternary superionic conducting AgIue5f8Ag2Oue5f8(SeO3 + MoO3) glasses with different compositions of the dopant salt AgI were prepared by rapid melt quenching. The compounds were characterised by X-ray diffraction, scanning electron microscopy and infrared spectroscopy, and glass-forming compounds were identified. The frequency dependence of electrical conductivity, σ, complex permittivity, ϵ ∗ , and electrical modulus, M ∗ , were studied for various compositions of the dopant salt AgI at different temperatures. At room temperature, 293 K, the highest conductivity, σ = 3.12 × 10−2 S/cm is obtained for the glass with 60% AgI. The ionic conductivity σ and the complex permittivity ϵ ∗ is described in terms of ion diffusion and polarisation. The composition and temperature dependence of the electric modulus M ∗ is studied and the results are reported.

Ionic transport in the Na2SO4Na2WO4 and Na2SO4M2(SO4)3 (M=La, Dy, Sm, In) systems

Abstract Premelted, predried Na2SO4, premelted Na2WO4, Na2SO4ue5f8Na2WO4 composites and Na2SO4ue5f8M2(SO4)3 (M = La, Dy, Sm, In) have been studied by means of X-ray diffraction, DTA and electrical conductivity measurements. The high temperature, highly conducting Na2SO4 phase I has been successfully stabilised at room temperature; the Na2SO4 containing 4 mole% La2(SO4)3 exhibits the highest conductivity (σ) and lowest activation energy (Ea) (σ=1.08 × 10−3ω−1 cm−1 at 290°C and Ea=0.50 eV) and therefore this system appears promising for further development.

A comparative study of crystallization kinetics between conventionally melt quenched and mechanochemically synthesized AgI–Ag2O–CrO3 superionic system

Abstract Non-isothermal crystallization kinetics in conventionally melt quenched versus mechanochemically synthesized amorphous AgI–Ag 2 O–CrO 3 superionic solids is discussed. The quenched as well as ball-milled samples exhibit glass ( T g ) and multiple amorphous⇒crystalline ( T c ) transitions. T g as well as T c are found to increase monotonically with heating rate. The activation energy for structural relaxation ( E s ) obtained using Moynihan equation is found to be higher for ball-milled samples that eventually suggests the relatively rigid and highly viscous structure of milled samples. The activation energy associated with nucleation and growth ( E c ) is obtained using Matusita–Sakka equation and its higher value confirms the higher rate of crystallization in ball-milled samples. The values of T c – T g and the enthalpy of phase transformation (ΔH) are also found higher for the ball-milled samples that confirm their comparatively high thermal stability. The electrical conductivity near the crystallization temperatures is studied as a function of time and temperature and these results confirm the presence of amorphous⇒crystalline transition temperatures in the ball-milled as well as in the melt-quenched samples.

Mechanochemically synthesized amorphous superionic systems for solid-state batteries

Solid-state electrochemical cells of the type Ag|a-SIC|I2 are fabricated using mechanochemically synthesized amorphous superionic conductors (a-SICs), AgI–Ag2O–MxOy (MxOy=B2O3, V2O5, CrO3) as electrolytes. The typical value of the conductivity of these systems is in the range 10−2 to 10−3 Ω−1 cm−1 and the transport number of Ag+ ions is near unity. n nThe mechanochemically synthesized powders were directly used as electrolytes. Constant load characteristics for various loads confirm the thermodynamic stability and competency of these materials as electrolytes. The cells are discharged at low (J∼65 μA/cm2) and high (J∼105 μA/cm2) drain currents and nominal cell capacities and energy densities were obtained. Nominal cell capacities for all these cells for low current discharge rates were found within 6–7% of the theoretical capacity of Ag/I2 cell. Whereas, for high discharge rates, ∼3–4% of the theoretical value. A current density of ∼500 μA/cm2 can be drawn safely without any serious polarization.

Characterization and electrochemical cell characteristics of mechanochemically synthesized AgI–Ag2O–MoO3 amorphous superionic system

Abstract Mechanochemically synthesized amorphous and thermally stable x AgI(100− x )[0.5Ag 2 O+0.5MoO 3 ] system for x =40, 50, 60 and 70 shows high ionic conductivity of ∼10 −2 –10 −3 xa0Ω −1 xa0cm −1 at room temperature. The highest ionic conductivity is achieved for 36xa0h milled sample containing 50xa0m/o AgI, which is more than three orders of magnitude higher than that of crystalline AgI at room temperature and comparable with the glassy fast ionic conductor of the same composition prepared by conventional quenching. The samples are thermally stable at least up to ∼70xa0°C with ionic transport number near unity. These amorphous samples are further characterized by distinct glass transition as well as amorphous⇒crystalline transition temperatures. The FT-IR spectra confirm the presence of MoO 4 2− and Mo 2 O 7 2− groups in the ball-milled systems. Investigations on galvanic cells of the type Ag|a-SIC|I 2 +C reveal that these mechanochemically synthesized amorphous samples are stable under battery conditions and the cell performance parameters are comparable with those of earlier investigated Ag/I 2 cells prepared using glassy SICs.

Automated measurement of ac impedance data: Acquisition and analysis

Abstract A set of algorithms (the IONICS systems) for the acquisition and analysis of ac impedance data in superionic conductivity research using an IBM PC compatible computer, have been developed and tested extensively. The algorithms can be used for acquiring ac impedance data at different frequencies as a function of temperature. The furnace can also be controlled simultaneously. The data can then be graphically displayed and numerically fitted with the appropriate curves, i.e., straight lines on Arrhenius plots and arcs of circles complex impedence plots. This paper describes the IONICS system, which consists of three modules ACQUIRE, ANALYSE and GENERATE.

PHASE DIAGRAM AND ENHANCED ELECTRICAL TRANSPORT IN KBR-NABR MIXED CRYSTALS

Abstract Phase diagram and ionic conductivity of KBr—NaBr mixed crystals system have been studied over the entire composition and stable temperature range. A maximum conductivity enhancement by a factor of 25 with respect to pure KBr, and with respect to pure NaBr, is obtained for the K 0.5 Na 0.5 Br solid solution at 500°C. The conductivity values are found to follow the melting (solidus) temperature in a correlated manner. The minimum in the solidus curve of the phase diagram is observed between 43 and 72 mole% NaBr at 615°C.

Aluminaless composite solid electrolytes I : enhanced electrical transport in β-Li2SO4-Na2SO4 system

Abstract β-Li 2 SO 4 -Na 2 SO 4 composites have been prepared by quenching the melt and characterized by DTA and complex impedance analysis to obtain dc electrical conductivity (ϐ dc ). The samples containing upto 50 m/o Na 2 SO 4 are actually mixtures of β-Li 2 SO 4 and LiNaSO 4 and those containing 50 to 100 m/o Na 2 SO 4 are essentially mixtures of Na 2 SO 4 and LiNaSO 4 . Both the mixtures exhibit enhanced ϐ by 1 to 2 orders of magnitude. The mechanism of enhancement in these mixtures appears to be similar to that in Al 2 O 3 -dispersed solid electrolytes, better known as “composite solid electrolytes”. Thus there is enormous scope for further studies on the so-called aluminaless composite solid electrolytes, and possibility of even developing them into commercial solid electrolytes.

Transport studies on mechanochemically synthesized amorphous AgI–Ag2 O–CrO3 superionic system

Abstract Mechanochemical synthesis, a relatively new technique in the field of solid state ionics, has been successfully used to obtain highly conducting amorphous materials of composition x AgI(100− x )[0.5Ag 2 O+0.5CrO 3 ] for x =50, 60 and 70. The ionic conductivity of ball-milled amorphous samples is found to be ∼10 −2 Ω −1 cm −1 at room temperature which is ∼3 orders of magnitude higher than that of AgI and comparable with superionic glass of the same compositions. The amorphous samples are thermally stable upto ∼100 °C. On annealing the samples at ∼110 °C there is precipitation of AgI and Ag 2 O nanocrystallites of size ∼10 nm. For all the samples the ionic transport number is found to be near unity. The galvanic cells of the type Ag|a-SIC|I 2 +C, fabricated using mechanically milled materials as electrolyte, are thermodynamically stable. The electrical, structural, electrolytic and thermal properties are very much close to those of AgI–Ag 2 O–CrO 3 and AgI–Ag 2 CrO 4 superionic glasses investigated earlier.

Conduction in CsCl-Al2O3 composite solid electrolytes: Evidence for interfacial space charge

Abstract Conductivity versus temperature studies reveal that the phase transition from B2 phase (CsCl structure) to a high temperature B1 phase (NaCl structure) is suppressed in the CsCl-Al 2 O 3 composites. However, DTA measurements on the composite samples show that both the B1↔B2 and the melting transition occurring in pure CsCl occur in the composites. Moreover, there is no shift in these transition temperatures due to dispersion of A1 2 O 3 , which suggests that A1 2 O 3 remains as a separate phase in the composites. These results indicate that the conduction in the composites is controlled by a single mechanism in the temperature regimes that are above and below the transition temperature, possibly through a high-conducting interfacial space charge region.