Low Room-temperature Resistivity Research Articles

Transparent-conducting, gas-sensing nanostructures (nanotubes, nanowires, and thin films) of titanium oxide synthesized at near-ambient conditions

A template-based, electroless wet-chemical method for synthesis of nanotubes and nanowires of nanocrystalline anatase titanium oxide (titania) at 45 °C is reported. Single-nanowire electrical property measurements reveal low dc resistivities (7–21 × 10−4 Ω cm) in these titania nanowires. In the presence of 1000 parts per million of CO gas at 100 °C, the resistivity is found to increase reversibly, indicating low-temperature gas-sensing capability in these titania nanowires. Thin films of nanocrystalline anatase titania, deposited using a similar wet-chemical method, also have low room-temperature dc resistivities (6–8 × 10−3 Ω cm), and they are transparent to visible light. Nanostructure-properties relations, together with possible electrical conduction, optical absorption, and gas-sensing mechanisms, are discussed. The ability to fashion transparent-conducting and gas-sensing nanocrystalline anatase titania into nanotubes/nanowires and thin films at near-ambient conditions could open a wider field of applications for titania, including nanoelectronics, chemical sensing, solar cells, large-area windows and displays, invisible security circuits, and incorporation of biomolecules and temperature-sensitive moieties.

Low-resistivity, stable p-type ZnO thin films realized using a Li–N dual-acceptor doping method

A Li–N dual-acceptor doping method has been developed to prepare p-type ZnO thin films by pulsed laser deposition. The lowest room-temperature resistivity is found to be ∼0.93Ωcm, much lower than that of Li or N monodoped ZnO films. The p-type conductivity of ZnO:(Li,N) films is very reproducible and stable, with acceptable crystal quality. The acceptor activation energy in ZnO:(Li,N) is about 95meV. ZnO-based homostructural p-n junctions were fabricated by depositing an n-type ZnO:Al layer on a p-type ZnO:(Li,N) layer, confirmed by secondary ion mass spectroscopy. The current-voltage characteristics exhibit their inherent rectifying behaviors.

Composition dependent resistivity of thick film Ni(1−x)CoxMn2O4: (0 ≤ x ≤ 1) NTC thermistors

The fritless thick film NTC thermistor of composition Ni(1−x)CoxMn2O4: (0≤x≤1), synthesized by oxalate co-precipitation, were prepared by screen-printing on alumina substrate. The composition dependent room temperature resistivities, thermistor constant are investigated. The relation between logarithm of resistivity (ρ) and reciprocal of absolute temperature for the prepared thick film thermistor was almost linear for all the compositions studied. The thick film thermistor with ceramic composition Ni0.6Co0.4Mn2O4 was the best with high sensitivity and lowest room temperature resistivity. Highly temperature dependent effects are also observed for temperature below 340 K.

Novel organic magnetic conductors based on organochalcogen donors and an organic magnetic anion, TEMPO-NHCOCH 2SO 3−

We report here four novel purely organic magnetic conductors containing common electron donors together with an organic anion including an organic free radical, (TTF)(TEMPO-NHCOCH 2SO 3)·0.5H 2O ( 1), (TMTSF)(TEMPO-NHCOCH 2SO 3)·2H 2O ( 2), (BEDT-TTF) 3(TEMPO-NHCOCH 2SO 3) 2·2H 2O ( 3), and α-(BEDT-TTF) 3(TEMPO-NHCOCH 2SO 3) 2·6H 2O ( 4). The TTF complex was prepared by metathesis of (TTF) 3(BF 4) 2 and the PPh 4 salt of the anion ( 5). The other complexes were obtained by electrocrystallization of the corresponding donors with the salt 5 as an electrolyte. Crystal structures of 2–4 have been solved. The salts 1 and 2 show relatively high resistivities (>10 7 and 5 × 10 3 Ω·cm), whilst the BEDT-TTF salts 3 and 4 have rather low room temperature resistivities (1.2 and 5.1 Ω·cm) with small activation energies (0.041 and 0.050 eV). The magnetic behavior of complex 1 obeys the Curie-Weiss law ( C = 0.379 emu·K/mol and θ = −3.29 K) and those of the other complexes obey the 1-D Heisenberg model ( C/ J = 0.366/−0.32 for 2, 0.373/−0.81 for 3, and 0.380 emu·K·mol −1/+0.42 K for 4). The relationships between the crystal structures and the physical properties are discussed.

Realization of p-type ZnO films via monodoping of Li acceptor

p-Type ZnO thin films have been realized via monodoping of Li acceptor by adopting DC reactive magnetron sputtering. The lowest room-temperature resistivity was found to be 17.6Ωcm with a Hall mobility of 3.47cm2V–1s–1 and carrier concentration of 1.01×1017cm−3 for Li-doped p-type ZnO film deposited on glass substrate. The Li-doped ZnO film possessed a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region. Moreover, the effects of Li content on the crystallinity, electrical and optical properties of p-type ZnO films were discussed.

Effects of reducing and oxidizing atmospheres on the PTCR characteristics of porous n-BaTiO3ceramics by adding polyethylene glycol

Donor doped BaTiO3 (n-BaTiO3) ceramics were fabricated by adding polyethylene glycol (PEG) at 20 wt %. The effects of reducing and oxidizing atmospheres on the PTCR characteristics of the porous n-BaTiO3 ceramics were investigated. The PTCR characteristics of the porous n-BaTiO3 ceramics is strongly affected by chemisorbed oxygen at the grain boundaries and are recovered as the atmosphere is changed from the reducing gas to oxidizing gas. The low room-temperature resistivity of the porous n-BaTiO3 ceramics in reducing atmospheres may be caused by the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In addition, the high room-temperature resistivity of the porous n-BaTiO3 ceramics in oxidizing atmospheres may be caused by the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries.

P -type conduction in N–Al co-doped ZnO thin films

p -type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3Ωcm with a Hall mobility of 0.43cm2∕Vs and carrier concentration of 2.25×1017cm−3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region.

Experimental determination of intrinsic parameters in double-donor-doped(Ba0.92Sr0.08)TiO3-based positive temperature coefficient ceramics

In the present work, double-donor-doped(Ba0.92Sr0.08)TiO3-based positive temperature coefficient (PTC) ceramics were fabricated for potentialapplication in an over-current protector. The electrical properties of the materials were alsocharacterized. Using the experimental results obtained, theoretical models onelectrical properties were studied, and the intrinsic parameters were determined. Therelationship between the intrinsic parameters, the materials variables and theelectrical properties was discussed. It was noted that the addition of double donorsY2O3+Nb2O5 could result in relatively low room-temperature resistivity, in particular,30 Ω cm at the sinteringtemperature of 1230 °C. Within the investigated working temperature range, the grain-boundary potential barrierat a fixed measuring temperature decreased with increasing sample sintering temperature.The evaluated effective donor concentration and surface acceptor state density werein good agreement with the reported values in the literature. A parameter wasintroduced based on these two variables to represent the resistivity jump. Thedepletion layer thickness was determined to be in the submicron range. The presentwork is believed to be the first report to determine, based on the experimentaldata, all the essential intrinsic parameters for double-donor-doped PTC ceramics.

Structural Model of Ni/Graphite/BaTiO3Composites

Ni/graphite/BaTiO3 composites are fabricated by adding Ni and graphite powders into BaTiO3 calcined powder to achieve low room-temperature resistivity and better PTC effect. The resistivity of the composites reduced greatly with the increase of Ni, but the PTC effect deteriorates accordingly. The resistivity reduced slightly with the increase of graphite as it was less than 0.5 wt%, however it rose greatly after the graphite content exceeded 0.5 wt%, the graphite content had an optimum in this experiment. From the experimental results and regularities obtained, a structural model of the composites is proposed in this article. By employing this model the electrical properties and PTC effect of the composites can be explained properly.

Compositional and processing effects on electrical properties of (Ba 0.85Pb 0.15)TiO 3-based positive temperature coefficient resistors

In the present work, (Ba 0.85Pb 0.15)TiO 3-based positive temperature coefficient resistors (PTCRs) with low room-temperature resistivities were produced, and the effects of composition and processing conditions on materials electrical properties were investigated. For (Ba,Pb)TiO 3-based ceramics, it was noted that an addition of double donors Sb 2O 3+Nb 2O 5 led to a lower room-temperature resistivity than that of single donor Nb 2O 5. The resistivity followed to reduce when properly increasing the donor concentration and decreasing the acceptor (MnO 2) concentration. The correlation of sintering temperature with ceramic parameters, namely, density, grain size, room-temperature resistivity, resistivity jump, and room-temperature breakdown strength, was discussed. Metal Cr was added to ceramic matrix to further reduce the resistivity. A reducing sintering and a subsequent oxidation treatment processes were employed, and the influences of metallic content and processing conditions on composite density, room-temperature resistivity, and room-temperature breakdown strength were analyzed. Low room-temperature resistivity PTCRs were produced by controlling both Cr content and the processes.

Low-Temperature Iodine Heat Treatment of Quasi-One-Dimensional KFeS2 Crystals

Low-temperature iodine heat treatment of quasi-one-dimensional KFeS2 crystals was carried out. The resistivity along the c-axis is seen to decrease rapidly with increasing iodine-heat-treatment temperature and the nonmetallic temperature dependence was found to become weaker. The fluctuation-induced tunneling model was used to explain the reduction of the potential barrier at grain boundaries caused by the iodine heat treatment. In particular, samples subjected to iodine heat treatment at 100°C for 2 days exhibited a low room-temperature resistivity of approximately 10-1 Ωcm and a corresponding positive temperature gradient, which are evidence of the first observation of metallic behavior. Furthermore, the room-temperature Seebeck coefficient of these iodine-heat-treated samples was small, a value of +5.8 µV/K, and exhibited an almost linear temperature dependence that showed the samples to be metallic. Finally, although KFeS2 normally exhibits metallic conduction, the presence of grain boundaries formed during crystallization from the molten state is thought to have led to the nonmetallic conductivity observed in our experiments.

Sintering behavior and electrical properties of porous (Ba,Sr)(Ti,Sb)O 3 ceramics produced by adding corn-starch

Porous (Ba,Sr)(Ti,Sb)O 3 ceramics were fabricated by adding corn-starch (1–20 wt.%) and effect of sintering condition on the PTCR characteristics of the porous ceramics was investigated. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O 3 ceramics was decreased and increased with increasing sintering temperature and sintering time, respectively. The former was attributed mainly to the increase of grain size as well as the partial decrease of the porosity, and the latter was due to the increase of the electrical barrier height of grain boundaries. The sample sintered at 1450 °C for 1 h showed the excellent PTCR characteristics, i.e. the lowest room-temperature resistivity (8.4×10 Ω cm) with the high ratio of maximum resistivity to minimum resistivity (6.3×10 5).

Growth and characterization of OsO 2 single crystals

Large osmium dioxide (OsO 2) 1 1 It should be warned that the volatile osmium oxide is highly toxic and must be handled with extreme caution. single crystals with mirror-like prismatic facets have been successfully grown by the oscillating chemical vapor transport method. X-ray diffraction spectroscopy showed the tetragonal rutile structure and the lattice parameters were determined. The field emission energy dispersive X-ray analysis revealed the atomic ratio of Os to O to be about 1:2. The X-ray photoelectron spectra for both the Os 4f and O 1s core-level features exhibited strongly asymmetric line-shapes character. Temperature-dependent resistivity measurements indicated metallic character and a low room-temperature resistivity of ∼15 μΩ cm for the OsO 2 single crystals. The residual resistivity ratio was determined to be 50.

Positive temperature coefficient of resistivity effect in niobium-doped barium titanate ceramics obtained at low sintering temperature

A positive temperature coefficient of resistivity (PTCR) effect was observed for low temperature sintered Nb-doped BaTiO 3 ceramics using YB 6 as a sintering aid. A semiconducting BaTiO 3 with good PTCR property was obtained at a low sintering temperature of 1150 °C because of the addition of YB 6. Addition of 0.2 mol% Nb 2O 5 and 1 mol% of YB 6 led to a low room-temperature resistivity and a resistivity jump of about 10 4. Room-temperature resistivity of the ceramics was controlled by the grain boundary resistivity which was related to the added amount of YB 6 and the sintering temperature. YB 6 only worked as a sintering aid to enhance the densification of semiconducting BaTiO 3 ceramics at low sintering temperature.

Semiconducting barium titanate ceramics prepared by using yttrium hexaboride as sintering aid

Nb-doped BaTiO 3 semiconducting ceramics have been prepared at 1150 °C by employing YB 6 as the sintering aid. The low-temperature sintered samples exhibit a positive temperature coefficient of resistivity (PTCR) effect. The room-temperature resistivity and the magnitude of PTCR effect depend on the amount of YB 6 added, the Nb 2O 5 content and the sintering conditions. At 1 mol% of YB 6, the sample has a low room-temperature resistivity and a resistivity jump of about 10 4. Transmission electron microscope studies revealed that the intergranular region between the crystalline BaTiO 3 grains has an amorphous structure. Y, Ba and Ti are present in the intergranular region but no Y is detectable in the crystalline grains. A liquid phase is believed to be present during sintering and it helps to facilitate the densification process and to enhance the PTCR effect.

Highly conducting single-component crystals based on tetrathiapentalene (TTP) derivatives with four long alkylthio chains

TTP derivatives with four long alkylthio chains, TTC n -TTP (n= 6-8; 1a-c), have been prepared. A single-component crystal of TTC 6 -TTP shows moderately low room-temperature resistivity (8x10 4 Ω cm). X-ray analysis of TTC 6 -TTP indicates that uniform-stacked donor arrangement is the origin of the low resistivity. On the other hand, TTC 2 -TTP also has a uniform-stacked structure, but the TTP molecule is bending, which is disadvantageous for conductivity.

A novel method of preparing Ba 0.96Ca 0.04Ti 1.02O 3 ceramics with low room temperature resistivity

Using an ordinary ceramic processing technique, a new method of preparing Ba 0.96Ca 0.04Ti 1.02O 3 positive temperature coefficient resistors (PTCR) ceramics with low room temperature (r.t.) resistivity is introduced. The ceramics are prepared by mixing powder A, containing low concentration of donor dopants (0.12 mol.% Sb 2O 3), with powder B, containing high concentration of donor dopants (0.24 mol.% Sb 2O 3). As the weight ratio of A to B, A/B, is 10:1, the samples of PTCR ceramics with r.t. resistivity (∼5.0 Ω cm) are obtained. The ceramics exhibit PTC properties with ρ max/ ρ 25 as great as 10 4.1, and breakdown voltage as high as 120 V mm −1, which are much higher than those of the conventional BaTiO 3 ceramics with the same r.t. resistivity prepared from one kind of PTCR powder. With the aid of grain boundary theory, the influence of microstructure on electrical properties is discussed.

P-type conduction in codoped ZnO thin films

p-type conduction in ZnO thin films was realized by the codoping method. Types of conduction and carrier density in codoped ZnO films were found to be dependent on the oxygen partial pressure ratios in the sputtering gas mixture. The lowest room temperature resistivity was found to be 11.77 Ω cm with a hole density of 9.0×1016 cm−3 for the films deposited in 60% of oxygen partial pressure ratio. Codoped films were found to be c-oriented and highly transparent.

Positive temperature coefficient of resistivity effect in Pb-doped KnbO3

KnbO3 is a ferroelectric material with a Curie temperature (TC) at 415°C, thus giving it the potential to be a material for high-temperature positive temperature coefficient of resistivity (PTCR) applications. In this study, we investigated the PTCR effect in donor-doped KnbO3 ceramics containing 0, 0.1, 0.2, and 0.3 mol% PbO. The donor-doped KnbO3 ceramics exhibited a PTCR anomaly with a relatively low room-temperature resistivity. The temperature of the tetragonal-to-cubic phase transition (TC) of the KnbO3 decreased with the amount of added PbO, while the orthorhombic-to-tetragonal phase transition (TOT) remained unchanged.

Effect of additives and powder preparation techniques on PTCR properties of barium titanate

PTCR barium titanate has been prepared following two different powder preparation techniques: the solid-state reaction route and co-precipitation route for the comparison of results. The co-precipitation route produces better PTCR characteristics with low room temperature resistivity and better reproducibility of prepared samples. The effects of different concentrations of additives like donors, acceptors and excess titanium have been optimized to get good quality PTCR samples.