O.K. Tan

Ethanol sensors based on nano-sized α-Fe2O3 with SnO2, ZrO2, TiO2 solid solutions

Abstract Nano-sized α-Fe 2 O 3 based solid solutions with different compositions of SnO 2 , ZrO 2 and TiO 2 , were prepared using high-energy ball milling technique. Their structural properties and ethanol gas sensing properties were characterized using XRD and gas sensing measurements. The experimental results show that the mechanical alloying processes for these powders are the same. The screen-printed thick film gas sensors made from these mechanically alloyed materials demonstrated very high relative resistance. A non-equilibrium structural model was proposed to explain sensing mechanism. The comparison of gas sensing properties was performed for different α-Fe 2 O 3 based solid solutions with optimized compositions of SnO 2 , ZrO 2 and TiO 2 . Among these three sensors, x TiO 2 –(1− x )α-Fe 2 O 3 type of gas sensor has much lower relative resistance value for ethanol. This can be elucidated by the different valence states exhibited by titanium ions.

Frequency and temperature dependent impedance spectroscopy of cobalt ferrite composite thick films

Cobalt ferrite (CoFe2O4) composite thick films consisting of two different sized CoFe2O4 particles were deposited on Pt/Ti/SiO2/Si substrate by a hybridized sol-gel processing and spin coating technique. X-ray diffraction analysis showed a pure spinel phase of CoFe2O4, which was also confirmed by micro-Raman spectra. Scanning electronic microscope indicated a dense microstructure with a thickness above 8 μm. The detailed electrical investigations were conducted in the frequency range of 100 Hz–1 MHz and temperature range between 25 and 200 °C. Real and imaginary parts of impedance (Z′ and Z″) in the above frequency and temperature domain suggested the coexistence of two relaxation regimes: one was induced by electrode polarization; while the other was attributed to the coeffect of grains and grain boundaries, which was totally different from its counterpart of bulks and also not reported in other ferrites. Electrical modulus (M′ and M″) further showed the crossover from grains effect to grain boundaries e...

Ferromagnetic, ferroelectric and dielectric properties of Pb(Zr0.53Ti0.47)O3/CoFe2O4 multiferroic composite thick films

Pb(Zr0.53Ti0.47)O3/CoFe2O4 (abbreviated as PZT/CFO) multiferroic composite thick films have been prepared on the Pt/Ti/SiO2/Si substrate by using a hybrid sol?gel process. The thick films were finally annealed at 650??C in air and the ferromagnetic, ferroelectric and dielectric properties of these films were investigated. Both the PZT pervoskite phase and the CFO spinel phase were detected from x-ray diffraction. The thickness of the film was measured to be about 3.55??m using a surface profiler and was confirmed with scanning electron microscopy. Effective saturated magnetization (Mes) estimated from the measured saturated magnetization (Ms) was defined and the result shows that the Mes of the film fabricated in this work is larger than that of the thickest PZT/CFO film reported so far, so are the coercive magnetic fields (Hc). The properties of the films can be attributed to less constraints from the substrate to the composite thick films. The remanent polarization (Pr) of 12.5??C?cm?2 is comparable to that of the thickest PZT/CFO film reported so far due to the lack of polyvinylpyrrolidone (PVP), while the coercive electric field (Ec) of 252.8?kV?cm?1 is much larger than it. In addition, the PZT/CFO thick films exhibit a smaller dielectric permittivity of about 200 within the whole measured frequency range due to the introduction of the low dielectric permittivity CFO phase. Furthermore, dielectric loss decreases to ~0.01 with increasing frequency to 1?MHz, even lower than that of the PZT thick films. Although the ferroelectric properties and the dielectric constant of the PZT/CFO thick film were degraded compared with a pure PZT thick film, the coexistence of a promising ferromagnetic and ferroelectric response with a lower loss indicated that the PZT/CFO film is a good candidate for future applications in a reasonably high frequency range.

Pb(Zr0.3Ti0.7)O3/PbTiO3 multilayer thin films for pyroelectric infrared sensor application

We report on the properties of Pb(Zr0.3Ti0.7)O3∕PbTiO3 (PZT/PT) multilayer thin films for pyroelectric infrared sensor application. The effect of various PZT and PT stacking sequences has been systematically studied. The optimal structure for pyroelectric infrared sensor application is found to be the multilayer 5PZT/4PT thin film which has the PZT and the PT layers deposited alternately. It is characterized that the 5PZT/4PT multilayer thin film has reduced dielectric constant and comparable pyroelectric coefficient and dielectric loss when compared with the pure PZT thin film. The interface induced space charge polarization and “pinched” ferroelectric hysteresis loop of the multilayer thin film have been discussed. A detectivity figure of merit of 2.1×10−5Pa−1∕2 at 30Hz has been characterized for the 5PZT/4PT thin film, which is better than that of the pure PZT thin film.

XPS characterization of xα-Fe2O3–(1−x)ZrO2 for oxygen gas sensing application

Abstract XPS characterization was carried out for the solid-solution xα-Fe2O3–(1−x)ZrO2 which is a potential low temperature oxygen gas sensing material. The significant variation of relative intensity of the two peaks in O 1s spectra provides us a unique way to estimate the oxygen vacancy concentrations of xα-Fe2O3–(1−x)ZrO2 with x ranging from 0 to 0.3. The estimation result from XPS coincides with the calculated values from the substitutional model. The relationship between the sensitivity of the fabricated oxygen gas sensor and the oxygen vacancy concentration in the sensing material was established.

Scanning homodyne interferometer for characterization of piezoelectric films and microelectromechanical systems devices

A scanning homodyne interferometer is developed based on the combination of a modulated Mach–Zehnder interferometer and a high-precision two-dimensional translation stage. Due to its ultrahigh out-of-plane resolution (down to pico meters) and wide bandwidth potential, the scanning interferometer is well suited for characterization of piezoelectric thin films as well as for vibration measurement of microelectromechanical systems devices such as micromachined ultrasonic transducers.

Alcohol sensor based on a non-equilibrium nanostructured xZrO2–(1−x)α-Fe2O3 solid solution system

Abstract Non-equilibrium nanostructured solid solution x ZrO 2 –(1− x )α-Fe 2 O 3 powders have been prepared using the high-energy ball milling technique, and their particle size, structural properties and alcohol gas properties have been systematically characterized using X-ray diffraction (XRD) and gas sensing measurements. Experimental results show that particle size of the powder is drastically milled down to less than 10 nm after 20 h of high-energy ball milling. Our modified structural model, □ 1/3 +ZrO 2 + y O 2 →Zr 1/3 4+ +2(1− y )O s 2− +4 y O s − ; with □ 1/3 denoting the 1/3 available octahedral sites of the corundum structure; for these non-equilibrium nanostructured solid solution x ZrO 2 –(1− x )α-Fe 2 O 3 materials can explain both the lattice expansion of these high energy milled sample as well as the charge neutrality in terms of additional oxygen dangling bonds at the particle surfaces. The screen-printed thick film gas sensors made from such mechanically alloyed materials demonstrate a very high gas sensitivity value of 1097 at 1000 ppm of alcohol gas in air. It is believed that the high gas sensitivity value is related to the enormous oxygen-dangling bonds at the particle surfaces.

Influence of annealing temperature on the band structure of sol-gel Ba0.65Sr0.35TiO3 thin films on n-type Si(100)

The annealing temperature effect on the band structure of Ba0.65Sr0.35TiO3 (BST) thin films coated on n-type Si(100) substrate was investigated by ellipsometry and x-ray photoelectron spectroscopy. The band energy shifts of sol-gel BST films annealed at different temperatures are dependent on their developed microstructure. Related to the amorphous BST films annealed at 600°C, the polycrystalline BST film annealed at 700°C exhibits narrow band gap, upwards-moved Fermi level, and downwards-moved conduction band minimum, which are believed to contribute the enhanced field emission of BST-coated Si field emitter arrays.

Hydrogen-sensitive I–V characteristics of metal–ferroelectric gas sensor device fabricated by sol–gel technique

Abstract A new type of metal–ferroelectric gas sensor device was fabricated on platinum-coated n-type silicon with an amorphous ferroelectric (Ba 0.67 Sr 0.33 )Ti 1.02 O 3 (BST) layer of thickness 125 nm prepared by the sol–gel technique and annealed at temperatures ranging from 400 to 650°C. A layer of palladium 50 nm thick was deposited through an evaporation mask, and a layer of gold 400 nm thick was then deposited to provide the electrical contact. Measurements were made for the current–voltage characteristics and their time dependence before and after the introduction of hydrogen gas. The devices showed good hydrogen sensitivity of greater than 30 nA at the operating temperature range of 230–270°C. The I – V curve shift of 750 mV to 4.6 V was detected under the influence of 5–1042 ppm of hydrogen gas in air. The effects of bias voltage and operating temperature on the response and recovery times were studied. A model is also proposed in this paper to explain the key role played by the hydrogen ions at the metal–ferroelectric heterostructure interface and the related large Schottky barrier height-lowering phenomenon.

Aging-induced giant recoverable electrostrain in Fe-doped 0.62Pb(Mg1∕3Nb2∕3)O3–0.38PbTiO3 single crystals

The aging effect of a 0.2mol% Fe-doped 0.62Pb(Mg1∕3Nb2∕3)O3–0.38PbTiO3 single crystal was investigated. The authors found that aging induced a double ferroelectric hysteresis loop and a giant recoverable electrostrain of 0.8% at an electric field of 1.2kV∕mm in the ⟨001⟩-oriented tetragonal crystal. This result is much higher than the strains obtained in crystals near the morphotropic phase boundary under the same field. Such aging effect is attributed to a point-defect-mediated reversible domain-switching mechanism. Large electrostrains (>0.55%) at 1.2kV∕mm for a broad temperature from room temperature to 160°C suggest that the aged single crystal is a promising electromechanical material.