Stands of Scots pine and European beech (centre) have significantly higher structural heterogeneity than monocultures of Scots pine (left) and European beech (right). The superior heterogeneity of ...mixed stands over monocultures increases from dry to moist sites (from top to bottom). Display omitted
•Scots pine and European beech monocultures differ significantly in structure.•Mixed stands of both have more heterogeneous structures than monocultures.•Stand density increases, vertical structure and tree morphology diversify.•Multiplicative mixing effects can further enhance additive effects on structure.•Superior heterogeneity over monocultures increases with water availability.
The mixing of tree species with complementary ecological traits may modify forest functioning regarding productivity, stability, or resilience against disturbances. This may be achieved by a higher heterogeneity in stand structure which is often addressed but rarely quantified. Here, we use 32 triplets of mature and fully stocked monocultures and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) located along a productivity and water availability gradient through Europe to examine how mixing modifies the stand structure in terms of stand density, horizontal tree distribution pattern, vertical stand structure, size distribution pattern, and variation in tree morphology. We further analyze how site conditions modify these aspects of stand structure. For this typical mixture of a light demanding and shade tolerant species we show that (i) mixing significantly increases many aspects of structural heterogeneity compared with monocultures, (ii) mixing effects such as an increase of stand density and diversification of vertical structure and tree morphology are caused by species identity (additive effects) but also by species interactions (multiplicative effects), and (iii) superior heterogeneity of mixed stands over monocultures can increase from dry to moist sites. We discuss the implications for analyzing the productivity, for modelling and for the management of mixed species stands.
Mixing of complementary tree species may increase stand productivity, mitigate the effects of drought and other risks, and pave the way to forest production systems which may be more resource-use ...efficient and stable in the face of climate change. However, systematic empirical studies on mixing effects are still missing for many commercially important and widespread species combinations. Here we studied the growth of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) in mixed versus pure stands on 32 triplets located along a productivity gradient through Europe, reaching from Sweden to Bulgaria and from Spain to the Ukraine. Stand inventory and taking increment cores on the mainly 60–80 year-old trees and 0.02–1.55 ha sized, fully stocked plots provided insight how species mixing modifies the structure, dynamics and productivity compared with neighbouring pure stands. In mixture standing volume (+12 %), stand density (+20 %), basal area growth (+12 %), and stand volume growth (+8 %) were higher than the weighted mean of the neighbouring pure stands. Scots pine and European beech contributed rather equally to the overyielding and overdensity. In mixed stands mean diameter (+20 %) and height (+6 %) of Scots pine was ahead, while both diameter and height growth of European beech were behind (−8 %). The overyielding and overdensity were independent of the site index, the stand growth and yield, and climatic variables despite the wide variation in precipitation (520–1175 mm year⁻¹), mean annual temperature (6–10.5 °C), and the drought index by de Martonne (28–61 mm °C⁻¹) on the sites. Therefore, this species combination is potentially useful for increasing productivity across a wide range of site and climatic conditions. Given the significant overyielding of stand basal area growth but the absence of any relationship with site index and climatic variables, we hypothesize that the overyielding and overdensity results from several different types of interactions (light-, water-, and nutrient-related) that are all important in different circumstances. We discuss the relevance of the results for ecological theory and for the ongoing silvicultural transition from pure to mixed stands and their adaptation to climate change.
The focus of this work is a comparative analysis of the mechanisms of shaping of the hollow billets, which affect the yield of the centrifugal and continuous casting of БрО8Н4Ц2 ...(Cu–8% Sn–4% Ni–2% Zn)- and БрО5Ц5С5 (Cu–5% Sn–5% Zn–5% Pb)-type bronzes. As revealed, the action of centrifugal forces causes the segregation of chemical elements of the alloys, depending on their density. The alloying elements of the alloy, Sn and Pb, which have an increased density, contribute to this process more actively as compared with other elements. The factors affecting the intensity of the segregation process for alloying elements, the density of which is several times higher than the density of other alloy components, are determined. The causes of the appearance of tin, as an element with a maximum density, surrounded by the compounds with a minimum density are investigated. As established, the process of segregation occurring during the centrifugal casting reduces the uniform distribution of alloying elements throughout the billet bulk and significantly increases the allowances for machining. As shown, in the process of continuous casting, the force of gravity acts on the melt, but does not cause the separation of denser alloying elements during the shaping of the billet. As revealed, only one factor determining the amount of allowance for machining for the continuous casting process is the depth of the deformation hole on the outer surface of the billet, which is necessary to engage and remove the billet from the mould. The depth of the deformation hole is established, at which the shear stress of the cut corresponds to the shear stress on the cut for samples of the studied bronze grades. A three-dimensional model is constructed and proposed for determining the yield of suitable continuous casting of hollow billets depending on both the surface temperature of the billets and the forces acting on the billet from the side of the withdrawal-roll set. To assess the effect of technological parameters on the yield of the continuous and centrifugal casting processes, the web method is applied, which allows to expand the understanding of the various mechanisms of formation of billets. The study shows that, in comparison with the known indicators of the continuous-casting yield of 95.06–96.04% for billets with outside dimensions of 80–150 mm, one could increase the yield of casting up to 97.29–97.72%. It is concluded on the expedient fabrication of billets from antifriction alloys by continuous casting, since it allows increasing the yield of suitable casting by 22.35%–47.72% for the studied standard size of billets.
Since in a contemporary metrological database, there is no clear method for evaluation of the quality of materials under high and shock loads, this article presents results of experimental studies of ...practical application of qualimetric methods for evaluation of the quality of materials under such conditions. The analysis of the characteristics of materials under shock loads is performed along with qualimetric methods, which are adapted for evaluation of the quality of high-loaded materials. Criteria for evaluation of quality of materials under extreme loads are determined. The algorithm and methodical recommendations for evaluation test of materials are developed along with realization of practical implementation of developed recommendations. The algorithm for performance of evaluation test for materials is presented and specifies a choice of test method for determining necessary qualimetric methods of the quality evaluation. Based on experimental data, it is established that the most appropriate for practical application is using so-called ‘cobweb’ of quality as a primary qualimetric method. In addition, one can use methods of pie charts and construction of the ranked data. The quality evaluation with a price ratio is quite effective for the market and competing materials such as metals and steel. For an impulsive load, the qualimetric evaluation method such as the construction of nomograms is effective. The problem of the quality gradation development, which is basic during the construction of nomograms, is solved.
Due to the fact that in the modern metrological database there is no clear methodology for estimating quality of materials under the conditions of shock and high-speed loads, in this work the ...analysis of dynamic properties of metallic materials at the impulsive loads is carried out and presented. It is shown that shock loads characterize material on a viewpoint of dynamic properties (i.e. fracture characteristics) are key and dominant at the impulsive loads. This means that shock loads characterize the quality of materials under the high-speed loads. Based on the analysis of the methods of assessment of the quality of materials, as well as the analysis of their behaviour in the conditions of shock load, it concluded that the main and governing characteristics defining their quality are the dynamic characteristics. Based on these characteristics, a system of indicators of the quality of the loaded materials is constructed. The analysis of results of the modern methods for assessing quality of objects in conditions of impact loads has shown series of problems due to the low-quality assessment of the materials. It is demonstrated that the results of the analysis of experimental methods for testing of materials via impact way, the basic characteristics and parameters affect the final properties of the materials. Based on the adduced arguments, we formulated the basic approach principle defining the main criteria of the loaded materials’ quantity (dynamical characteristics). Analysing contemporary experience in the using of complex-quantitative methods, the most suitable methods for assessment of materials under the conditions of high load are selected. Presented list of materials contributed to the solution of the problem of creating an alternative mechanism for evaluating the quality of the loaded materials (along with expert way). Complex-quantitative methods are the best suited for use in experimental tests, and are obvious and easily reproducible. If there are recommendations on application of the methods, the latter can be easily implemented in the production and education process.
Kinetic analysis of nanodiamonds oxidation Zabuga, Viktor Ya; Bochechka, Oleksandr O.; Tsapyuk, Galyna G. ...
Molecular Crystals and Liquid Crystals,
09/2018, Letnik:
673, Številka:
1
Journal Article
Recenzirano
This study examines the oxidation kinetics of nanodiamonds (NDs) and carbon black (CB), taken for comparison, in air oxygen . The oxidation rate constants and parameters of the Arrhenius equation ...were determined under isothermal conditions within a temperature range from 430 to 560 °C. Thermal behavior of both carbon nanomaterials was investigated using non-isothermal thermogravimetric analysis (TGA) experiments in air and argon. Powders of NDs and CB were characterized by nitrogen adsorption, scanning electron microscopy and energy dispersion X-ray microanalysis, ATR Infrared spectroscopy, and the total titration. The effect of various parameters of the carbon nanomaterials on the oxidation rate, such as oxygen content, surface area, ash and admixture contents, and functional groups coverage, was investigated. Overall, this study confirms that the oxygen-containing surface groups generating active sites upon thermal decomposition have a major impact on the oxidation ability of NDs and CB.
The oxidation kinetics of the ACM5 0.1/0 statically synthesized diamond nanopowder in the presence of tungsten in the amounts of 1 and 5 wt %, introduced into the sample by wetting it with the (NH
4
...)
2
WO
4
water solution was investigated. It is established that at the chemical modification of a diamond nanopowder with tungsten by adsorption from a solution the inhibitory action of tungsten on the kinetics of the diamond oxidation also shows. This agrees with the suggested kinetic scheme of the reaction, which contains the stage of the diamond interaction with the gaseous tungsten oxide.
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