Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is D_b≈6×10⁻¹¹exp(−0.29/(k_BT )) cm²/s. Received: 18 May 1999 / Accepted: 8 September 1999 / Published online: 23 February 2000     

Coupled motion of [10−10] tilt boundaries in magnesium bicrystals

Magnesium bicrystals were grown with symmetric and asymmetric tilt boundaries about the [10–10] axis using the vertical Bridgman technique. Isothermal constant load tensile tests were conducted on these bicrystals in the temperature range 300–500°C and relative displacements of the two grains were measured to obtain an appreciation for grain boundary motion characteristics. Coupled grain boundary motion was noted in almost all cases with the degree of tangential motion versus migration changing with tilt misorientation, temperature and applied stress. Specifically, within the family of symmetric bicrystals evaluated, a minimum in grain boundary displacement in the specimen plane was observed at a tilt misorientation of 20°. In specific stress/temperature regimes, rigid body sliding was observed for the particular case of a 35° asymmetric tilt misorientation. The ease of basal and prism slip in magnesium at the temperatures considered and the consequential impingement of intragranular dislocations on the bicrystal boundary and their decomposition and motion along the boundary are thought to play an important role in the observed coupled motion of these tilt boundaries.     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

Nanostructural analysis of ZnO:Al thin films for carrier-transport mechanisms

The carrier mobility of sputter-deposited Al-doped ZnO transparent-conducting (ZnO:Al) thin films was controlled between 22 and 48 cm²/Vs by varying the ZnO:Al seed layer. The statistical distribution of the [001] grain misorientation was characterized from the X-ray diffraction rocking curve in the range from 0.043 (2.5°) to 0.179 rad (10.2°). The grain-boundary energy barriers (E_b) from Seto's model [1] clearly exhibit linear dependence on the grain-boundary misorientation angle (ω) according to the equation E_b = 78 ± 4 + 173 ± 32 ω meV.     

Self-diffusion at grain boundaries with a disordered atomic structure

A change in the free energy of a grain boundary is analyzed in the case when lattice vacancies come to the boundary and are then delocalized in its disordered atomic structure. It is shown that the free energy of the boundary is minimized at some excess atomic volume Δv _b =Δv _b ^* , whose value depends on the energy of vacancy formation in the crystal lattice and the boundary energy. The formation of a metastable localized grain-boundary vacancy as a result of thermal fluctuations of the density in a group of n ₀=\gM_v/Δv _b atoms (\gM_v is the vacancy volume), followed by the jump of an adjacent atom into this vacancy, is taken as an elementary event of grain-boundary diffusion. Expressions for the activation energy of diffusion and the diffusion coefficient are derived for equilibrium (Δv _b =Δv _b ^* ) and nonequilibrium (Δv _b >Δv _b ^* ) boundaries.     

Thermodynamic and Kinetic Properties of Grain-Boundary Ridges on Tilt $$[11\bar 20]$$ Grain Boundaries in Zinc

Thermodynamic and kinetic properties characteristic of grain-boundary ridges on tilt \([11\bar 20]\) grain boundaries in zinc are studied experimentally. Temperatures are determined for faceting–defaceting and roughening phase transitions in grain-boundary ridges for tilt \([11\bar 20]\) grain boundaries with 35°, 57°, and 85° angles of misorientation. Mobilities and enthalpies of activation are obtained for grain-boundary ridges on the same boundaries. Parameters of inhibition are also calculated for grain-boundary ridges, according to the observed shapes of grain-boundary loops.     

Nucleation of dynamic recrystallization and variant selection in copper bicrystals

Orientation-controlled copper bicrystals containing [001] symmetrical tilt boundaries aligned parallel to the loading axis were deformed in tension at 923?K and a strain rate of 4.2?×?10^?4?s^?1. The nucleation of dynamic recrystallization (DRX) was investigated along the grain boundary. For this purpose, both optical and orientation imaging microscopy methods were used. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply indented grain boundary regions. The critical strain for nucleation was about one-quarter to one-half of the peak strain and depended on the misorientation angle. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, all the primary twin traces were parallel to those of the inactive slip planes of the parent single crystals. Crystallographic analysis revealed the important role of the direction of GBM on twinning-plane variant selection. The characteristics of grain boundary nucleation depended sensitively on grain boundary character and on grain boundary mobility. The observed DRX nucleation mechanism is discussed in relation to the occurrence of GBM and twinning.     

Effect of High-Temperature Structure and Diffusion on Grain-Boundary Diffusion Creep in fcc Metals

Molecular dynamics simulations of high-energy twist and tilt bicrystals of fcc palladium reveal a universal, liquid-like, isotropic high-temperature diffusion mechanism, characterized by a rather low self-diffusion activation energy that is independent of the boundary type or misorientation. Medium-energy grain boundaries exhibit the same behavior at the highest temperatures; however, at lower temperatures the diffusion mechanism becomes anisotropic, with a higher, misorientation-dependent activation energy. Our simulations demonstrate that the lower activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures. We demonstrate that the existence of such a transition has important consequences for diffusion creep in nanocrystalline fcc metals. In particular, our simulations reveal that in the absence of grain growth, nanocrystalline microstructures containing only high-energy grain boundaries exhibit steady-state diffusion creep with a creep rate that agrees quantitatively with that given by the Coble-creep formula. Remarkably, the activation energy for the high-temperature creep rate is the same as that characterizing the universal high-temperature diffusion in high-energy energy bicrystalline grain boundaries.     

Mixed conductivity of gadolinium titanate-based pyrochlore ceramics: The grain boundary effects

In order to reveal the role of grain boundaries on the ionic and electronic conduction processes, the transport properties of Gd_2−xGa_xTi₂O_7−δ (x=0.10–0.14) pyrochlore ceramics, pure and with SiO₂ additions, were studied at 700–950 °C by impedance spectroscopy and faradaic efficiency measurements. The oxygen ion transference numbers of “pure” materials in air vary in the range of 0.95–0.97, increasing when temperature decreases. For silica-containing ceramics having, as expected, highly resistive grain boundaries, the ion transference numbers were considerably lower, 0.76–0.88, and increase with temperature. This behavior suggests that grain boundaries in these oxygen ion-conducting ceramics have a larger limiting effect on ionic transport than on electronic conduction. Increasing boundary resistivity may increase the relative role of electronic conductivity in solid oxide electrolytes, thus preventing their potential use in electrochemical cells at low temperatures. Also, the presence of even small electronic contributions to the total conductivity may lead to significant errors in the grain-boundary resistance values estimated from impedance spectroscopy data. The evaluation of the grain boundary exact contribution should be based on a clear knowledge of the magnitude of transference numbers. Paper prestented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Volume and grain boundary diffusion of implanted 113Sn in aluminium

Volume and grain boundary diffusion of ¹¹³Sn in aluminium was investigated with the radiotracer method. The implantation technique was used for tracer deposition to avoid problems of tracer hold-up caused by the oxide layer always present on aluminium. The diffusion penetration was chosen large enough to permit serial sectioning of samples with the aid of a microtome.The temperature dependence of the volume diffusivity was determined as D(T)=4.54×10^–5×exp[–(114.5±1.2)kJmol^–1/RT] m ² s ^–1. This confirms previous measurements from our group which already showed that Sn is the fastest foreign metal diffusor so far investigated in aluminium.Grain boundary diffusion of ¹¹³Sn in Al polycrystals was measured in the type-B kinetic regime. The grain boundary diffusion product P=sD _gb (s=segregation factor, =grain boundary width, D _gb=grain boundary diffusivity) was found to be strongly affected by the impurity content of aluminium. For Al polycrystals of 99.9992% nominal purity we obtained P _5N(T)=1.08×10^–8exp [–(96.9±7.5) kJ mol^–1/RT] m³ s^–1 and for less pure Al polycrystals of 99.99% nominal purity P _4N(T)=3.0×10^–10 exp [–(90.1±4.2) kJ mol^–1/RT] m³ s^–1 was determined. The grain boundary diffusion product in the purer material is more than one order of magnitude higher than in the less pure material. Very likely this is an effect of co-segregation of non-diffusant impurities into the grain boundaries.     

Atomic configurations and diffusion mechanisms near the asymmetric grain boundaries in tetragonal CuAu I alloy with <100 > and <001 > tilt axes

The molecular dynamics method is used to investigate an atomic configuration in the structure of asymmetric tilted grain boundaries for the ordered CuAu I alloy with FCT symmetry and L10 structure. Investigations are performed for three misorientation angles: 7, 16, and 22° and grain boundaries with <100 > and <001 > tilt axes. The distinctive features of the structural grain boundary reorganization and diffusion mechanisms are elucidated at different temperatures.     

Grain Boundary Diffusion and Segregation in Interstitial Solid Solutions Based on BCC Transition Metals: Carbon in Niobium

Bulk and grain boundary (GB) diffusion of ¹⁴C in Nb has been studied by the radiotracer serial sectioning technique. B and C kinetic regimes were realized for GB diffusion in the temperature range from 800 to 1173 K. The values of P = sD _gb, D _gb and s follow the Arrhenius dependencies: P = 5.15 × 10^–15 exp[–(83.1 kJ/mol)/RT] m³/s (973–1173 K), D _gb = 2.3 × 10^–6 exp[–(133.0 kJ/mol)/RT] m²/s (800–950 K), and s = 4.7 exp[(49.9 kJ/mol)/RT].The increase in the GB diffusion compared with self-diffusion is very large despite the probable retardation effect due to the strong segregation.The results for GB diffusion of C in Nb as well as for other interstitial solutes (P, S) in bcc transition metals (- Fe, Mo) are discussed in the framework of the transition state theory. It is assumed that GB segregation decreases the energy of the ground state whereas the change in the diffusion mechanism (e.g. from vacancy to interstitial) leads to a strong decrease of the transition state energy. This change in the diffusion mechanism results in a fast GB diffusion of interstitial solutes in spite of their large tendency to segregate to GBs.     

Intergranular Melting of Ultrafine Grained Nd2Fe14B Studied by Means of Radiotracer Diffusion

Grain-boundary (Gb) diffusion was studied in ultrafine grained Nd₂Fe₁₄B-based permanent magnets below and above the melting transition of the Nd-enriched intergranular phase using the radiotracer technique with the isotope ⁵⁹Fe. The product D _Gb of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in -Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour D _Gb = 1.53 × 10^–11 exp(–1.74 eV/kT) m³ s^–1 below the intergranular melting transition. Similar values D _Gb are observed for ultrafine grained Nd-Fe-B with reduced Nd excess in the grain boundaries. The diffusion characteristics are compared with the kinetics of the hot-deformation which is of technical relevance for the processing of high-performance permanent magnets.     

Diffusion study of nitrogen implanted into <Emphasis Type="Italic">α</Emphasis>-Hf using the nuclear resonance technique

The diffusion of nitrogen in α-Hf was studied in the temperature range of (823–1123) K using the ion implantation and nuclear resonance techniques. The measurements show that the diffusion coefficients follow the Arrhenius behavior D(T)=D ₀exp (−Q/RT) with D ₀=(5.5±2.1)×10⁻⁷ m²/s and Q=(228±1) kJ/mol. A comparison of the present results with the previous one is done.     

Grain growth in ultrafine titanium powders during sintering

Grain growth behaviour of fine (∼3 μm) and attrition milled nanocrystalline (∼32 nm) titanium powers during sintering have been studied. The activation energies of grain growth (Q _g) in fine titanium were found to be 192.9 and 142.4 kJ/mol at lower and higher temperature ranges, respectively. The nanocrystalline titanium showed very low values of Q _g (54.6 kJ/mol) at lower temperatures and it increased to 273.2 kJ/mol at higher temperatures. The constant (n) in nano Ti system was found to have unusually very high values of 6.5–8.2. The grain boundary rotation along with the diffusional processes could be the grain growth mechanism in nanocrystalline and in fine titanium powders.     

Diffusion of silicon in titanium nitride films. Efficiency of TiN barrier layers

Two kinds of reactively evaporated titanium nitride films with columnar (B ₀ films) and fine-grained film structure (B ₊ films) have been examined as diffusion barriers, preventing the silicon diffusion in silicon devices. The silicon diffusion profiles have been investigated by 2 MeV ⁴He⁺ Rutherford backscattering spectrometry (RBS) after annealing at temperatures up to 900° C, in view of application of high-temperature processes. The diffusivity from 400 to 900° C: D (m² s^–1)=2.5×10^–18 exp[–31 kJ/mol/(RT)] in B ₀ layers and D (m² s^–1)=3×10^–19 exp[–26 kJ/mol/(RT) in B ₊ TiN layers. The diffusivities determined correspond to grain boundary diffusion, the difference being due to the different microstructure. The very low diffusivity of silicon in B ₊ TiN layer makes it an excellent high-temperature barrier preventing silicon diffusion.     

Compensation Effect for Grain Boundary Diffusion in a Bicrystalline Experiment

The paper is devoted to the problem of the compensation effect for grain boundary (GB) diffusion, i.e. the linear dependence of the pre-exponential factor of the GB diffusion coefficient on the activation energy. Specific features of GB diffusion as a thermally activated process namely, the influence of segregation factor, K, and variation of the GB width, d, on the diffusion rate are discussed. A special diffusion experiment was designed to estimate the contribution of the separate component parts of the triple product, KdD_GB (D_GB is the GB diffusion coefficient). The experiment was performed with Al bicrystals. The variation of the GB width d, and a value of the segregation factor K, due to GB structure change are estimated. It is concluded that D_GB is the main GB structure-sensitive parameter in the triple product. This circumstance allows us to consider the GBs in Al bicrystals as a series of uniform objects and to describe the kinetics of GB diffusion in terms of the compensation temperature T_c and a “barrier” phase. The value of T_c for GB diffusion of Zn and Ge in Al bicrystals is practically the same and equals 709 and 706 K, respectively. The character of the “barrier” phase is discussed.     

Radiation-induced changes in the atomic structure of grain boundaries in tungsten

Field-ion microscopy is used to study changes in the structure of the grain boundaries induced by intergrain adsorption of point defects created by ion bombardment of tungsten bicrystals. It is found that irradiation at temperatures below the threshold of grain-boundary relaxation causes a local expansion of the boundaries. Computer simulation using molecular dynamics shows that intergrain adsorption of vacancies can lead to the formation of three-dimensional grain-boundary structures. Fiz. Tverd. Tela (St. Petersburg) 41, 383–385 (March 1999)     

双晶铝晶界弛豫的转变温度

测量了夹角分别为60°和153.4°的[110]对称倾侧铝双晶晶界的扭转滞弹性蠕变曲线,指出了晶界的弛豫强度在低于温度T₀时变为零,此温度(T₀)约等于该金属的熔点温度(T_m)的二分之一,这说明晶界的内部结构在这个温度的附近发生了显著的变化。 关键词：     

Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3ⁿ CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D₀). The fraction of Σ3ⁿ CSL boundaries sharply rose to approximately 0.5 in the range of D/D₀ from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3ⁿ CSL boundaries slowed down significantly in the range of D/D₀ > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3ⁿ CSL boundaries in the recrystallized microstructure.