• Hua Wang

      Articles written in Bulletin of Materials Science

    • Effects of Bi doping on dielectric and ferroelectric properties of PLBZT ferroelectric thin films synthesized by sol–gel processing

      Hua Wang Li Liu Ji-Wen Xu Chang-Lai Yuan Ling Yang

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      [Pb0.95(La1−𝑦Bi𝑦)0.05][Zr0.53Ti0.47]O3 (PLBZT) ferroelectric thin films have been synthesized on indium tin oxide (ITO)-coated glass by sol–gel processing. PLBZT thin films were annealed at a relatively low temperature of 550 °C in oxygen ambient. Effects of Bi doping on structure, dielectric and ferroelectric properties of PLBZT were investigated. Bi doping is useful in crystallization of PLBZT films and promoting grain growth. When the Bi-doping content 𝑦 is not more than 0.4, an obvious improvement in dielectric properties and leakage current of PLBZT was confirmed. However, when the Bi-doping content is more than 0.6, the pyrochlore phase appears and the remnant polarization 𝑃r of PLBZT thin films is smaller than that of (Pb1−𝑥La𝑥)(Zr1−𝑦 Ti𝑦)O3 (PLZT) thin films without Bi doping. PLBZT thin films with excessive Bi-doping content are easier to fatigue than PLZT thin films.

    • Structural evolution, electrical and optical properties of AZO films deposited by sputtering ultra-high density target

      Jiwen Xu Zupei Yang Hua Wang Xiaowen Zhang

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      Aluminum-doped zinc oxide (AZO) target was fabricated using AZO nanopowders synthesized by co-precipitation method and then the AZO films with different thicknesses were deposited on glass by d.c. magnetron sputtering at room temperature. AZO target is nodules free and shows homogeneous microstructure, ultra-high density and low resistivity. ZnAl2O4 phase appears in AZO target and disappears in AZO films. All AZO films show c-axis preferred orientation and hexagonal structure. With increasing film thickness from 153 to 1404 nm, the crystallinity was improved and the angle of (002) peak was close to 34.45°. The increase in grain size and surface roughness is due to the increase in film thickness. The decrease of resistivity is ascribed to the increases of carrier concentration and Hall mobility. The lowest resistivity is 9.6 × 10-4 𝛺.cm. The average transmittance of AZO films exceeds 80%, and a sharp fundamental absorption edge with red-shifting is observed in the visible range. The bandgap decreases from 3.26 to 3.02 eV.

    • Bipolar resistive switching behaviours in ZnMn2O4 film deposited on p+-Si substrate by chemical solution deposition

      Jiwen Xu Zupei Yang Yupei Zhang Xiaowen Zhang Hua Wang

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      ZnMn2O4 active layer for resistance random access memory (RRAM) was deposited on p+-Si substrate by chemical solution deposition. The bipolar resistive switching behaviours of the Ag/ZnMn2O4/p+-Si capacitor are investigated. The bipolar resistive switching is reproducible and shows high ON/OFF ratio of > 102 and long retention times of > 105 s. The conduction mechanism of the Ag/ZnMn2O4/p+-Si capacitor in the low-resistance state (LRS) is ohmic conduction, whereas that of the device in high-resistance state (HRS) successively undergoes Ohm’s law, trap-filled-limited and Child’s law conduction procedure at room temperature.

    • Synthesis and resistive switching behaviour of ZnMnO3 thin films with an Ag/ZnMnO3/ITO unsymmetrical structure

      Hua Wang Shu-Ming Gao Ji-Wen Xu Chang-Lai Yuan Xiao-Wen Zhang

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      Single-phase MnZnO3 films were prepared on glass substrates coated with the use of indium tin oxide (ITO) as transparent bottom electrode via the sol–gel method. The effects of annealing temperature on structure, resistance switching behaviour and endurance characteristics of the ZnMnO3 films were investigated. The stable resistive switching behaviour with high resistance ratio in Ag/ZnMnO3/ITO unsymmetrical structure was observed. No second phase is detected, and the crystallinity of the MnZnO3 films is improved with the increase in annealing temperature from 350 to 400°C. The MnZnO3 films annealed at 350–450°C with an Ag/MnZnO3/ITO structure exhibit bipolar resistive switching behaviour. Ohmic and space-charge-limited conductions are the dominant mechanisms at low and high resistance states, respectively. $V{}_{\text{ON}},\ \text{V_{OFF}}$ and $R_{\text{HRS}}/R_{\text{LRS}}$ of theMnZnO3 films increase with the increase in annealing temperature. Improved endurance characteristics are observed in the samples annealed at 350 and 400°C. The endurance of the MnZnO3 films degrades when annealed at >450°C.

    • Structure and properties of (1−x)[(K$_{0.5}$Na$_{0.5}$)NbO$_3$–LiSbO$_3$]– xBiFe$_{0.8}$Co$_{0.2}$O$_3$ lead-free piezoelectric ceramics


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      Lead-free piezoelectric ceramics $(1−x)$[0.95(K$_{0.5}$Na$_{0.5}$)NbO$_3$–0.05LiSbO$_3$]–$x$BiFe$_{0.8}$Co$_{0.2}$O$_{3}$(KNN–LS–$x$BFC) were prepared by a conventional sintering technique. The effect of BFC content on the structure, piezoelectricand electrical properties of KNN–LS ceramics was investigated. The results reveal that the BFC is effective in promoting the sinterability and the electrical properties of the ceramics sintering at low temperature of 1030$^{\circ}$C. Theceramics show a single perovskite structure, in which the tetragonal phase decreases while the orthorhombic phase increases with the increase of $x$. The more the BFC content is, the smaller and homogeneous grains were formed.With the increase of $x$, the $d_{33}$ and the $k_p$ increase to a maximum value and then slightly decrease, but the $Q_m$ increases continuously. As BFC content increases, the Curie temperature $T_c$ and remnant polarization $P_r$ decrease, but the diffusivity of phase transition in KNN–LS ceramics will intensify and the coercive field $E_c$ fluctuate between 1.16 and 1.51 kV mm$^{−1}$. The samples with $x =0.004$ exhibit optimum electrical properties at room temperature ($d_{33} = 268 pC$ N$^{−1}$, $k_p =52$%, $\epsilon_r = 1366$, $\tan \delta =2.11$%, $T_c = 325^{\circ}$C, $P_r = 20.4$ $\mu$C cm$^{−2}$, $E_c =1.16$ kV mm$^{−1}$).

    • Effects of Zn doping concentration on resistive switching characteristics in Ag/La$_{1−x}Zn$_x$MnO$_3$/p$^+$-Si devices


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      Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^+$-Si devices with different Zn doping contents were fabricated through sol–gel method. The effects of Zn doping concentration on the microstructure of La$_{1−x}$Zn$_x$MnO$_3$ films, as well as on the resistance switching behaviour and endurance characteristics of Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^{+}$-Si were investigated. After annealing at 600$^{\circ}$C for 1~h, the La$_{1−x}$Zn$_x$MnO$_3$ ($x = 0.1$, 0.2, 0.3, 0.4, 0.5) are amorphous and have bipolar resistance characteristics, with RHRS/RLRS ratios $>$103. However, the endurance characteristics show considerable differences; $x = 0.3$ shows the best endurance characteristics in more than 1000 switching cycles. The conduction mechanism of the Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^{+}$-Si is the Schottky emission mode at high resistance state. However, the conduction mechanism at low resistance state varies with Zn doping concentration. The dominant mechanism at $x = 0.1$ is filamentary conduction mechanism, whereas that at $x \ge 0.2$ is space-charge-limited current conduction.

    • Bipolar resistive switching behaviour in Mn$_{0.03}$Zn$_{0.97}$O/amorphous La$_{0.7}$Zn$_{0.3}$MnO$_3$ heterostructure films


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      Mn$_{0.03}$Zn$_{0.97}$O (MZO)/amorphous La$_{0.7}Zn$_{0.3}$MnO$_3$ (LZMO) heterostructures were deposited on p$^+$-Si substratesthrough sol–gel spin coating. Ag/MZO/LZMO/p$^+$-Si and Ag/LZMO/MZO/p$^+$-Si devices exhibit a bipolar, reversibleand remarkable resistive switching behaviour at room temperature. The ratio of the resistance at high-resistance state (HRS)to that at low-resistance state (LRS) ($R_{\rm HRS}/R_{\rm LRS}$) in the Ag/LZMO/MZO/p$^+$-Si device is approximately five orders of magnitude, and is maintained after over 10$^3$ successive switching cycles or over a period of $2\times 10^6$ s, indicating good endurance property and retention characteristics. Conversely, the ratio in the Ag/MZO/LZMO/p$^+$-Si device began to decrease after 100 successive switching cycles. The LZMO/MZO interface could play an important role in the resistive switching behaviour of the devices. The dominant conduction mechanism of the two devices is charge-trap emission.

    • Rectifying resistance-switching behaviour of Ag/SBTO/STMO/p$^+$-Si heterostructure films


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      The Sr$_{0.88}$Bi$_{0.12}TiO$_3$/SrTi$_{0.92}$Mg$_{0.08}$O$_3$ (SBTO/STMO) heterostructure films were prepared on p$^+$-Si substratesby sol–gel spin-coating technique, and the films had good crystallinity and uniform grain distribution. The heterostructure films with a structure of Ag/SBTO/STMO/p$^+$-Si exhibited a bipolar, remarkable resistance-switching characteristic, and $R_{\rm HRS}/R_{\rm LRS} \sim 10^4$. More importantly, the heterostructure films showed rectifying characteristic in the low resistance state (LRS), and the rectification ratio can reach 10$^2$ at $\pm$1 V. The dominant resistive-switching conduction mechanism of high resistance state (HRS) was Ohmic behaviour, and the LRS changed to space charge-limited current(SCLC).

    • Resistance-switching properties of Bi-doped SrTiO$_3$ films for non-volatile memory applications with different device structures


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      SrTiO$_3$ and Bi-doped SrTiO$_3$ films were fabricated with different device structures using the sol–gel method for non-volatile memory applications, and their resistance-switching behaviour, endurance and retention characteristics were investigated. SrTiO$_3$ and Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si or Pt have the same phase structure, morphologies and grain size; however, the grain size of the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si is slightly larger than those of the SrTiO$_3$ films grown on Si and the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Pt. The SrTiO$_3$ or Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si or Pt all exhibitbipolar resistive-switching behaviour and follow the same conductive mechanism; however, the Ag/Sr$_{0.92}$Bi$_{0.08}$TiO$_3$/Si device possesses the highest $R_{\rm HRS}/R_{\rm LRS}$ of 10$^5$ and the best endurance and retention characteristics. The doping of Bi is conducive to enhance the $R_{\rm HRS}/R_{\rm LRS}$ of the SrTiO$_3$ films; meanwhile, the Si substrates help improve the endurance and retention characteristics of the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films.

    • Influence of Ni doping on the structural, ferroelectric, magnetic and optical properties of Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ thin films


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      Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ ($x = 0.025–0.125$) thin films were synthesized by applying a sol–gel method on fluorine-doped tin oxide substrates. The influence of Ni doping concentration on the structure, leakage current, ferroelectric, magnetic and optical properties of Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ thin films was investigated. Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ thin films are polycrystalline films that present a single perovskite structure without any impurity phase when the Ni doping concentration is below 0.1 and present a Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ phase when the Ni doping concentration is above 0.1. The grain size of the films and their holes gradually decrease with an increase in the Ni doping amount. The saturation magnetization of Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ thin films increases with Ni content.However, appropriate Ni doping concentration can decrease the leakage current and enhance the ferroelectric polarization and optical transmittance of the films. Meanwhile, the absorption edge has a slight red shift. Bi$_{0.85}$Nd$_{0.15}$Fe$_{1−x}$Ni$_x$O$_3$ thin films possess better combination properties at a leakage current density of $4.27 \times 10^{−9}$ A cm$^{−2}$, ferroelectric polarization of 28.58 $\mu$C cm$^{−2}$, saturation magnetization of 2.08 emu cm$^{−3}$ and transmittance of over 85% when the Ni doping concentration, $x$ is 0.05.

    • High piezoelectric properties of 0.82(Bi$_{0.5}$Na$_{0.5}$)TiO$_3$–0.18(Bi$_{0.5}$K$_{0.5}$)TiO$_3$ lead-free ceramics modified by (Mn$_{1/3}$Nb$_{2/3}$)$^{4+}$ complex ions


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      The complex ions (Mn$_{1/3}$Nb$_{2/3}$)$^{4+}$ doped 0.82BNT–0.18BKT (BNKT-xMN) ceramics were prepared by conventional solid-state sintering. The effects of the MN content on the structural and electrical properties of the BNKT-$x$MN ceramics were investigated. The grain size decreases sharply after doping MN. With the increase of the MN content, the phase structure changes from the rhombohedral and tetragonal phase to the tetragonal phase, then to the pseudo-cubic phase. The ferroelectric phase transforms to the relaxor phase. At critical phase (x = 0.03), the maximum positive bipolar strain and unipolar strain are 0.38 and 0.386%, respectively. The corresponding $d^*$$_{33}$ and $d_{33}$ are 767 pm V$^{–1}$ and 158 pC N$^{–1}$, respectively. Meanwhile, the dielectric constant gradually decreases with the increase of the MN content, which flattens the permittivity curves. The large piezoelectric responses are closely associated with the reversible relaxor ferroelectric phase transformation.

    • Enhancement of the up-conversion luminescence performance of Ho$^{3+}$-doped 0.825K$_{0.5}$Na$_{0.5}$NbO$_3$-0.175Sr(Yb$_{0.5}$Nb$_{0.5}$)O$_3$ transparent ceramics by polarization


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      In this study, Ho$^{3+}$ doped 0.825K$_{0.5}$Na$_{0.5}$NbO$_3$-0.175Sr(Yb$_{0.5}$Nb$_{0.5}$)O$_3$ luminescence transparent ceramics were prepared via the traditional solid-state sintering method. The structure and optical properties of the ceramics before and after polarization were studied at 40 kV cm$^{-1}$ for 0.5 h. With the increase of Ho content, the phase structure of the ceramics changed from a pseudo-cubic phase to the tripartite and the orthorhombic phases, and the light transmittance decreased. The ceramics demonstrated an up-conversion luminescence characteristic under the excitation of a 980 nm laser, and the emission wavelengths were 550 and 670 nm. The best up-conversion luminescence performance was obtained when the Ho content was 0.1%. Moreover, the polarization markedly enhanced the luminescence performance of the 0.825K$_{0.5}$Na$_{0.5}$NbO$_3$-0.175Sr(Yb$_{0.5}$Nb$_{0.5}$)O$_3$-0.1%Ho ceramics due to the increased possibility of energy-level radiative transition of rare-earth Ho$^{3+}$ ions and reduction of the $E$$_g$ value of the ceramic.

    • Effects of Er$^{3+}$ doping on the structure and electro-optical properties of 0.94(K$_{0.5}$Na$_{0.5}$)NbO$_3$–0.06Sr(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ ceramics


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      The traditional solid-phase reaction method was used to dope the 0.94(k$_{0.5}$Na$_{0.5}$)NbO$_3$–0.06Sr(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (0.94KNN–0.06SZN) with rare-earth Er$^{3+}$, showing that the transparent ferroelectric ceramics have both up-conversion luminescence. Also the changes in the phase structure, optoelectronic properties of the ceramics after Er$^{3+}$ doping were investigated. The results show that the doping of Er$^{3+}$ has no significant effect on the phase structure, dielectric constant, coercivity field and residual polarization intensity of the ceramics. With the increase of Er$^{3+}$ content, the saturation polarization intensity shows a trend of decreasing and then increasing, and the dielectric constant first decreases and then stabilizes. The large amount of Er$^{3+}$ also greatly reduced the light transmission of the ceramics. In addition, the doping of Er$^{3+}$ gives the ceramics new properties. Under 980 nm laser excitation, the ceramics exhibit luminescent emission bands at 533, 554 nm (green) and 672 nm (red). The luminous intensity of the ceramic first strengthens with the increase of Er$^{3+}$ content and then weakens, and the strongest luminous intensity is obtained when the Er$^{3+}$ content is 1.00% mol. Transparent ferroelectric ceramics with light-emitting functions will have a broad application prospect in the field of photoelectric crossover.

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