Low Room-temperature Resistivity Research Articles

Preparation of bixbyite phase (Mn xFe 1− x) 2O 3 for NTC thermistor applications

Spinel compounds like doped Mn 3O 4 and ferrites are conventionally used as negative temperature coefficient (NTC) thermistors. Incidentally, Mn 2O 3 has got two orders of magnitude lower room temperature resistivity than that of Mn 3O 4 and it is expected that doped Mn 2O 3 should perform better than doped Mn 3O 4 as NTC thermistor. However, Mn 2O 3 can not be fabricated in the ceramic form as it decomposes to Mn 3O 4 above 900 °C. We have found that iron, as solid solution, can stabilize Mn 2O 3 as bixbyite (Mn x Fe 1− x ) 2O 3 phase even at high temperatures We have successfully prepared phase-pure nano-size powders of (Mn x Fe 1− x ) 2O 3 (where x=0.98, 0.66 and 0.37) by citrate–nitrate gel method. After sintering the precursor powders at high temperature (1500 °C), their electrical behaviour was studied. It has been found that the NTC sensitivity index ( β) of the bixbyite phase (Mn x Fe 1− x ) 2O 3 (where x=0.37 and 0.66) can be as high as 6000 K (in the temperature range of 50–150 °C) which is appreciably higher than that of conventional NTC materials.

Effect of additives on the electrical properties of a (Ba0.92Sr0.08)TiO3-based positive temperature coefficient resistor

Abstract In the present work, the effects of the additives (BN, Y 2 O 3 +Nb 2 O 5 , Sb 2 O 3 +Nb 2 O 5 , and Cr) on the electrical properties of (Ba 0.92 Sr 0.08 )TiO 3 -based positive temperature coefficient resistor (PTCR) were investigated. It was noted that the addition of BN lowered the sintering temperature of the material system studied. A proper amount of BN could enhance the PTC effect, but the room-temperature resistivity would increase with increasing the content of BN. The addition of double donor Sb 2 O 3 +Nb 2 O 5 could lead to a lower room-temperature resistivity than that of Y 2 O 3 +Nb 2 O 5 . Metal Cr was also added to further reduce the resistivity. The low room-temperature resistivity and PTC behavior was achieved by controlling both the metallic Cr content and the process of reducing sintering and oxidation treatment.

The Variable Thermal Sensitivity of Strontium-Lead Titanate Semiconducting Ceramics

Strontium-lead titanate semiconducting ceramics with low room temperature resistivity (ρRT) and variable NTCR-PTCR composite effects were fabricated at different sintering conditions. Both ρRT and the NTCR effect decreased at the higher heating rate or doping excess of PbO. After the heat-treatment at 950°C, the transformations from PTCR to NTCR-PTCR characteristics were observed in excess PbO-doped samples. According to the results, it is assumed that the lead vacancies (VPb″) in the grain boundary layers degenerate the conductivity of strontium-lead titanate semiconducting ceramics and the NTCR behaviors below the Curie temperature are ascribed to the electron detrapping of acceptor defects with increasing temperature.

Influence of microstructure on the properties of Sr 0.5Pb 0.5TiO 3 V-shaped PTCR

Lanthanum-doped Sr 0.5Pb 0.5TiO 3 semiconducting ceramics were prepared by the solid state reaction technique with excess PbO additive. Samples' resistivities rise about 5 orders of magnitude above the Curie temperature ( T c≈115 °C) and their negative temperature coefficient of resistivity (NTCR) increases with an increase of the sintering temperature. The crystalline structure, microstructure and compositional distribution were investigated using XRD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with X-ray energy dispersive analyses (EDAX), respectively. The distribution of Pb, Sr and Ti in the grain boundary layer has been shown to be similar to that in the grain interior. It is assumed that the PbO additive compensates Pb loss and keeps the Pb/Ti ratio stable in the grain boundary layers, which results in the low room temperature resistivity and weak NTCR effects.

Processing characteristics of PTCR ceramics with low sintering temperature

The processing behavior of PTCR ceramics of (Ba,Sr,Ca,Pb)TiO3 solid solution composition with additives of lanthanum oxide (La2O3) and boron nitride (BN) was studied. The ceramics can be sintered at temperatures as low as 1100 °C and possess rather low room-temperature resistivity with good PTCR effect. The sample ball milled with de-ionized water exhibits a more uniform microstructure compared to the sample ball-milled with alcohol. Particle size of less than 1 μm was found to be adequate for preparing the ceramics and the finer particles (≤0.45 μm) do not significantly improve the PTCR behavior. The performance of the PTCR sample is not sensitive to the sintering parameters such as the sintering time and cooling rate. This may be ascribed to the presence of excess BaO in the sample and the low sintering temperature, thereby eliminating the effect of Ba ion vacancies on the properties of the PTCR sample.

Preparation and properties of PTCR ceramics with low resistivity sintered at low temperature

Two types of positive temperature coefficient of resistance (PTCR) ceramics with relatively low room-temperature resistivity were prepared using the four-component system (Ba, Sr, Ca, Pb)TiO3 with lanthanum oxide (La2O3) as the donor dopant and boron nitride (BN) as the sintering aid. The first type of materials was sintered at 1080 °C for 2 h. It has a Curie point \(T_c \sim 120 ^\circ C\), a room-temperature resistivity \(\rho _v\) of 58 Ω cm and a resistivity jump Δρ of \(3 \times 10^4 \Omega cm\) around \(T_c\). For the second type of materials that were sintered at 1100 °C for 20 min, \(T_c \sim 120 ^\circ C\), \(\rho _v = 19\Omega cm\) and \(\Delta \rho = 10^4 \Omega cm\). These PTCR ceramics are considered to be suitable materials for fabricating multilayer PTCR devices by the co-firing process. Factors associated with the composition that influence the PTCR property of the materials are discussed.

TiCl 3-doped Ba 0.92Ca 0.08TiO 3 positive temperature coefficient resistance ceramics with low room temperature resistivity

The influence of TiCl 3 solution on the room temperature resistivity and the electrical properties of Ba 0.92Ca 0.08TiO 3 positive temperature coefficient resistance (PTCR) ceramics were studied. The results indicate that the PTC effect can be improved significantly when an appropriate amount of TiCl 3 in solution is added to the raw materials. The carrier concentration ( N d), the surface charge density ( N s), and the barrier height at grain boundaries ( φ) were calculated by AC complex impedance analysis. The segregation of Ti 3+ at the grain boundaries is believed to have enhanced the PTC effect.

A structural model of Cr/(Ba,Pb)TiO 3 positive temperature coefficient composite

The over-current protector is one of the main applications of the positive temperature coefficient (PTC) thermistor. Low room-temperature resistance and a PTC effect are required for the use of the over-current protection. As a result, lowering the room-temperature resistivity of PTC materials becomes very important. From a Japanese patent, the method of adding metal to BaTiO3-based PTC ceramics to form composites has shown good results. But in recent publications, few papers were related to this area. Furthermore, in the limited literature, the resistance–temperature curve of the material expressed a negative temperature coefficient (NTC) effect when metal was present. In the present work, chromium (Cr), was added to (Ba,Pb)TiO3 ceramics to form PTC composite with higher Curie temperature (TC = 180°C). Under a given composition and method, the prepared composite had low room-temperature resistivity (p = 1.33Ωcm) and PTC effect (Pmax/Pmax = 10). From the experimental results obtained, a structural model of the composite is proposed. The co-function of metal and ceramics, and sintering atmosphere factor on the PTC effect are discussed in this model. By employing this model, the resistance–temperature properties of the composites can be explained satisfactorily.

On the growth of icosahedral Al-Pd-Mn quasicrystals from the ternary melt

Abstract Icosahedral Al71Pd21,Mn8 quasicrystals have been grown from the ternary melt via a ‘self-flux’ technique. The nucleation is exceptionally well controlled, and individual grains can have masses of up to almost 30 g (limited only by the dimensions of the growth crucible). The quasicrystals grown by this technique are readily separated from the melt by decanting and clearly manifest a dodecahedral morphology with pentagonal facets. X-ray and electron-diffraction data indicate a remarkably well-ordered structure. with a correlation length of over 1000 A. The magnetic susceptibility has a temperature-independent diamagnetic component and a temperature-dependent paramagnetic term. For temperatures greater than approximately 60 K, the paramagnetic term follows a Curie-Weiss temperature dependence characterized by an average effective moment of 0.62 ± 0.02μB/Mn and a Weiss temperature of −12.8 ± 0.1 K. Deviations from the Curie-Weiss temperature dependence for temperatures less than approximately 60 K may be due to the formation of local (short range) magnetic correlations. The electrical resistivity has a broad maximum at approximately 90 K and a room-temperature value of 1.57 ± 0.05 mΩ cm. We note that the previously observed empirical trend that a very high resistivity is associated with the highest quality Al–Pd–Mn quasicrystals does not seem to be in general true and that these high-quality (as demonstrated by X-ray and electron diffraction) ‘flux-grown’ samples have a relatively low room-temperature resistivity. In addition to the ternary icosahedral Al–Pd–Mn quasicrystals, a novel pseudo-quaternary icosahedral quasicrystal, Al67,Ga4Pd21,Mn8, is described.

Effect of BaO and SiO2 addition on PTCR BaTiO3 ceramics

The influence of Ba-excess and liquid phase sintering with SiO2 on the electrical conduction and microstructure in PTCR BaTiO3 has been investigated. Dense (95–96%), small grain (5–10 μm) PTCR materials were obtained in Ba-excess (Ba/Ti = 1.006) BaTiO3. The materials exhibit low room temperature resistivity ρRT (100–102 Ω·cm) and high PTCR respond (more than 5 orders). Solid state sintering was found to inhabit the semiconducting and PTCR behavior in Ba-excess materials. Liquid phase sintering, using SiO2 in the Ba-excess BaTiO3, resulted in low ρRT and significant PTCR response. Through domain observation, interior “Polaron deficient zones” were found in samples which exhibit limited liquid phase sintering, leading to non-uniform directional domains and low charge carrier mobility. Proper control of the SiO2 concentration was found critical for obtaining uniform directional domain microstructures and low ρRT.

Atomic-Scale Structure and Properties of Thin Epitaxial SrRuO3 Films Grown on SrTiO3 (100) by Pulsed Laser Deposition

AbstractTransition metal oxides with perovskite structure exhibit a wide range of electrical properties. SrRuO3 (SRO), a perovskite oxide with high conductivity, finds applications as an electrode layer in ferroelectric devices and as a barrier layer in superconductor-normal metal-superconductor (SNS) Josephson junctions involving high Tc superconductors. Here we report on the structure and properties of thin SRO films on SrTiO3 (100) substrates grown by pulsed laser deposition. AFM investigations revealed that the SRO films exhibit island growth. The island size increases with thickness, and at a typical thickness of about 20 nm a connected network of islands is formed. The resistivity of the SRO films grown at different substrate temperatures (700 °C - 850 °C) exhibits metallic behavior with a ferromagnetic transition at 150 K. The films with typical thicknesses of 20 nm, grown at a substrate temperature of 775 °C exhibited the lowest resistivity of 200 μΩ-cm at 300 K, whereas the films grown at lower (700 °C) and higher (850 °C) temperatures showed a much higher resistivity at 300 K. By AFM and high resolution TEM investigations, this behavior is correlated with the atomicscale structure of the films. The film with the lowest room temperature resistivity showed an almost defect-free orthorhombic structure, whereas the film grown at 850 °C contained structural defects in high density, such as SrO-rich antiphase boundaries and twins located at the boundaries between the islands. The films grown at 700 °C showed a cubic perovskite structure, which is obviously the reason for their high resistivity.

Role of substrates for heteroepitaxial growth of low room-temperature resistivity RuO2 thin films deposited by pulsed laser deposition

Conductive RuO2 thin films were grown on different substrates by pulsed laser deposition. Films deposited on MgO are of (110) orientation and of polycrystalline nature, while highly crystalline RuO2 films can be heteroepitaxially grown on LaAlO3 and yttria-stabilized zirconia (YSZ) substrates under proper processing conditions. The crystalline RuO2 thin films show metallic resistivity versus temperature characteristics and have a room-temperature resistivity of 35±2 μΩ cm. A residual resistivity ratio (ρ300 K/ρ4.2 K) of around 5 has been achieved for the RuO2 thin films grown at a substrate temperature of 700 °C on both LaAlO3 and YSZ.

Characterization of highly oriented (110) TiN films grown on epitaxial Ge/Si(001) heterostructures

The characteristics of epitaxial growth of titanium nitride films on Ge/Si(001) have been studied. The growth of titanium nitride and germanium films on (001)Si was carried out in situ in a high vacuum chamber (<10−7 Torr) using a multitarget stage in a pulsed laser deposition system. Electrical resistivity, stoichiometry, crystallinity, and epitaxial relationships as a function of deposition temperature have been studied. Electrical resistivity of the titanium nitride films grown at deposition temperatures in the range of 450 °C–750 °C was measured using a four-point probe. The stoichiometry of these films was investigated using Auger electron spectroscopy and Raman spectroscopy. The crystalline quality and epitaxial nature of TiN films grown at different substrate temperatures were characterized using x-ray diffraction and transmission electron microscopy. Highly oriented titanium nitride films with (110) orientation were obtained on Ge(001) film when the substrate temperature was maintained between 550 °C and 650 °C. The epitaxial growth of the titanium nitride films was found to be a function of two-dimensional or three-dimensional growth mode of germanium film on silicon (001) substrate. Titanium nitride films grown at a substrate temperature of 650 °C exhibited the lowest room temperature resistivity (26 μΩ-cm), highest nitrogen content (close to stoichiometry), and the best epitaxiality with the Ge(001) films on Si(001). The epitaxial relationships for the TiN/Ge/Si(001) heterostructure are found to be [001]TiN‖ [110]Ge‖ [110]Si and [110]TiN‖ [110] Ge‖ [110]Si. To explain the epitaxial growth in a large mismatch system (∼28%) such as TiN/Ge(001), the domain matching mechanism is proposed. Domains of size four (001)TiN by seventeen (220)TiN in the titanium nitride film match closely with domains of size three (220)Ge by sixteen (220)Ge in the germanium film, respectively. The lattice matching epitaxy involving a 4% mismatch between Ge and Si provides a mechanism for epitaxial growth of Ge on Si(001).

Heteroepitaxial growth of highly conductive metal oxide RuO2 thin films by pulsed laser deposition

Highly conductive ruthenium oxide (RuO2) has been epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The RuO2 film is (h00) oriented normal to the substrate surface. The heteroepitaxial growth of RuO2 on LaAlO3 is demonstrated by the strong in-plane orientation of thin films with respect to the major axes of the substrate. High crystallinity of RuO2 thin films is also determined from Rutherford backscattering channeling measurements. Electrical measurements on the RuO2 thin films demonstrate a quite low room-temperature resistivity of 35±2 μΩ cm at deposition temperatures of above 500 °C.

BaTiO3-based positive temperature coefficient of resistivity ceramics with low resistivities prepared by the oxalate method

Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4Ω cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm−1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature.

Influence of the preparation conditions on the magnetic properties and electrical resistivity of Fe 73.5Nb 3Cu 1Si 13.5B 9 nanocrystalline alloys

We present the magnetic properties and the resistivity values of a series of Fe 73.5Nb 3Cu 1Si 13.5B 9 alloys quenched from different melting temperatures and using different spin velocities. Some nanocrystals with a strong preferred orientation can be obtained directly from the melt at low velocities. Such partially crystalline samples show a lower room-temperature magnetostriction and resistivity than the amorphous ones. They also show a retardation in the nanocrystallization process, but the magnetic properties are poorer than those of the high-velocity samples, both in the as-quenched state and after complete nanocrystallization.

ErSb/GaSb(001) and GaSb/ErSb/GaSb(001) heterostructures and [ErSb,GaSb] superlattices: Molecular beam epitaxy growth and characterization

Successful growth of ErSb(001) single crystal layers on GaSb(001) substrates has been demonstrated. The reflection high-energy electron diffraction patterns show a (4×4) surface reconstruction. Reflection high-energy electron diffraction oscillations, x-ray diffraction, and Rutherford backscattering with channeling indicate single crystal monolayer-by-monolayer growth and continuity of the Sb sublattice at the ErSb/GaSb interface. The ErSb has a low room temperature resistivity equal to 30 μΩ cm but may be used as a metallic reflector only for wavelengths greater than 2.4 μm. The overgrowth of GaSb on ErSb leads to mirrorlike surfaces but the overlayers contain symmetry-related defects. On the contrary, nearly perfect GaSb overlayers were grown on [ErSb,GaSb] superlattices which exhibit metallic behavior.

An investigation of the stability of copper germanide thin films in the presence of Si and SiO2

Copper germanide (Cu3Ge) has received interest in recent years as a potential metallization for VLSI applications due to its exceptionally low room temperature resistivity. We have investigated the thermal stability of Cu3Ge thin films on both silicon and thermally oxidized silicon wafers. Films were deposited by electron beam evaporation of sequential layers of Ge and Cu and exposed to an annealing schedule ranging from 100°C to 450°C. Secondary ion mass spectrometry (SIMS) analysis has revealed an interaction of the film with the silicon substrate. At temperatures as low as 200°C, diffusion of silicon into the copper germanide film was observed with the concentration and depth of penetration scaling with increased annealing temperature. Results on controlling this interaction will also be presented and correlation is made with resistivity measurements.

Subpicosecond carrier response of unannealed low-temperature-grown GaAs vs temperature

The subpicosecond carrier lifetime of an unannealed molecular beam epitaxial layer of GaAs grown at ∼210°C has been demonstrated between 10 and 290K through optoelectronic switching and all-optical pump-probe measurements. The low room-temperature resistivity of the as-grown layers, believed to arise from hopping conductivity through defect sites, has been observed to increase as the sample temperature was lowered, allowing ultrafast switching measurements to be performed using the low-temperature-as-grown GaAs as a photoconductive element. After illumination by 100 femtosecond optical pulses, photogenerated carriers in the sample have rapidly relaxed, returning the material to its high-resistivity state in less than 1 ps. This indicates that the precipitates present in post-annealed samples are not required for the fast relaxation of photoexcited carriers in low-temperature-grown GaAs. Ultrafast switching measurements on a post-annealed version of the GaAs film also resulted in the generation of similar, 0.6 ps full width at half maximum pulses.

Superconducting Bi-Sr-Ca-Cu-O films prepared by DC magnetron sputtering on new polycrystalline substrates

Superconducting thin films of Bi-Sr-Ca-Cu-O have been deposited for the first time onto several new stabilized binary oxide solid solution systems:MgO:CaO, MgO: ZrO2, ZrO2:Y2O3, and ZrO2:CaO. The films were grown at substrate temperatures of up to 750 degrees C. Immediately after deposition on polished substrates the films were crystalline and had quite a low room temperature resistivity; they developed superconductivity after heat treatment at temperatures in excess of 840 degrees C for a few minutes. The resistivity data indicated zero values between 45 K and 75 K depending on the substrate material and annealing conditions. It is concluded that stabilized zirconia and magnesia ceramics provide suitable substrates for the preparation of good quality films.