Permafrost and terrain conditions at northern drilling-mud sumps: Impacts of vegetation and climate change and the management implications

Permafrost can provide a containment medium for drilling wastes deposited to in-ground sumps, but tall shrubs may proliferate on covers causing snow to accumulate, active layers to deepen and the ground to thaw. We evaluate these effects using a 2-dimensional heat transfer model to simulate the thermal evolution of sumps in warm (− 3.0 °C mean annual ground temperatures (MAGT)) and cold (− 6.0 °C MAGT) permafrost under varying snow and climate conditions characteristic of the Mackenzie Delta region. Application of climate and snow normals for Inuvik, Northwest Territories, south of treeline, and Tuktoyaktuk, on coastal tundra, maintained wastes within frozen ground at temperatures below − 1.5 °C in warm permafrost and − 3.0 °C in cold permafrost, respectively. A gradual increase in snow depth from 0.17 m to 1.5 m simulating the effect of shrub growth on snow accumulation, caused thawing by the third decade. In the absence of shrub growth and increasing snow, moderate climate warming (0.09 °C/year) also caused sump thawing after 35 years for the warm scenario, but for the cold scenario wastes remained below − 2 °C through to year 40. Climate warming and increasing snow depths hasten thermal degradation. Modeling results indicating sump degradation due to deepening snow were corroborated by snow and ground temperature measurements, observations of collapsed shrub covered sumps in the Mackenzie Delta region and the local absence of permafrost where deep snow accumulates over mineral soils. Although thawing increases the mobility of sump contents, the associated subsidence of the sump and adjacent areas may inhibit lateral movement of the wastes. Several factors combine to influence the integrity of sumps in permafrost indicating the need for a long-term management strategy.     

In-situ test study on the cooling effect of two-phase closed thermosyphon in marshy permafrost regions along the Chaidaer-Muli Railway, Qinghai Province, China

More than half of the recently built 142-km long Chaidaer-Muli Railway (CMR) in northern Qinghai Province, China travels across warm (≥−1 °C), ice-rich permafrost in wetlands on the southern flank of the Qilian Mountains. In comparison with the Qinghai-Tibet Railway from Golmud to Lhasa, the CMR traverses mostly across wetlands underlain by more ice-rich permafrost. Warm and ice-rich permafrost is sensitive to human activities and environmental changes, which can result in changes in the active layer thickness and permafrost temperatures, inducing instability and failure of infrastructures in permafrost regions. Thermosyphons were adopted in a quarter of the whole CMR route. For studying the cooling effect of the thermosyphon technique, two monitoring sites with different mean annual ground temperatures were installed since 2007. According to analysis of the ground temperature monitoring results from 2007 to 2010, the thermosyphon technique cooled the underlying permafrost and raised the permafrost table. The CMR has been put in operation since February 2010. The deformation monitoring data from 2008 to 2010 showed that the maximum accumulated settlement was 0.08 m and the minimum was 0.01 m. The settlements mainly happened in the initial months after the embankment construction was finished. In-situ monitoring results indicate that the thermosyphon technique has effect on cooling down the underlying permafrost and keeping the thermal stability of embankment in the unstable, marshy and ice-rich cold regions.     

The thermal effect of differential solar exposure on embankments along the Qinghai-Tibet Railway

The difference in solar radiation produces a predictable thermal effect on the sunny and shaded slopes of embankments constructed in permafrost regions of the Qinghai-Tibet Highway and Railway, which results in differences in soil temperatures and the permafrost table under the shoulder. From the period 2005 to 2008, a systemic network of 42 sites was established along the Qinghai-Tibet Railway to monitor permafrost conditions and embankment performance. Soil temperatures up to 20 m in depth under the embankment were continuously measured hourly. In this article, we investigate daily mean soil temperatures under embankments for 25 observed sites and the temperature difference under the shoulder for both the sunny and shaded slopes of the embankments. We found significant differences in the thermal effect from the sunny and shaded slopes of the embankments along the Qinghai-Tibet Railway. On the sunny slope of the embankment the cooling process of soils under the shoulder is shorter and the thawing process is longer by 15-30 days than on the shaded slope. The multiyear average temperatures under the shoulder for the sunny slope are higher—0.23-1.58 °C with an average of 0.86 °C—than those for the shaded slope. The temperature differences in winter (DJF) are much larger than those in summer (JJA). The multiyear mean permafrost table under the shoulder for the sunny slope of the embankment is larger than for the shaded slope, ranging from 0.1 to 3 m, with an average of 1.13 m. However, engineering measures can effectively reduce the thermal effect between the sunny and shaded slopes of embankments, resulting in a decreasing temperature difference that ranges from 0.46 to 0.71 °C, with an average of 0.58 °C.     

Modelling temperature-dependent heat production over decades in High Arctic coal waste rock piles

Subsurface heat production from oxidation of pyrite is an important process that may increase subsurface temperatures within coal waste rock piles and increase the release of acid mine drainage, AMD. Waste rock piles in the Arctic are especially vulnerable to changes in subsurface temperatures as the release of AMD normally is limited by permafrost. Here we show that temperatures within a 20 year old heat-producing waste rock pile in Svalbard (78°N) can be modelled by the one-dimensional heat and water flow model (CoupModel) with a new temperature-dependent heat-production module that includes both biological and chemical oxidation processes and heat source depletion over time. Inputs to the model are meteorological measurements, physical properties of the waste rock material and measured subsurface heat-production rates. Measured mean annual subsurface temperatures within the waste rock pile are up to 10 °C higher than the mean annual air temperature of −5.8 °C. Subsurface temperatures are currently decreasing with 0.5 °C per year due to decreasing heat production, which can be modelled using an exponential decay function corresponding to a half-life period of pyrite oxidation of 7 years. Simulations further suggest that subsurface temperatures two years after construction of the pile may have been up to 34.0 °C higher than in 2009 and that the release of AMD may have been more than 20 times higher. Sensitivity simulations show that maximum temperatures in the pile would have been up to 30.5-32.5 °C lower and that the pile would have been frozen 12-27 years earlier if the pile had been initially saturated with water, constructed with a thickness half of the original or a combination of both. Simulation show that the pile thickness and waste rock pyrite content are important factors controlling the internal build up of heat leading to potential self-incineration. However, site specific measurements of temperature-dependent heat production as well as simulation results show that the heat produced from pyrite oxidation alone cannot cause such a temperature increase and that processes such as heat production from coal oxidation may be equally important.     

Design and construction of a large-diameter crude oil pipeline in Northeastern China: A special issue on permafrost pipeline

The design and building of a pipeline in permafrost regions challenge engineers and scientists in many regards, and the geohazards resulting from the (differential) frost heaving and thaw settlement of the pipeline foundation soils present one of the most daunting tasks. The China-Russia Crude Oil Pipeline, a spur line from the Siberia-Pacific Pipeline System, presented unique scientific and engineering problems because of: 1) extensive presence of the more ice-rich permafrost in boreal forests and swamps; 2) an insistence on a buried construction mode because of concerns about the potential for frequent forest fires and other safety issues; 3) great uncertainties in the temperatures of oil being transported although the given estimated oil temperature of − 6.4 to + 3.6 °C entering the Mo'he Pump Station, and the estimated oil temperatures could vary from about − 6 to + 10 °C along the southward pipeline route; 4) the limited lead time for detailed surveys on engineering geology along the pipeline routes and for engineering design; 5) very much limited investment and a limited number of engineers experienced in designing and building a major pipeline in an area where about one-half of its length would be impacted by generally warm (− 3 to 0 °C) permafrost. Nevertheless, the pipeline engineers and permafrost scientists strived to economically build and satisfactorily operate the first major crude oil pipeline in the boreal ecosystem in China. The major results on the formation mechanisms and mitigative measures for the (differential) frost heave and thaw settlement were presented in the eight papers in this special issue on permafrost pipeline, and one additional paper on the Golmud-Lhasa Oil Products Pipeline on the Qinghai-Tibet Plateau was also included. They may provide insights to the understanding of pipeline-permafrost interactions and benefit the future design and construction of pipelines in similar northern environments.     

Characteristics and mechanisms of embankment deformation along the Qinghai-Tibet Railway in permafrost regions

Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai-Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT <− 1.5 °C, embankments only experienced seasonal frost heaves, and of which the magnitudes were not significant. So, the embankments in the regions performed satisfactorily. Whereas in regions with MAGT ≥− 1.5 °C, both traditional embankment and crushed rock embankment experienced settlements, but characteristics and mechanisms of the settlements were different for the two kinds of embankment. For crushed rock embankment, the magnitudes of settlement and differential settlement between right and left embankment shoulders were not significant and increased slowly. In respect that upwards movements of permafrost tables and better thermal stability of permafrost beneath embankment, mechanism of settlements on the embankment was inferred as creep of warm and ice-rich layer often present near permafrost table. While for traditional embankment, particularly in warm and ice-rich permafrost regions, the magnitudes of settlement and differential settlement between right and left embankment shoulders were significant and still increased quickly. Considering underneath permafrost table movements and permafrost temperature rises, mechanisms of settlements on the embankment included not only creep but also thawing consolidation of underlying permafrost. Therefore, some strengthened measures were needed to ensure long-term stability of these traditional embankments, and special attention should be paid on temperature, ice content and applied load within the layer immediately beneath permafrost table since warming and thawing of the layer could give rise to considerable settlement.     

Effects of post-deposition surface treatment on the optical, structural and hydrogen sensing properties of TiO2 thin films

TiO₂ thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO₂ thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO₂ thin films were tested as sensors of hydrogen at concentrations 10,000-1000 ppm and operating temperatures within the 180-200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.     

Cooling effects study on ventilated embankments under the influence of the temperature differences between the sunny slopes and the shady slopes

Based on theories of heat and mass transfer, three-dimensional theoretical and numerical models were presented to analyze the temperature characteristics of embankments in permafrost regions. Conditions of air convective within crushed rock layer and the background of the climate warming in the future were considered. In addition, temperature differences between the sunny and shady slopes of embankments, as well as the uneven distribution of air temperatures inside the ventilated duct were also considered in the models. Cooling effects and temperature field characteristics of three kinds of embankments were analyzed and compared. Results of traditional embankment indicated that there were significant differences in permafrost tables between the sunny and shady slopes, and the underlying permafrost was in serious degradation. A large asymmetric 0 °C melting bulb was formed in and beneath the embankment over winter time. The analysis indicated that the duct-ventilated embankment could lower the temperature of underlying permafrost, and elevate the permafrost table under the embankment. The duct-ventilated embankment could also adjust the differences of permafrost tables, and improve the asymmetry of the temperature fields between the sunny and shady slopes. Due to climate warming, however, a 0 °C melting bulb still developed at the sunny slope foot of the duct-ventilated embankment. To prevent the 0 °C melting bulb from occurring, a closed crushed rock revetment was applied on the sunny slope, and the results indicated that the cooling effect of duct-ventilated embankment with the closed crushed rock revetment was better than that without the closed crushed rock revetment. Soils in and beneath the duct-ventilated embankment with closed crushed rock revetment could refreeze completely over winter time and this kind of embankment could also effectively adjust the differences of permafrost tables between the sunny and shady slopes.     

Estimates on thermal effects of the China-Russia crude oil pipeline in permafrost regions

Using the finite element method, thermal effects of the Chinese-Russian crude oil pipeline in permafrost regions were estimated under two construction modes (conventional burial and aboveground embankment), two thermal control techniques (bare and insulated pipe), and three climate conditions along the route (marked under the natural mean ground surface temperatures of − 0.5, − 1.0, − 1.5 °C, respectively). The results show that with the silt embankment on gravel berm and the insulated pipe, the thaw of permafrost under the pipeline could be prevented during its service life. However, thawing of the permafrost under the conventionally buried pipeline cannot be prevented regardless of climate conditions or thermal insulation around the pipe. So it is strongly recommended that over-excavation and/or thermosyphons be applied for the conventional burial construction mode, especially in the warm and ice-rich permafrost sections to mitigate frost heave and thaw settlement in the subsoil.     

Effects of annealing temperature on crystallisation kinetics and properties of polycrystalline Si thin films and solar cells on glass fabricated by plasma enhanced chemical vapour deposition

Solid-phase crystallisation of Si thin films on glass fabricated by plasma enhanced chemical vapour deposition is compared at different annealing temperatures. Four independent techniques, optical transmission microscopy, Raman and UV reflectance spectroscopy, and X-ray diffraction, are used to characterise the crystallisation kinetics and film properties. The 1.5 μm thick films with the n+/p−/p+ solar cell structure have incubation times of about 300, 53, and 14 min and full crystallisation times of about 855, 128, and 30 min at 600 °C, 640 °C, and 680 °C respectively. Estimated activation energies for incubation and crystal growth are 2.7 and 3.2 eV respectively. The average grain size in the resulting polycrystalline Si films measured from scanning electron microscopy images gradually decreases with a higher annealing temperature and the crystal quality becomes poorer according to the Raman, UV reflection, and X-ray diffraction results. The dopant activation and majority carrier mobilities in heavily doped n+ and p+ layers are similar for all crystallisation temperatures. Both the open-circuit voltage and the spectral response are lower for the cells crystallised at higher temperatures and the minority carrier diffusion lengths are shorter accordingly although they are still longer than the cell thickness for all annealing temperatures. The results indicate that shortening the crystallisation time by merely increasing the crystallisation temperature offers little or no merits for PECVD polycrystalline Si thin-film solar cells on glass.     

Influence of annealing temperature on the structural, mechanical and wetting property of TiO2 films deposited by RF magnetron sputtering

TiO₂ films have been deposited on silicon substrates by radio frequency magnetron sputtering of a pure Ti target in Ar/O₂ plasma. The TiO₂ films deposited at room temperature were annealed for 1 h at different temperatures ranging from 400 °C to 800 °C. The structural, morphological, mechanical properties and the wetting behavior of the as deposited and annealed films were obtained using Raman spectroscopy, atomic force microscopy, transmission electron microscopy, nanoindentation and water contact angle (CA) measurements. The as deposited films were amorphous, and the Raman results showed that anatase phase crystallization was initiated at annealing temperature close to 400 °C. The film annealed at 400 °C showed higher hardness than the film annealed at 600 °C. In addition, the wettability of film surface was enhanced with an increase in annealing temperature from 400 °C to 800 °C, as revealed by a decrease in water CA from 87° to 50°. Moreover, the water CA of the films obtained before and after UV light irradiation revealed that the annealed films remained more hydrophilic than the as deposited film after irradiation.     

Forecasting the oil temperatures along the proposed China-Russia Crude Oil Pipeline using quasi 3-D transient heat conduction model

The China-Russia Crude Oil Pipeline (CRCOP) faces significant challenges due to differential frost heaving and thaw settlement resulting from significant variations of oil temperatures along the pipeline. Oil temperature distribution along the pipeline during the long-term operation period is a very important factor in pipeline foundation design under future climate warming and various frozen soil conditions. It is important for the assessment and prediction of differential frost heave and thaw settlement of the pipeline foundations soils, forecasting the development of the seasonal and inter-annual frozen and thawed cylinders around the operating pipeline, stress-strain analysis of the pipeline, and mitigation of subsequent frost hazards. A quasi three-dimensional computational model was developed to predict the oil temperature along the pipeline. It was verified by analytic solutions of the minimum oil temperatures along the route provided by the Daqing Oilfield Engineering (DOE) Co. The oil temperatures were predicted and analyzed for two proposed annual oil flow rates of 15 million tons (0.3 mbpd) and 30 million tons (0.6 mbpd) with and without mitigative measures (only pipe insulation was considered here) during the operation period. Also, the inter-annual variations of oil temperature at key typical locations were investigated to understand the impact of climate warming. The results indicated that the maximum oil temperature cools southwards, but the minimum oil temperature warms southwards (with the inlet oil temperatures from − 6 to +10 °C). However, the average annual oil temperature decreases southwards in the northern part of the pipeline, then it starts to slowly increase. The amplitudes of oil temperature change will decrease southwards. Oil temperatures will slightly increase with elapsing time due to the imposed boundary conditions of climate warming. The oil temperatures with a lower flow rate vary more significantly than that with a higher flow rate because the oil temperature with a low flow rate is more affected by the thermal regime of the surrounding soils and the external environments. Insulation around the pipeline tends to reduce the oil temperature variations along the pipeline during pipeline operation period. Therefore, pipe insulation can effectively reduce the development of frozen and thawed cylinders in the permafrost zone. The phase change of water in soils around the pipeline has a distinct influence on the oil temperature during the freeze-thaw transition periods. The oil temperature tends to be equal to the ambient ground temperature around the pipeline with southward distance and with elapsing operation time. The pipeline oil temperature is controlled by the incoming oil temperature and the surrounding ground temperature before the equalization. It would be mainly controlled by the ground temperature around the pipeline afterwards.     

Cracking behavior of evaporated amorphous silicon films

The residual stress in amorphous silicon films deposited by evaporation is investigated with different substrate temperatures. The stress measured from all the films studied in this paper is tensile. The level of stress decreases from 580 MPa to 120 MPa with increasing substrate temperature from 60 °C to 350 °C. When the film becomes thicker, strain increases and cracks are formed for stress relaxation. 10 µm thick amorphous Si films are deposited at 350 °C without cracks. This cracking behavior is theoretically studied and confirmed by experiment.     

Seafloor temperature and conductivity data from Stefansson Sound, Alaska

Overconsolidated sediments, seasonal seafloor freezing, and ice-bonded permafrost are unique features in shallow arctic coastal waters. They are related to low seawater temperatures and varying salinities. Seabed temperatures can be less than −1.0°C for much of the year, with noticeable warming occurring only during the summer months. Observations from recent deployment of three instruments in Stefansson Sound and data from an earlier deployment, which included sites in Harrison Bay, showed decreasing mean annual seafloor temperatures with increasing water depth, ranging from −0.9°C in 4.4 m of water to −1.6°C in 14 m of water. Salinities also varied seasonally, with noticeable freshening developing during the summer and highly uniform values occurring during the winter. Periodic temperature and salinity measurements at sites in Stefansson Sound, made during August 1987 and August 1989, also helped verify the data obtained with the seabottom instruments.Seasonal freezing of the seabed can begin in late September and may noticeably change its engineering properties. In areas of coarse-grained sediments, ice bonding and strengthening of the seabed can result. In areas of fine-grained sediments it appears that seasonal freezing of the seafloor can cause overconsolidation of the seabed sediments. This densification process can result in a significant permanent increase in strength.     

Annealing effects on microstructure and mechanical properties of sputtered multilayer Cr(1 − x)AlxN films

Multilayer Cr_(1 − x)Al_xN films with a total thickness of 2 μm were deposited on high-speed steel by medium frequency magnetron sputtering from Cr and Al-Cr (70 at.% Al) targets. The samples were annealed in air at 400 °C, 600 °C, 800 °C and 1000 °C for 1 hour. Films were characterized by cross-sectional scanning electron microscopy and X-ray diffraction analysis. The grain size of the as-deposited multilayer films is about 10 nm, increasing with the annealing temperature up to 100 nm. Interfacial reactions have clearly changed at elevated annealing temperatures. As-deposited films' hardness measured by nanoindentation is 22.6 GPa, which increases to 26.7 GPa when the annealing temperature goes up to 400 and 600 °C, but hardness decreases to 21.2 GPa with further annealing temperature increase from 600 to 1000 °C. The multilayer film adhesion was measured by means of the scratch test combined with acoustic emission for detecting the fracture load. The critical normal load decreased from 49.7 N for the as-deposited films to 21.2 N for the films annealed at 1000 °C.     

State of the art of high temperature power electronics

High temperature power electronics has become possible with the recent availability of silicon carbide devices. This material, as other wide-bandgap semiconductors, can operate at temperatures above 500 °C, whereas silicon is limited to 150-200 °C. Applications such as transportation or a deep oil and gas wells drilling can benefit. A few converters operating above 200 °C have been demonstrated, but work is still ongoing to design and build a power system able to operate in harsh environment (high temperature and deep thermal cycling).     

Preparation and characterization of pulsed laser deposited CdTe thin films at higher FTO substrate temperature and in Ar + O2 atmosphere

Pulsed laser deposition (PLD) is one of the promising techniques for depositing cadmium telluride (CdTe) thin films. It has been reported that PLD CdTe thin films were almost deposited at the lower substrate temperatures (<300 °C) under vacuum conditions. However, the poor crystallinity of CdTe films prepared in this way renders them not conducive to the preparation of high-efficiency CdTe solar cells. To obtain high-efficiency solar cell devices, better crystallinity and more suitable grain size are needed, which requires the CdTe layer to be deposited by PLD at high substrate temperatures (>400 °C). In this paper, CdTe layers were deposited by PLD (KrF, λ = 248 nm, 10 Hz) at different higher substrate temperatures (T_s). Excellent performance of CdTe films was achieved at higher substrate temperatures (400 °C, 550 °C) under an atmosphere of Ar mixed with O₂ (1.2 Torr). X-ray diffraction analysis confirmed the formation of CdTe cubic phase with a strong (1 0 0) preferential orientation at all substrates temperatures on 60 mJ laser energy. The optical properties of CdTe were investigated, and the band gaps of CdTe films were 1.51 eV and 1.49 eV at substrate temperatures of 400 °C and 550 °C, respectively. Scanning electron microscopy (SEM) showed an average grain size of 0.3–0.6 μm. Thus, under these conditions of the atmosphere of Ar + O₂ (15 Torr) and at the relatively high T_s (500 °C), an thin-film (FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe: Cu/Ag) solar cell with an efficiency of 6.68% was fabricated.     

Effects of preheating temperature on cold cracks,microstructures and properties of high power laser hybrid welded 10Ni3CrMoV steel

Laser hybrid welding has become one of the most promising welding methods for high strength low alloy steels due to combining the advantage of the laser and arc. A novel Y-groove cold cracking test adapted to laser hybrid welding is designed to assess the weldability of 10Ni3CrMoV steels at room temperature and different preheating temperatures. The experimental results show that the orientation of the predominant root cracks generally follows the contour of the fusion line. As the temperature increases from 25 °C to 150 °C, at first the root crack rate decreases and then slightly increases at 150 °C. The root crack rate obtained at 120 °C is the lowest. The fracture model changes from a brittle cleavage fracture to a mixture fracture with quasi-cleavage facets and dimples. The thermal cycle curves of laser hybrid welding obtained by temperature measurement systems are used to evaluate the crack resistance and microstructure transformation. The microstructures of welded joints obtained at different temperatures are analyzed by optical microscope (OM). The results reveal that the microstructures of the coarse grained region and the fusion zone at 120 °C have higher cold crack resistance and good impact toughness. Mechanical properties of the welded joint obtained at 120 °C and 150 °C are comprehensively evaluated by microhardness test, uniaxial tensile test and charpy V-notch impact test with side notches. Fractographs of the impact specimens are studied by scanning electron microscopy (SEM). The test results show that the welded joints obtained at 120 °C have satisfactory mechanical properties that can meet the technical requirements for shipbuilding industry.     

Development of freezing-thawing processes of foundation soils surrounding the China-Russia Crude Oil Pipeline in the permafrost areas under a warming climate

The proposed China-Russia Crude Oil Pipeline (CRCOP) will be subjected to strong frost heave and thaw settlement of the surrounding soil as it traverses permafrost and seasonally frozen ground areas in Northeastern China. The freezing-thawing processes, the development of the maximum frozen cylinder in taliks and thawed cylinder in permafrost areas, and the variations in the maximum freezing depths under the pipeline in taliks and thawing depths in different permafrost regions near Mo'he station, the first pumping station in China, were studied in detail using numerical methods in this paper. The inlet oil temperature at Mo'he station was assumed to vary from 10 to − 6 °C in a sine wave form during the preliminary design phase. Research results showed that the freezing-thawing processes of soils surrounding the buried pipeline had distinct differences from those in the undisturbed ground profile in permafrost areas. In summer, there was downward thawing from the ground surface and upward and downward thawing from the pipeline's surface once the temperature of the oil rose above 0 °C. In winter, downward freezing began from the ground surface but upward and downward cooling of the cylinder around the pipeline didn't begin until the temperature of the oil dropped below 0 °C. However, in the undisturbed ground profile, downward thawing from the ground surface occurred in summer and downward freezing from the ground surface and upward freezing from the permafrost table occurred in winter. The maximum thawing depths and thawed cylinder around the pipeline in warm permafrost enlarged with elapsing time and decreasing water content of the soils. In taliks, the maximum freezing depths and frozen cylinder around the pipeline kept shrinking with elapsing time and increasing water content of the soils. The freezing-thawing processes and development of the thawed and frozen cylinders around the pipeline were muted by any insulation layer surrounding the pipeline. Insulation had better thermal moderating on the heat exchange between the pipeline and the surrounding soils during the early operating period. But its role slowly weakened after a long-term operating. Research results will provide the basis for assessment and forecasting of engineering geological conditions, analysis of mechanical stability of the pipeline, foundation design, and pipeline construction and maintenance.     

Structural, optical and electrical study of undoped GaN layers obtained by metalorganic chemical vapor deposition on sapphire substrates

We investigate optical, structural and electrical properties of undoped GaN grown on sapphire. The layers were prepared in a horizontal reactor by low pressure metal organic chemical vapor deposition at temperatures of 900 °C and 950 °C on a low temperature grown (520 °C) GaN buffer layer on (0001) sapphire substrate. The growth pressure was kept at 10,132 Pa. The photoluminescence study of such layers revealed a band-to-band emission around 366 nm and a yellow band around 550 nm. The yellow band intensity decreases with increasing deposition temperature. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies show the formation of hexagonal GaN layers with a thickness of around 1 μm. The electrical study was performed using temperature dependent Hall measurements between 35 and 373 K. Two activation energies are obtained from the temperature dependent conductivity, one smaller than 1 meV and the other one around 20 meV. For the samples grown at 900 °C the mobilities are constant around 10 and 20 cm² V⁻¹ s^− 1, while for the sample grown at 950 °C the mobility shows a thermally activated behavior with an activation energy of 2.15 meV.