Partho Sarkar

Functionally graded ceramic/ceramic and metal/ceramic composites by electrophoretic deposition

Constant current electrophoretic deposition (EPD) has been used to synthesise Al2O3/YSZ,Al2O3/MoSi2, A12O3/Ni and YSZ/Ni functionally graded materials (FGM). EPD is a cheap and simple technique to fabricate complicated ceramic shapes. By this technique it is possible to synthesize step FGMs as well as continuous-profile FGMs. The profile can be controlled precisely by controlling the deposition current density, second component flow rate, suspension concentration, etc. The microstructures of the FGMs produced were characterized by optical and electron microcopy and micro-indentation was used to track the Vickers hardness and fracture toughness variation across the composition profiles.

Fabrication of textured Bi-Sr-Ca-Cu-O thick film by electrophoretic deposition

Bi‐Sr‐Ca‐Cu‐O thick films (≊5–40 μm) were electrophoretically deposited on silver foil using ethyl alcohol as the suspending liquid. The deposition voltage was ≊70 V/cm and the current ≊550 μA/cm2. X‐ray diffraction of the as‐deposited sample shows texturing. This texturing was enhanced by pressing the film uniaxially at ≊82 MPa. Samples were sintered at 810 °C in a 10% oxygen‐in‐argon atmosphere. Liquid at the sintering temperature promotes textured grain growth. The critical current density (Jc) of the film determined by magnetic measurement at 5 K in a 2 kOe field is ≊1.3×104 A/cm2.

Electric field relaxation studies in the CeO2-Y2O3 system

Abstract Electrical relaxation in the (CeO 2 ) 1− x -(Y 2 O 3 ) x [where x = 0 to 0.2] system has been studied as a function of temperature (~ 100–600°C) using the electric modulus formalism in the frequency range 5–10 7 Hz. Two relaxation processes are observed in low Y 2 O 3 (up to ~2 m o ) doped samples. One is due to long range migration of free oxygen vacancies (Process A) and the other is due to orientation relaxation of the (Y Ce V o ) . charged associated defects (Process B). The conductivity relaxation process is analysed using the non-exponential decay function φ(t) = exp [−( t τ o ) gb ] for 0 , of the electric field. The relaxation parameters of undoped CeO 2 do not change with temperature suggesting the vacancies are frozen-in on cooling pure CeO 2 from sintering temperatures so guaranteeing the vacancy concentration does not change at the experimental temperatures used for the modulus studies. Undoped CeO 2 has a relaxation process close to a single relaxation but, with increasing dopant concentration, the electric modulus relaxation peak broadens. The observed activation enthalpy minimum (activation enthalpy vs m o Y 2 O 3 ) for Process A is explained using the concept of partial dissociation of V o o from (Y Ce V O ) . defect associates and formation of higher-order defect clusters at higher m o Y 2 O 3 .

ac Conductivity and conductivity relaxation studies in the CeO2−Y2O3 system

Abstract The electric modulus and ac conductivity of CeO 2 −Y 2 O 3 solid solutions were studied as a function of Y 2 O 3 concentration (0.05 to 20 m/o Y 2 O 3 ) and temperature (∽400–650 K). A minimum relaxation time and maximum conductivity were identified at ∽ 3 m/o Y 2 O 3 in CeO 2 .

The mechanical and electrical properties of ZrO2-Naβ″-Al2O3 composites

ZrO2-Naβ″-Al2O3 composites were prepared by a conventional method using two different powder routes and different milling liquids. The retained tetragonal-phase ZrO2 was 85 to 90% for composites with 2.4 to 15 vol% ZrO2. The fracture toughness (Klc) and strength increased with increasing ZrO2 content. At 20 vol % ZrO2,Klc and bend strength were 4.35 M Pa m1/2 and 390 MPa, respectively. Stress-induced transformation toughening is the predominant toughening mechanism. Dispersion toughening also contributes to the increase ofKlc. Surface strengthening was found to be an effective strengthening method for low ZrO2 levels. The critical tetragonal ZrO2 grain size was found to increase from 0.86 to 1.02gmm as the ZrO2 content increased from 2.5 to 15 vol %. A detailed study of the ionic conductivity of the 15 vol % ZrO2 dispersed sample was conducted by an a.c. technique between −124° C and ∼ 300° C. The bulk and total conductivities were calculated via complex-plane analysis. The total (grain and grain-boundary) ionic specific resistivity was ∼ 9 Ω cm at 300° C. The activation enthalpies of the bulk and total conductivity processes were 0.30 and 0.32 eV, respectively.

Magnetically enhanced reaction sintering of textured YBa2Cu3Ox

Liquid phase reaction sintering of YBa2Cu3Ox electrophoretically deposited thick films has been conducted in a magnetic field (∼1 T). The magnetic field influences the crystallographic orientation of the final microstructural grains (texturing). It is suggested that the magnetic field does not influence the orientation of nuclei during the nucleation process but induces physical rotation of the monograins in the sintering liquid and influences their grain growth.

The kinetics of ultrasonic/electrical-field assisted hydronium ion exchange of K+-β′′-Al2O3

Abstract The rate of electrical-field-assisted, hydronium-ion-exchange of K + -β″-Al 2 O 3 ceramics was investigated. A model based on ion migration from a decreasing source was proposed to interpret the kinetics of the exchange process. Results of the ion exchange experiment with ultrasonic field superimposition plus a cycling dc field are also reported.

Thermodynamic analysis of NH3-H2O∣NH+4-H3O+-β″/β-Al2O3 fuel cells

Abstract Thermodynamic equilibria and equilibrium species distribution were calculated for the systems NH 3 -H 2 , NH 3 -H 2 O and NH 3 -H 2 -H 2 O. The simulation calculation suggested that if NH 3 -gas mixtures were used for NH + 4 -H 3 O + -β″ /β-Al 2 O 3 solid electrolyte fuel cells operated > 100°C, the EMF would be comparable with that of H 2 -O 2 fuel cells.

Transport properties of H3O+-β″/β-Al2O3 ceramic electrolytes

Abstract The ionic transference number and electronic conductivity of H 3 O + -β″/β-Al 2 O 3 ceramic electrolytes were measured at 293–429 K using the electrochemical cells: Pt, H 2 , HCl (0.1 m) ∣H 3 O + -β″/β-Al 2 O 3 ∣ HCl (0.01 m), H 2 , Pt; Pt, H 2 +H 2 O(g) ∣H 3 O + - β″/β-Al 2 O 3 ∣Pt; Pt, H 2 , H 2 O∣H 3 O + -β″/β-Al 2 O 3 ∣O 2 , H 2 O, Pt. The ionic transference number is 0.99 0.02. The electron conductivity is described by 3.9×10 -3 exp(-0.38 eV/ kT ), confirming the ionic transport nature of H 3 O + -β″ /gb-Al 2 O 3 .

Electrical Relaxation Studies in Fluorite Oxides

Electric relaxation in CeO 2 -M 2 0 3 (M 34 sY 3+ , La 3+ ) solid solutions has been investigated as a function of temperature (373K-673K) using the electric modulus formalism in the frequency range 5 to 10 7 Hz. Two relaxation processes are observed in dilute solid solutions. The low frequency process is identified as a long range migration of free oxygen-vacancies (Process A) and the high frequency process is due to reorientation relaxation of the (MceVo) charged associates (Process B). The relaxation process is analysed using a non-exponential decay function, o(t)=exp[-(t/τo) B ] for O 2 0 3 .