On laws governing the splitting failure of flat metallic specimens subjected to thermal shock

Computed and experimental data on the splitting failure of copper, nickel, titanium, molybdenum, brass, and bronze metallic foils from 0.005 to 1 mm thick under thermal shock initiated by the x-radiation of a nuclear explosion are presented. It is proposed that the concepts “average energy liberated over the thickness (mass) of the specimen” ε, “specific absorbed energy” W, and “splitting strength of the material” σ be used as criterial characteristics of failure thresholds of optically thin flat metallic specimens (foils). It is demonstrated that the critical average energy liberation ε_*, which results in splitting, decreases logarithmically (ε_* =A _*-B _*log Δ) with increasing thickness Δ of the irradiated specimens in the interval Δ≈0.001–1 mm, and the critical specific potential energy W_* reguired to effect splitting increases with increasing optical mass m of the specimen under the law W_*=−αmlog (βm), where A_*, B_*, α, and β are certain parameters. It is shown that the longevity of the copper, nickel, titanium, molybdenum, brass, and bronze under radiation-induced thermal shock decreases exponentially with increasing amplitude of the failing stress (splitting strength) and can be described on the basis of the kinematic concept of strength. Deceased. Translated from Problemy Prochnosti, No. 1, pp. 37–47, January–February, 1997.     

Power generation of the touching Cu/Bi-Te/Cu composites under the periodically alternating temperature gradients

The thermo-emf ΔV of the touching p- and n-type Cu/Bi-Te/Cu composites with different thicknesses of t _Bi-Te and t _Cu was measured as a function of time by alternating the temperature difference ΔT at periods of T = 20, 60, 120, 240 and ∞ sec, where t _Bi-Te was varied from 0.1 to 2.0 mm and t _Cu from 0 to 4.0 mm. As a result, ΔV changes significantly with t _Bi-Te, t _Cu and T. The effective thermo-emf ΔV _eff increases significantly with an increase of 1/T and exhibited a local maximum at 1/T = 1/240 s⁻¹. The resultant | α | and the effective temperature difference ΔT _eff were increased significantly by optimizing t _Bi-Te and t _Cu at 1/T = 1/240 s⁻¹. The power generation ΔW _eff (= ΔV _eff²/4R _calc) estimated using the measured ΔV _eff and calculated R _calc also exhibited a local maximum at 1/240 s⁻¹ for an optimum combination of t _Bi-Te = 0.1 mm and t _Cu = 2.0 mm, so that the maxima ΔW _eff at 1/T = 1/240 s⁻¹ for the p- and n-type composites were 2.28 and 2.92 times higher than those obtained at 1/T = 0 s⁻¹. This significant increase in ΔW _eff is owing to both the increase in ΔT _eff and the increase in ZT due to the increase in |α|. The power generation was thus found to be enhanced significantly by imposing the alternating temperature gradients on touching Cu/Bi-Te/Cu composites.     

Unified model of nucleation and growth of fatigue macrocracks. Part 2. Application of deformational parameters of the fracture mechanics of materials in the stage of crack initiation

We propose a new approach to the experimental estimation of local strains at the tip of a concentrator. The approach is based on the measurement of displacement of certain points in the vicinity of the tip of a notch, which is further associated with the effective radius of the notch. Various concentrators in structures are simulated within a wide range of variation in radii of notches (ρ=0.1−6.5mm) and in the geometry of specimens. We establish the main dependences between the value of the range of local strains Δɛ^* and the periodN _i prior to the nucleation of a fatigue macrocrack. Thus, by using the experimental quantity Δɛ^*, we can estimate the periodN _i prior to the nucleation of a fatigue crack of lengtha _i=d^* for structural elements of a complicated geometry. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv. Vol. 34, No. 3, pp. 55–66, May–June, 1998.     

Cyclic Crack Resistance of High-Strength Cast Irons at the Stages of Initiation and Growth of a Macrocrack

On investigating high-strength cast irons of the austenitic, ferritic, and pearlite-ferritic classes, we have experimentally corroborated that the relations _y ^* vs. N _i, d ^* vs. N _i, and ^* vs. N _i obtained at the stage of initiation of a fatigue macrocrack in air and a corrosive medium (3.5% NaCl solution) can be transformed into the relations da/dN vs. K and da/dN vs. ^*. The latter describe the stage of macrocrack propagation within the framework of the force and strain approaches, respectively. We have shown that the strain parameters, as compared with the force ones, are 1.5–2 times more sensitive to the influence of a corrosive medium on the cyclic crack resistance of high-strength cast irons.     

Effect of the thickness of Bi–Te compound and Cu electrode on the resultant Seebeck coefficient in touching Cu/Bi–Te/Cu composites

The resultant Seebeck coefficient α of the touching p- and n-type Cu/Bi–Te/Cu composites with different thicknesses of t _Bi–Te and t _Cu was measured as a function of t, where t _Bi–Te was varied from 0.1 to 2.0 mm, t _Cu from 0.3 to 4.0 mm and t is the lapse time after imposing the voltage. The temperature difference ΔT is produced by imposing a constant voltage of 1.70 V on two Peltier modules connected in series. The resultant α of composites was calculated from the relation α = ΔV/ΔT, where ΔV and ΔT were measured with two probes placed on both end coppers. ΔV decreases abruptly with an increase of t below t = 5 min, while above t = 7 min, it tends to saturate to a constant value. The resultant α and saturated ΔV vary significantly with changes in t _Cu and t _Bi–Te. When a composite has a combination of t _Cu = 1.0 mm and t _Bi–Te=0.1 mm, the generating powers ΔW (=(ΔV)²/4R) estimated using the saturated ΔV and calculated electrical resistance R for the p- and n-type composites have great local maximum values which are 4–5 times as large as those obtained for the conventional combination of t _Bi-Te = 2.0 mm and t _Cu = 0.3 mm. It is surprising that the generating power ΔW is enhanced significantly by sandwiching a very thin Bi–Te material between two thick coppers, unlike the conventional composition of thermoelectric modules. On the other hand, when a composite has a combination of t _Bi–Te = 0.1 mm and t _Cu = 0.3 mm, the resultant α of the p- and n-type composites exhibited great values of 711 and −755 μV/K, respectively, so that the maximum resultant ZT of the p- and n-type composites reached extremely large values of 8.81 and 5.99 at 298 K. However, the resultant ZT decreases rapidly with an increase of t _Cu or t _Bi–Te. The resultant ZT is thus found to be enhanced significantly not only in superlattice systems but also in macroscopic composites. The present enhancement in ZT is attributed to the large barrier thermo-emf generated in the Bi–Te region shallower than 50 μm from the boundary.     

Cyclic fatigue-crack propagation in a silicon carbide whisker-reinforced alumina composite: role of load ratio

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=K _min/K _max); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from ∼10⁻¹¹–10⁻⁵ m cycle⁻¹ at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔK _TH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on K _max(da/dN ∞ K _max ³⁰ ) compared to ΔK(da/dN ∞ ΔK ⁵), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of timedependent crack bridging involving either matrix grains or unbroken whiskers.     

Unified model of nucleation and growth of fatigue cracks. Part 1. Use of force parameters of fracture mechanics of materials in the stage of crack nucleation

According to the phenomenological model of nucleation of a fatigue macrocrack, the process is considered as a two-parameter process. The process is described by the local stress or strain range and a certain linear parameter of the material. We propose the corresponding parameters, namely, the local stress range Δσ _y ^* and the characteristic sized ^* of the prefracture zone. The formation of this zone is caused by the anomaly of the yield strength of the material in subsurface layers, the microstructure, the loading amplitude, the cyclic strain hardening, and the environment. The quantityd ^* is a constant of the material, which is independent of the geometry of notch and specimen. the boundary of the prefracture zone is considered as a macrobarrier that determines the growth of microstructurally short and physically small cracks. The moment when a physically small crack oversteps the boundary of the prefracture zone is defined by the quantitative criterion (a ₀=d ^*) of the initial sizea ₀ of a macrocrack in the material. The proposed dependences of (Δσ _y ^* ,N _i ),N _i ) and (d ^*,N _i ) can be regarded as a basis for the determination of characteristics of resistance of the material to the nucleation of a fatigue macrocrack. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 7–21, January–February, 1998.     

Effect of crack closure on the experimentally measured characteristics of cyclic crack-growth resistance of steels

On the basis of the proposed approach and an original procedure for determination of the current value of crack tip opening displacement, we obtain new characteristics of cyclic crack-growth resistance for which the effect of crack closure disappears. We experimentally discovered a phenomenon of natural pulsations of crack closure with constantly decreasing amplitude and duration of pulses to the minimum stress intensity factor K _min in a cycle for any positive asymmetry of cycles. We construct a model of damped pulsations of crack closure depending on the maximum stress intensity factor K _max in a cycle and asymmetry of cycles. This model is used to show that the kinetic diagrams of fatigue fracture are invariant for any positive asymmetry of cycles within the limits of the presence of the effect of crack closure. The observed effect of asymmetry quantitatively takes into account the range of pulsating stress intensity factors of crack closure ΔK _cl ^op in the range of stress intensity factors ΔK in a cycle. Practical experience shows that the procedure for quantitative investigation of the kinetics of growth of fatigue cracks extensively used in linear fracture mechanics and based on analysis of the coefficients of crack opening displacement U and γ and functional dependences of the stress intensity factor of crack opening displacement K_op on k _max is inconsistent. We propose a new experimental approach to the qualitative analysis of the kinetics of growth of fatigue cracks based on evaluation of the effect of crack closure and expressed in terms of the effective range of stress intensity factors ΔK_eff in the case of its simple determination from the kinetic diagrams of fatigue fracture together with K_op. We hope that the accumulated experimental data and the proposed model of damping pulsations of crack closure will lay a foundation for a new understanding of the natural resistance of materials to fracture under cyclic loading. Scientific and Engineering Center of Materials-Science Support of the Production and Certification of Equipment of Nuclear Power Plants at the Institute for Problems of Strength of the National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 161 – 171, March – April, 1998.     

Combined effect of staining substances on the discoloration of esthetic Class V dental restorative materials

The purpose of this study was to determine the combined effect of an organic substance (mucin as a substitute for salivary organic substances), chlorhexidine, and an iron compound/tea solution on the changes in the color of esthetic Class V dental restorative materials. Color of a glass ionomer, resin-modified glass ionomer, compomer and flowable resin composite of A2 shade, respectively, was determined according to the CIELAB color scale relative to the standard illuminant D65. Color was measured at baseline, and after sequential immersion in the following substances: Step-1, mucin in PBS (MCP) for 48 h; Step-2, chlorhexidine (CHX) for 24 h; Step-3, iron compound (IRN) or tea solution (TEA) up to 7 days; and Step-4, ultrasonic cleaning for 1 h. Color change (ΔE _ab *) was calculated by the equation: Δ E _ab* = [(Δ L*)² + (Δ a*)² + (Δ b*)²]^1/2, of which ΔL ^∗ indicates changes in value, Δa ^∗ indicates changes in red-green parameter and Δb ^∗ indicates changes in yellow-blue parameter. Δ E _ab* values after immersion in MCP and CHX were compared, and Δ E _ab* values after immersion in IRN or TEA, and subsequent ultrasonic cleaning were compared with respect to the restorative material and immersion substance. Δ E _ab* and changes in the color parameters (ΔL ^∗, ΔC _ab* and ΔH _ab*) were analyzed by repeated measures, analysis of variance and a post-hoc test at the 0.05 level of significance. Color changes after immersion in MCP were acceptable (Δ E _ab* < 3.3), and those after immersion in CHX were generally acceptable. The range of Δ E _ab* values after immersion in IRN was 3.1–19.6, and that after ultrasonic cleaning was 2.4–9.6. The range of Δ E _ab* values after immersion in TEA was 10.7–21.1, and that after ultrasonic cleaning was 11.9–14.5. Color changes of four Class V restorative materials after combined treatment with mucin, chlorhexidine and an iron compound/tea solution were not acceptable. Colors did not recover to their original values after ultrasonic cleaning. Modifications on the surface of a restoration should be considered to reduce stain accumulation.     

Induction time approach to surface induced crystallization in polyethylene/poly (ɛ-caprolactone) melt

The surface induced crystallization of poly (ɛ-caprolactone) (PCL) on an ultra-high modulus polyethylene (PE) fibre was investigated using a new approach based on the induction timet _i. This approach allows estimation of the free energy difference function Δσ as it appears in the theory of heterogeneous nucleation. The classical approach based on the rate of heterogeneous nucleationl is not applicable to transcrystallization because the nucleation density at the fibre surface cannot be measured. A relationship betweenl andt _i is proposed and a theoretical justification is presented. Good agreement between the two approaches is obtained for a verification case where bothl andt _i can be determined. A transcrystalline growth rate study yields an estimate of the parameter σσ_e for PCL of 680 erg²cm⁻⁴. The maximum growth rateg ^* is also obtained. The results obtained indicate the influence of certain parameters on the appearance of transcrystallinity. It is also shown how the interfacial morphology can be controlled by the knowledge of the variations of the induction time with temperature.     

Rubber toughening of plastics

The tensile behaviour of a rubber-toughened polyamide 6,6 (RTPA66) is compared with that of the corresponding untoughened polyamide (PA66). At constant strain rate, moistureconditioned specimens of RTPA66 show a substantial increase in volume ΔV with increasing extension ε, whereas PA66 becomes denser. At ε=40%, ΔV is 60% in RTPA66, but −1.4% in PA66. In creep experiments on both polymers, extension follows the Andrade equation ε(t)=ε(0)+bt ^1/3. Eyring plots of logb against applied stress σ are linear for the PA66, but the RTPA66 shows a sharp increase ind logb/dσ above 30 MPa, where significant dilatation begins. Scanning electron micrographs of tensile and impact specimens reveal that dilatation in RTPA66 is due to formation of voids within the rubber particles, leading to fibrillation of the nylon matrix at high strains. It is concluded that this cavitation accelerates shear yielding in the nylon matrix.     

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part IThe deformation of three aluminum alloys at 473 K to 833 K

Constant-load indentation tests were performed on wrought-2024, P/M-2024, and wrought-1100 aluminum alloys to assess the capability of the microindentation testing technique for measuring the high-temperature deformation rate controlling parameters of these alloys. The three alloys all display threshold indentation stress σ_th below which the indentation strain rate ε_ind approaches zero. The nominal inter-obstacle spacing, ℓ*, calculated from σ_th, increases with temperature in a way that is consistent with the known temperature dependence of the inter-particle spacing and dislocation cell size. The measured activation energy ΔG_o of ɛ_ind increases with temperature but remains within the range that is typical of deformation that occurs by dislocation glide limited by weak particles or dislocation/dislocation interactions. The three alloys tested show different trends of ΔG_o versus ℓ* and the trends are consistent with the known temperature dependence of the obstacles to dislocation glide. This study demonstrates that high-temperature indentation tests are sufficiently precise to detect changes in the operative deformation parameters between different alloys of the same general composition. This lays the groundwork for the use of this technique as a general tool for studying the local high-temperature deformation of a wide range of metal-based systems.     

A correlation of ΔK th-value with the exponent,m, in the equation of fatigue crack growth for various steels

Literature (mainly in Japan) relating to fatigue-crack-growth-data at R=0 in an air environment for a wide range of steels is reviewed with particular attention to the threshold stress intensity, K _th. The collected data are analyzed in terms of the exponent, m(the slope of the linear portion of the log(da/dN)-logD relationship) by taking account of microstructure, material strength, fracture toughness and specimen thickness. The mean rate of fatigue crack growth for ductile steels in the range from the intermediate growth rate to threshold level and the relevant threshold values at R=0, K _th0, can be represented asda/dN = 1.700 × 10^-4(K103.6)^m - 10^-6andK _th0 = 103.6(5.88 × 10^-3)^1/m,where (da/dN) and K are measured by the units of mm/cycle and kgf/mm^3/2, respectively. Contrary to this, in the case of extremely brittle steels with K _IC-value below 200 kgf/mm^3/2 (the fracture occurs by the intergranular separation), the relationships are given byda/dN = 2.893 × 10^-5(K/49.94)^m - 2.5 × 10^-7andK _th0 = 49.94(8.64 × 10^-3)^1/m.

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Résumé La littérature, principalement japonaise, relative aux données sur la propagation des fissures de fatigue pour R=0 dans un environnement d'air et pour une large catégorie d'aciers fait l'objet d'une revue avec une attention particulière pour l'intensité de contrainte de seuil, K _th. Les données collectées sont analysées en terme de l'éxposant m (qui représente la pente de la portion linéaire de la relation log(da/dN)-log K, en tenant compte de la microstructure, de la résistance du matériau, de la ténacité à la rupture et de l'épaisseur de l'éprouvette. La vitesse moyenne de propagation d'une fissure de fatigue dans le cas des aciers ductiles dans la fourchette entre la vitesse intermédiaire de fissuration et le niveau critique, et les valeurs correspondantes de seuil à R=0, K _th0 peuvent être représentées par la relation:da/dN = 1.700 × 10^-4(K103.6)^m - 10^-6etK _th0 = 103.6(5.88 × 10^-3)^1/m,où (da/dN) et K sont mesurés en unités de mm/cycle et en kgf/mm^3/2 respectivement. En contraste, dans le cas d'aciers extrèmement fragiles avec des valeurs K _IC en dessous de 200 kgf/mm^3/2 (la rupture se produit par une séparation intergranulaire), ces relations sont données par:da/dN = 2.893 × 10^-5(K/49.94)^m - 2.5 × 10^-7etK _th0 = 49.94(8.64 × 10^-3)^1/m.

     

A method and apparatus for precision measurements of the absorption coefficient of wide-aperture microwave radiators in free space

A method and apparatus for measuring the absorption coefficient χ _f (the relative radiation coefficient ε _f ) of wide-aperture radiators in free space are described. The requirements on the values of ε _f and the error Δε _f , which ensure the necessary accuracy when calibrating these radiators, are discussed. The results of measurements of ε _f and Δε _f of the radiating element of a cryogenic wide-aperture radiator in the 10–43.2 GHz frequency band are presented.     

Evaluation of cyclic elastoplastic strains near notches

On the basis of direct measurements of the opening displacements of notch contours, we develop an experimental-numerical procedure for the evaluation of the range of local elastoplastic strain Δε_τ. The experimental values of Δε_τ are compared with the data of numerical calculations performed by using formulas known from the literature (Δε). It is shown that, for Δε_τ(Δε)<2%, the experimental and theoretical values are in good agreement but only in the case where the gradients of strains near the tips of the notches are taken into account. For Δε_τ>2%, the theoretical values of Δε are higher than experimental Δε_τ and the smaller the radius of the notch ρ, the greater the difference between these values. Therefore, it reasonable to use the experimentally evaluated and approved values of the parameter Δε_τ. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 2, pp. 62–72, March–April, 1997.     

Temperature dependence of the local Seebeck coefficient near the boundary in touching Cu/Bi–Te/Cu composites

The thermo-emf ΔV and temperature difference ΔT across the boundary were measured as functions of r and I for the touching p- and n-type Cu/Bi–Te/Cu composites composed of t _Bi–Te = 2.0 mm and t _Cu = 0.3 mm, where r is the distance from the boundary and I is a direct current producing ΔT which flows through two Peltier modules connected in series. The resultant Seebeck coefficient α across the boundary is obtained from the relation α = ΔV/ΔT. As a result, the resultant |α| of the touching p- and n-type composites have a great local maximum value at r ≈ 0.03 mm and decrease rapidly with further increase of r to approach the intrinsic |α_Bi–Te|. The maximum resultant α of the p- and n-type composites reached great values of 1,043 and −1,187 μV/K at 303 K corresponding to I = 0.8 A and of 1,477 and −725 μV/K at 360 K corresponding to I = 2.0 A. Reflecting the temperature dependence of the intrinsic α_Bi–Te, the maximum α of the p-type composite increases with an increase of T, while that of the n-type one decrease with an increase of T. Surprisingly, the maximum α of the p- and n-type composites have great gradients of 8.36 and −7.15 μV/K² in the range from 303 to 366 K, respectively, which are 21.8 and 134 times larger in absolute value than 0.383 and −0.0535 μV/K² of the intrinsic p- and n-type α_Bi–Te, so that the maximum resultant α was thus found to be much more sensitive to temperature than the intrinsic α_Bi–Te. Moreover, the local Seebeck coefficient α _l (r) derived analytically from the resultant α(r) is enhanced significantly in the narrow region below r ≈ 0.05 mm and the maximum α _l values of the p- and n-type composites were found to have extremely great values of approximately 1,800 μV/K at 360 K and −1,400 μV/K at 303 K, respectively, which are approximately 7.3 and 6.5 times higher in absolute value than the intrinsic p- and n-type α_Bi–Te at the corresponding temperatures.     

Thermodynamic Properties of Matrine in Ethanol

In this paper, the enthalpies of dissolution of matrine in ethanol (EtOH) were measured using a RD496-2000 Calvet microcalorimeter at 309.65 K under atmospheric pressure. The differential enthalpy (Δ_dif H _m) and molar enthalpy (Δ_sol H _m) of dissolution of matrine in ethanol were determined. And the relationship between heat and the amount of solute was also established. Based on the thermodynamic and kinetic knowledge, the corresponding kinetic equation that described the dissolution process was determined to be \fracdadt=2.36×10^-4(1-a)^1.09{\frac{{\rm d}\alpha}{{\rm d}t}=2.36\times 10^{-4}(1-\alpha )^{1.09}} . Moreover, the half-life, t _1/2 = 48.89 min, Δ_sol H _m = −12.40 kJ · mol⁻¹, Δ_sol S _m = −354.7 J · mol⁻¹ · K⁻¹, and Δ _sol G _m =  97.43  kJ · mol⁻¹ of the dissolution process were also obtained. The results show that this work not only provides a simple method for the determination of the half-life for a drug but also offers a theoretical reference for the clinical application of matrine.     

The Interplay of Phonon and Magnetic Mechanism of Pairing in Strongly Correlated Electron System of High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Cuprates

We consider magnetic mechanism of superconducting pairing in the effective low energy t−t′−t″−J ^* model with all parameters calculated ab initio. Interaction of strongly correlated electrons with different phonon modes is also incorporated. In a BCS type theory, the d_x²-y²d_{x^{2}-y^{2}} gap is given by a sum of magnetic and phonon contributions. The main contribution to the only fitting parameter G is determined by a competition of the breathing and buckling modes. Fitting the parameter G from the isotope effect, we obtain that magnetic and phonon contributions to the critical temperature T _c work together and are of the same order of magnitude.     

Fatigue crack growth in 15KH2MFA steel subjected to aging in high-temperature hydrogen

We study the influence of high-temperature aging in gaseous hydrogen on the cyclic crack-growth resistance of 15Kh2MFA steel by taking into account the effect of fatigue crack closure. We show that aging in hydrogen promotes the intense pulverization and homogenization of the structural components. Changes in the structure of steel and a decrease in the effective fatigue threshold ΔK _{th eff} on holding in hydrogen are caused primarily by the action of hydrogen and the load is an additional factor accelerating this process. We discovered that the effect of internal hydrogen on the effective fatigue threshold is ambiguous depending on the level of stresses in the process of holding in hydrogen. Thus, ΔK _{th eff} increases on holding at σ=120 MPa as compared with the degassed metal but decreases on holding at σ=240 MPa. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 4, pp. 121–126, July–August, 1997.     

Glass transition and fragility of telluro-vanadate glasses containing antimony oxide

The activation energy (ΔH ^*) of the glass transition and the heating-rate dependence of the glass transition temperature (T _g) of V₂O₅–Sb₂O₃–TeO₂ glasses were determined using differential scanning calorimetry technique. Non-isothermal measurements were performed at different heating rates φ (=3, 6, 9, 10, 13 K/min). The heating rate dependence of T _g was used to investigate the applicability of different theoretical models describing the glass transition. The application of Moynihan and Kissinger et al. models to the present data led to different values of (ΔH ^*) at each different heating-rate regions. This behavior was attributed to the strong heating rate dependence of the activation energy of the process. The fragility parameter (m = ΔH ^*/RT _g) were ≲90, suggesting that these glasses may be classified as strong glasses. The viscosity, η, calculated at a few selected temperatures near the glass transition region increased with increasing Sb₂O₃ content at any given temperature, which is also expected. Also the compositional dependence of T _g and ΔH ^* was investigated.