INCREASED CONTACT RESISTANCE IN ELECTRICAL NETWORKS AS A CAUSE OF FIRES

The occurrence of intermittences in electrical circuits can result in serious reliability problems particularly where low signal voltages and currents are used. It is well known that differential thermal expansion or vibration in electrical connectors can result in micromotion resulting in fretting corrosion. This can lead to an increase in contact resistance and eventual loss of electrical contact. The occurrence of electrical intermittences has been considered a precursor to contact failure associated with fretting corrosion and can cause disruption in digital circuit signals. We report the development of instrumentation that simultaneously measures the occurrence of electrical intermittences along with contact resistance during the fretting of electrical contacts. In addition, the measurement system records contact friction and normal force dynamically. Intermittences can be counted and timed with durations from 20 ns to milliseconds as a function of fretting cycles and correlated with the increase in contact resistance. All systems are integrated under LABVIEW computer control software. Measurements were made on Cu-Cu and Sn-Sn rider/flat combinations. Results will be interpreted in terms of the influence of wear debris on the electrical properties of the contacts.

One-inch test samples of bright and matte tin-lead platings over both copper and nickel substrates were examined after heat aging for various times at 150 degrees C in forming gas. The intermetallic compounds formed are somewhat harder and more brittle than the platings. Therefore, as the intermetallic growth approaches the outer surface of the platings, various surface properties may be altered. Generally, the coefficient of friction decreases with a corresponding increase in wearability and contact resistance. The formation of the intermetallic continues as long as there are sufficient amounts of the alloying constituents present. Samples aged to simulate a period of time for storage exhibited an increase in contact resistance when compared to as-received samples.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

It has been demonstrated that the formation of insulating material by mechanochemical reaction during make-break operations can lead to a rapid increase of electrical contact resistance in Pd contacts. To further examine this phenomenon the changes at contact surfaces during make-break were observed and the effects of impact and wipe, which exist at the closure of contacts, were studied separately. Observation shows the wear track spreads from a circle to an ellipse, and the frictional powder is scraped from wear track and gathers from particles to clusters. Additionally the results of impact and wipe experiments indicate that the formation of insulating material depends on wipe rather than impact. Frictional powder strongly affects the increase of contact resistance.

Although a surface oxide film on platinum-metal contacts is effective in preventing these contacts from polymerizing organic materials, it causes an increase in contact resistance. It is important to determine the thickness of surface oxide film which effectly prevents organic polymerization without causing a significant increase in contact resistance. Auger Electron Spectroscopy is used here to determine the thicknesses of the surface oxide films formed on Ru and Rh plated contacts at various temperatures in synthetic air. These are compared with results for untreated Ru and plated Rh contacts. Contact voltage versus contact resistance and contact force versus contact resistance characteristics of plated Ru and Ru oxidized after plating are measured by the crossed-rod method to study the relationship between contact resistance and thickness of a surface oxide film. The extent of organic polymerization on Ru and Rh plated contacts is determined by operating the contacts unloaded (dry) and monitoring the contact resistance after the treated and untreated contacts are exposed to benzene vapor. !t is found that, when the surface of the contacts is covered by a thin oxide film in either the case of plated Ru or plated Rh, the contact resistance can be low and stable, whereas contacts cleaned by Ar+ bombardment prior to sealing will be high in resistance. From these experimental results, it is determined that the thickness range of surface oxide film which prevents polymerizing without any measurable increase in contact resistance is approximately 5A ~25A.

This paper analyzes the effect of sliding speed on the electrical conductivity and friction properties of the contact pair of an on-load tap changer (OLTC). Reciprocating current-carrying tribological tests were carried out on a rod–plate–copper–tin–copper contact galvanic couple at different sliding speeds in air and insulating oil media. The results show that as the sliding speed increases from 24 mm/s to 119 mm/s, the average contact resistance in air increases from 0.2 Ω to 0.276 Ω, and the average contact resistance in insulating oil also increases from 0.2 Ω to 0.267 Ω. At 119 mm/s, the maximum contact resistance in insulating oil reaches 0.3 Ω. The micro-topography images obtained by scanning electron microscopy show that with the increase in sliding speed, the wear mechanisms in the air are mainly abrasive wear and adhesive wear, and the wear mechanisms in oil are mainly layered wear and erosion craters; high sliding speed and arcing promote contact surface fatigue and crack generation. X-ray photoelectron spectroscopy was used to analyze the surface. The copper oxide in the air and the cuprous sulfide in the insulating oil cause the surface film resistance, and the total contact resistance increases accordingly. In addition, the test shows that 119 mm/s in air and 95 mm/s in insulating oil are the speed thresholds. Below these speed thresholds, the increase in contact resistance is mainly caused by mechanical wear. Above these thresholds, the increase in contact resistance is mainly caused by arc erosion and chemical oxidation processes. Non-mechanical factors exacerbate the deterioration of the contact surface and become the main factor for the increase in contact resistance.

The article examines the change of the probability of an emergency mode (failure) of the vehicle's electrical wiring depending on the time of its operation. We revealed necessity of establishing the wear and operation time of electrical wiring in a vehicle. Emergency modes in the vehicle electrical network have been identified and analyzed. The behavior of some contact connections of a vehicle was studied for an increase in contact resistance. It is concluded that it is necessary to develop deterministic and stochastic mathematical models that describe the state of the vehicle’s electrical network in order to assess its fire hazard.

The thermal stability of nonalloyed Mo/Au and Ti/Pt/Au contacts on n+ InGaAs, ranging in thickness from 3 to 50 nm, was investigated for a heterojunction bipolar process. Layers of highly silicon doped InGaAs were grown on n+ GaAs by molecular beam epitaxy, without a compositionally graded layer between the InGaAs and GaAs. The as-deposited contact resistance exhibits a strong dependence on InGaAs thickness. For In0.5Ga0.5As, as the InGaAs thickness increases from 6 to 20 nm, the contact resistance decreases from 8×10−7 Ω cm2 to as low as 2×10−7 Ω cm2. Ohmic contacts with various InGaAs thicknesses, InGaAs composition, and contact metallurgies were heat treated at 300 °C for times of 2 h or more. The contact resistivity of MoAu on In0.5Ga0.5As contact layers that are thinner than 20 nm shows poor thermal stability, increasing as much as 160% for a 6 nm thick layer of InGaAs. Contacts on a pseudomorphic InxGa1−xAs layer of the same thickness, but with a lower In mole fraction of x=0.35, exhibit better thermal stability, increasing by only 15% in contact resistivity with heat treatment. Stable contacts can also be obtained by making the In0.5Ga0.5As thickness at least 20 nm. Cross-sectional transmission electron microscopy (TEM) of 20 nm InGaAs films heat treated at 300 °C for 2 h reveals a virtually unchanged Mo–InGaAs interface. TEM analysis of TiPtAu contacts shows that the Ti/InGaAs interface and the InGaAs layer thickness become increasingly nonuniform with thicker titanium layers. This physical degradation correlates to an increase in the contact resistivity. The increase in contact resistivity can be minimized by reducing the Ti thickness to 5 nm or less. For extended anneals at 300 °C for accumulated times of 397 h, MoAu on 20 nm of In0.5Ga0.5As demonstrates better stability in contact resistance than Ti (5 nm)/Pt/Au on 20 nm of In0.5Ga0.5As.

Mo/Al/Ti or TiN/TiSi2ohmic contacts were fabricated on AlGaN/GaN HEMT structures having various thicknesses of AlGaN layers. Optimum thicknesses depending on annealing temperature were found, which should be correlated with the contact formation mechanism. AlGaN/GaN HEMTs (high electron mobility transistors) have drawn lots of attention for high frequency and power applications owing to high mobility of 2DEG. One of the important issues of the devices is formation of ohmic contacts with low contact resistance. Since contact metal layers are usually placed on the AlGaN layer under which two-dimensional electron gas (2DEG) is induced, current passes through the AlGaN layer, a barrier layer for electrons, should be formed. Various models of the contact formation mechanism, such as the intrusion of metal into 2DEG [1] and formation of n-type region due to N vacancies [2], have been discussed. However, comprehensive understanding for the mechanism is still insufficient so far. We expect that optimum AlGaN thickness is present to obtain low contact resistance because of a trade-off between increase in the resistance by increasing the thickness due to thicker barrier and that by decreasing the thickness due to depletion of 2DEG. In this work, we revealed the prospect experimentally, and discussed mechanism based on the phenomenon. The wafers used in this study consist of 30-nm-thick undoped AlGaN with GaN layer epitaxially grown on Si (111) substrates. The top AlGaN layers were thinned by the cyclic etching in which oxidation in O3 ambient and removal by hydrofluoric acid were carried out, so that samples with various AlGaN thicknesses were prepared. Contact electrodes, Mo/Al/Ti (35/60/15nm) or TiN/TiSi2 (45/20nm) [3], were formed by sputtering followed by subsequent annealing (400-1100 oC for 1min. in N2) on the samples, as shown in Fig. 1. Contact resistances were evaluated by the TLM method. A typical characteristic of the contact resistance vs. AlGaN thickness for the case of relatively low annealing temperature (650oC) is shown in Fig.2. The optimum AlGaN thickness to get the lowest resistance, 11.6 nm in this case, was clearly observed. On the other hand, in the case of higher annealing temperature (950oC or 1100oC), the monotonous decrease in contact resistance with the thickness was observed as shown in Fig.3. The increase in contact resistance in the region thinner than 11.6 nm is speculated to be due to depletion of 2DEG, which is common to the both low and high temperature cases. While, the increase in contact resistance for thicker layers at low temperature annealing, as shown in Fig.2, is probably attributed to high resistivity in the AlGaN layer. However, in the case of high temperature annealing, as shown in Fig.3, some mechanism of the metal intrusion and/or the conversion to n-type to overcome the thick barrier layer should work effectively. In conclusion, dependences of contact resistance on thickness of the AlGaN layer were observed, and they indicated existence of the optimum thickness. Discussion of these properties correlated with mechanism of ohmic contact formation will be useful for further understanding of the contact technology for AlGaN/GaN HEMTs.

An increase in the contact resistance of a no-insulation (NI) high-temperature superconducting coil was observed in high-field tests, which may be related to the mechanical deformation and the separation between adjacent turns in the coil. The large electromagnetic force generated in the high magnetic field can causeseparation between adjacent turns of the NI coil, which can affect the contact resistance of the magnet. An electromagnetic–mechanical model is built to study the effect of separation on the contact resistance and field delay time of an NI layer-wound coil. The numerical results show that the large electromagnetic force generated in the high field leads to the local separation between adjacent turns and the increase in contact resistance of the NI layer-wound coil. Moreover, a higher external field or target current can result in a larger area of separation, a higher contact resistance and a shorter characteristic field delay time. An overband can restrain the mechanical deformation and separation between turns of the NI coil in the high field, which suppresses the increase of turn-to-turn contact resistance.

AlGaN/GaN high electron mobility transistors (HEMTs) have gained more attention recently because of the superior mobility (~1400 cm2/V-s) and high breakdown voltage. The quality of Ohmic contacts are key characteristics to guarantee the high mobility. During fabrication, Ohmic metals would be exposed to chemicals such as Buffer Oxide Etchant (BOE). BOE is a mixture of HF and NH4F, which is known to be reactive to metals. As a result, it is necessary to understand the effect of BOE treatment on Ohmic metals. The HEMT structures consisted of a 25 nm AlGaN barrier layer, a 0.8 µm C doped GaN buffer layer, a 1.4 µm AlGaN transition layer and a thin AlN nucleation layer on 300 µm Si substrate. HEMT fabrication began with Ti/Al/Ni/Au Ohmic metallization and rapid thermal annealing in N2 at 850ºC for 1 min. Ohmic metals were exposed to BOE for 3 mins. The resistance was monitored every 15 sec by transmission line method (TLM). Scanning electron microscopy (SEM) was used to observe the surface morphology change after BOE exposure. Energy dispersive X-ray spectrum (EDX) and Auger electron analysis was used to characterize the elemental change after BOE exposure. Figure 1 shows the effect of BOE exposure to contact resistance. As shown in Figure 1, the contact resistance increased from 1×10-4 Ω-cm2 to 2×10-4 Ω-cm2 after exposure to BOE for 2 minute. BOE solution is a solution which composed of NH4F and HF. Since the reaction of Ti, Al, and Ni to HF is spontaneous, the increase of contact resistance is expected. The sheet resistance doesn’t change much with BOE treatment time, indicating the GaN is more inert to BOE exposure. Table I shows the elemental change after BOE treatment analyzed by EDX. As shown in TABLE 1, Au decreased by 7% although it supposes to be not reactive to BOE. The content of Ti decreased by 0.1% and the contents for the rest of elements, Al, Ni and N, increased by 0.2, 1.3 and 0.6%, respectively. For EDX, the X-rays are generated in a region about 2 microns in depth of the sample. The thickness of the Ti/Al/Ni/Au Ohmic metallization was around 0.3 µm, and thus the EDX signals of Ga and N should be dominated by the bulk GaN even with some Ga or N out-diffusing through the alloyed Ohmic metallization. Thus, it is reasonable to assume that the Ga and N contents did not change significantly. The percent increases of Ga and N content after BOE treatment were due to the decrease of the Au content. By comparing the increases of percent content of Al, it was less than the percent increase of Ga or N. Therefore, besides Ti and Au, the content of Al also decreased after BOE treatment. However the increase of Ti/Al/Ni/Au Ohmic metallization contact resistance should be mainly caused by the decrease of Au content in the BOE treated sample. Although Au is inert to BOE, Au could be removed by the etching of Al underneath this layer. SEM is also used to observe the surface morphology change after BOE treatment. The Ohmic contacts can be further divided into three different regions, island, ring, and field area. Each island is around 3-5 µm and surrounded by a 1 µm-width Au ring. After BOE treatment, the width became thinner and the island became shorter. With the EDX analysis, Au/Al was found to mainly be distributed in ring area and Ni/Al in the island. Because Al is reactive to BOE, it might be etched and thus peeled off Au in the ring area. The etching of Au might be the reason of thinner width and the increase of contact resistance. In summary, the degradation of Ohmic metallization exposed in BOE was studied. The contact resistance of Ohmic metallization increased significantly after treatment in BOE for 2 minutes. Moreover, after annealing, there were island-like structures surrounded by Au-Al alloy rings and a field area between the islands. The BOE etching occurred mainly at the island and ring areas instead of the field area between the islands. The increase of sheet resistance was due to the etching of surface Al and Ti and the loss of Au in the island and ring areas. Figure 1

This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.

The surface degradation of c-axis oriented YBa2Cu3O7−δ thin films due to air, CO2, N2, O2, and vacuum exposure has been studied with reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy, and contact resistivity measurements. The formation of an amorphous surface reaction layer upon exposure to air and CO2 is monitored with RHEED and correlated with an increase in contact resistivity. The contact resistivity of samples exposed to air increases with time t as ρc = (1.0 × 10−7 Ω cm2)e√t/640 min. Surfaces exposed to CO2 show a similar degradation while surfaces exposed to N2 showed a slightly different degradation mechanism. Vacuum exposed surfaces how little increase in contact resistivity, indicating no long-term surface oxygen loss.

전기 분 배전반의 내부 회로 인 부스 바는 충전부의 노출로 인해 전기 사고의 원인이 되며 이로 인한 감전 사고와 단락 사고가 발생된다. 또한 동물 및 이물질 침투로 인한 전기 화재 사고가 발생되는 원인이 되기도 한다. 또한 확장이 불가능한 구조로 무리하게 연결하여 사용하므로 접촉 저항 증가 발생으로 과열 사고를 유발시키기도 하며, 절단과 절곡 등의 제조 공정상의 어려움을 가지고 있기 때문에 주문생산방식을 택할 수밖에 없는 현실이며, 긴급전기 사고 발생시 복구하는데 지연되는 원인이 되기도 한다. 본 논문에서는 이와 같은 문제점을 개선하기 위해서 분 배전반의 모선을 절단하여 개별 블록으로 절연 난연 케이스와 격벽 구조화로 안전성을 높여 모듈 블록화 하였으며, 확장성과 유연성을 구현하였으며, 쉽게 확장이 가능한 구조이며, 또한 쉽게 분 배전반을 조립할 수 있다는 것이며, 이와 같이 전기적 위험 요소를 제거한 전기 분 배전반의 내부 구조는 블록형 부스 바 구조를 가지고 메인차단기와 분기차단기간의 회로 형성 구현 방식의 분 배전반를 제안하고 공인시험기관의 시험을 통해 그 유용성을 입증 하였다. The internal circuit of the bus-bar for an electrical switch board is a prime cause of electric shock and short circuit accidents due to the exposure of live parts. Electrical fires can also be caused by animals and foreign substances in the switchboard that connect the components with a difficult structure resulting in overheating due to an increase in contact resistance. Preventing these types of accidents is a prime concern in the manufacturing process, such as cutting and bending. In this study, the cutting bus bar of a switch board contained improved modules as a flame retardant that isolates a separate blocks to prevent such problems. This was implemented as a scalable and flexible means of reducing electrical switchboard hazards to offer a safe switch board bus-bar structure of a new connecter type

In order to improve the long term reliability of lead-salt diode lasers, ohmic contacts of multilayer, thin-film structures consisting of In plus Au, Pt, Ni, and Pd have been studied. Diode lasers of PbSnTe fabricated with a variety of contacts were tested during room-temperature storage and during accelerated aging tests. The results show that contact reliablility can be improved when multiple overlapping films are used. After 4500 h of baking at 60 °C, lasers with In-Au-Pd-Au contacts on both sides showed the least resistance increase (10%). For lasers with In-Au-Pt-Au contacts, 1 h of baking at 60 °C is equivalent to 2 d storage at room temperature. Extrapolating these results, a 70% increase in contact resistance is expected for this type of laser after 9000 d of storage at room temperture. Our data also suggests that a smaller increase in contact resistance can be expected for lasers fabricated with In-Au-Ni-Au and In-Au-Pd-Au contacts.

This paper deals with a problem encountered during the development of a new gate electrode structure for MOS VLSI (Metal-Oxide-Semiconductor Very Large Scale Integrated Circuits), namely, the increase in contact resistance between the two layers in a Mo/poly Si gate electrode structure after annealing at around 1273 K. This study was carried out by using ESCA (Electron Spectroscopy for Chemical Analysis) to understand the above phenomenon and find the means to prevent this contact resistance increase.It was found that (1) the formation of Si oxide at Mo/poly Si interface was the cause of the increase in contact resistance, (2) the O (oxygen) necessary for this oxide formation was supplied from the Mo in contact with the poly Si, and (3) the oxide formation can be prevented by the addition of Si to the Mo, allowing the Si atoms to bond with the O atoms inside the Mo layer but not at the Mo/poly Si interface.