• T K Dey

      Articles written in Bulletin of Materials Science

    • Temperature dependence of thermal conductivity of vanadium substituted BPSCCO system between 10 and 150 K

      A K Dhami T K Dey

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      Thermal conductivity of a set of (Bi0.8Pb0.2−yVy)2Sr2Ca2Cu3O10+δ (0 =y ≤ 0.05) pellets in the temperature range between 10 and 150 K is reported. Vanadium substitution influences strongly the magnitude of thermal conductivity (λ,) over the entire temperature range. But the nature of λ(T) dependence remains similar to that generally observed for HTSCs. The electronic contribution to the total λ in the normal state is estimated to be ∼ 25%. We have attempted to examine our data, assuming the role of both electrons and phonons in the origin of the λ(T) behaviour belowTc. Observed temperature variation of λ(T) for the present set of samples could be explained very well assuming this electron + approach. Some of the microscopic quantities estimated from the best-fit parameters give reasonable values.

    • A sensitive magnetic field sensor using BPSCCO thick film

      S Vijay Srinivas Abhijit Ray T K Dey

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      A highly sensitive magnetic sensor operating at liquid nitrogen temperature and based on BPSCCO screen-printed thick film, is reported. The sensor resistance for an applied magnetic field of 100 × 10–4T(100 gauss) exhibits an increase by 360% of its value in zero field at 77.4 K. The performance of the sensor in presence of magnetic field, the hysteretic features and the effect of thermal cycling, has been discussed.

    • Non-destructive evaluation of defects in ferromagnetic plates using a sensitive magnetic sensor based on second harmonic response of superconducting Bi1.6Pb0.4Sr2Ca2Cu3O10+𝛿 pellet

      Abhijit Ray T K Dey

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      The characteristics of a magnetic sensor, based on the non-linear electromagnetic response of the weak links present in polycrystalline BPSCCO superconductor are reported. The second harmonic response of the sensor in an alternating magnetic field at 40 kHz and at 77 K being a strong linear function of low d.c. field is utilized for magnetic field sensing. The noise limited resolution of the sensor is found to be 3.16 × 10-9 T/√ Hz for 𝐻a.c. = 16 G and frequency 40 kHz. The magnetic sensor has been applied for non-destructive detection of various types of flaws in ferromagnetic plates and also for detection of small magnetic inclusions in a non-magnetic matrix. Our results suggest that the 2𝑓 response based BPSCCO superconductive magnetometer has potential for its appication in the area of non-destructive evaluation of defects in ferromagnetic materials.

    • A novel method for sensing rotational speed, linear displacement and current using superconducting BPSCCO magnetic sensor

      T K Dey A Ray S K Ghatak

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      For many decades, magnetic sensors have been of great assistance to mankind in variety of functions that include simple compass based navigational systems to devices that monitor the invisible biological activities. In industries magnetic sensors are in great demand for control and measurement of linear and rotary position sensing etc, because of its non destructive and contact less way of detection. Consequently, newer, smarter and cheaper materials are continuously being explored to suit the varied needs of technological requirements. In the present communication, the characteristics of a magnetic sensor, based on the non linear electromagnetic response of the weak links present in the polycrystalline BPSCCO superconductor are reported. The second harmonic response of sintered superconducting BPSCCO pellet in an alternating magnetic field at 40 kHz and 77 K being a strong linear function of low d.c. magnetic field has been utilized for the development of highly sensitive magnetic field sensors. The noise limited resolution of the sensor is found to be 3.16 × 10–9 T/√ Hz for $H_{a.c.}$ = 16 Oe and frequency 40 kHz. We further demonstrate that such HTSC based magnetic sensors are capable of sensing the rotational speed, small displacement and direct current with good resolution. The experimental methods and results obtained are discussed.

    • Role of spin polarized tunneling in magnetoresistance and low temperature minimum of polycrystalline La1–𝑥K𝑥MnO3 (𝑥 = 0.05, 0.1, 0.15) prepared by pyrophoric method

      Soma Das T K Dey

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      The low temperature resistivity and magnetoresistance of bulk samples of La1–𝑥K𝑥MnO3 has been investigated between 10 K and 300 K with and without the magnetic field (𝐻 = 0.8 T). All the samples show metal–insulator transitions with Curie temperature (𝑇C) ranging between 260 K and 309 K. At temperature below 60 K, the K-doped manganites exhibit a shallow minimum, which disappears under an applied field of 0.8 T. This field dependent minimum in resistivity, observed in K-doped lanthanum manganites is explained in the light of intergrain tunneling of the charge carriers between anti-ferromagnetically coupled grains of the polycrystalline samples. The field variation of magnetoresistance below 𝑇C follows a phenomenological model which considers spin polarized tunneling at the grain boundaries. The intergranular contribution to the magnetoresistance is separated out from that due to spin polarized tunneling part at the grain boundaries. The temperature dependence of intrinsic contribution to the magnetoresistance follows the prediction of the double exchange model for all values of field at 𝑇 < 𝑇C.

    • Percolation based enhancement in effective thermal conductivity of HDPE/LBSMO composites

      Madhusree Kole D Tripathi T K Dey

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      Thermal conductivity of composites with electrically conducting La0.7Ba0.15Sr0.15MnO3 (LBSMO) filler of nanometric grain size in HDPE matrix is investigated. Volume fraction of LBSMO fillers was varied between 0 and 0.30. SEM photographs of the composites show the presence of clusters and percolative paths, particularly for composites prepared with higher filler volume fractions. The effective thermal conductivity of the composites displays significant enhancement with increasing filler content in HDPE. A maximum enhancement of ∼65% compared to that for pure HDPE has been observed for composite with 0.30 volume fraction of LBSMO filler. Most of the models those are generally used to predict the properties of two phase mixtures, has been found either to under/overestimate the measured effective thermal conductivity of the composites. We confirm that the observed rapid increase in the effective thermal conductivity of HDPE/LBSMO composite over the studied range of filler volume fraction (viz. 0–0.30), is predicted very well, considering the effect of percolation as proposed by Zhang et al (2009).

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