Anton Trník

Determination of Young's Modulus of Ceramics from Flexural Vibration at Elevated Temperatures

Summary In this paper we describe the construction of a laboratory-made resonant apparatus for the modulated force thermomechanical analysis (mf-TMA) and estimate the uncertainty of the measurement of Young’s modulus of a porcelain sample. The porcelain sample was measured while increasing the temperature up to 1000 C/5 C/min. Thermodilatometry was performed to determine the actual dimensions of the sample in the same temperature regime. We assessed the influence of the repeatability of the directly measured values, the accuracy of the used measurers and di erent exceptions from the ideal experimental arrangement on the Young’s modulus measurement uncertainties of types A and B. We found that the majority of the total uncertainty (80%) is determined at the room temperature. The remaining sources of uncertainty can be attributed to thermal expansion of the sample and inhomogeneity of the temperature field. The relative expanded uncertainty of the Young’s modulus at the elevated temperatures was 0.9%.

Effective thermal conductivity of hollow bricks with cavities filled by air and expanded polystyrene

Effective thermal conductivity of hollow bricks with the cavities filled by either air or expanded polystyrene is analyzed using a hybrid experimental–computational approach. The experimental setup involves an application of a thermal insulation box and a set of temperature and heat flux probes placed at characteristic positions of the specimen-insulating box system. Using the measured heat fluxes and temperatures, the heat loss of the system is determined. A computer simulation tool based on the finite element principle is then used for modeling the temperature fields and heat losses in the studied system. Finally, the effective thermal conductivity is identified using an iterative procedure. Experimental results show that the application of expanded polystyrene as cavity filler instead of air leads to ∼30% decrease in the effective thermal conductivity of hollow brick blocks.

Mechanical Properties of Kaolin during Heating

Flexural strength (MOR) and Young’s modulus (YM) of Sedlec kaolin were measured using the three point-bending method and modulated force thermomechanical analysis (mf-TMA). Thermal analyses DTA, thermogravimetry and thermodilatometry (TDA). An escape of the physically bound water (20 – 250 °C) strengthens the sample and YM and MOR increase their values significantly. MOR and YM lower their values as dehydroxylation starts at 400 °C. Both quantities, MOR and YM, pass through minimum in the dehydroxylation region (400 – 650 °C). Their next increase is probably caused by the van der Waals forces acting between metakaolinite crystals and by the starting of the solid-state sintering. YM steeply increases above 950 °C as a consequence of the solid-state sintering. A Weibull’s modulus passes through the sharp maximum at the interval 300 – 400 °C.

Mechanical Properties of Kaolin-Base Ceramics During Firing

Firing of silicate ceramics, which are made of clays with high contents of kaolinite, transforms a green body into a ceramic product [1, 2]. The green body exhibits significant changes of its properties resulting from dehydration at low temperatures, phase changes during dehydroxylation and high-temperature reactions, and densification during sintering [3, 4]. All these changes significantly influence mechanical properties of the fired body as well as its other physical properties. To save time and energy, it is desirable to conduct the firing in the shortest time possible without damage to the fired ceramic body. Calculating the safe upper limit of the heating or cooling rate of the large ceramic bodies (e.g. high-voltage insulators) is a complex task that requires knowing five material quantities: mechanical strength (MOR), Young’s modulus (YM), Poisson’s ratio, thermal conductivity, and coefficient of the linear thermal expansion (CLTE). All of these quantities must have to be known as functions of the actual temperature at the firing. The upper limit of the heating rate according to [2] is

Chiral segregation, an unusual racemic phase, and a residual entropy for a lattice system of model chiral molecules

We study low temperature phases of a 2D model molecular system that contains an equimolar mixture of a chiral molecule and its mirror image. The molecules lie on a thin film in the close-packed regime, occupying the sites of a honeycomb lattice, and neither enantiomer is externally favored. We show that, in one range of interactions, chiral segregation into two ordered homochiral phases containing a single enantiomer occurs. An ordered racemic phase occurs in which one sublattice is occupied by one enantiomer and the other sublattice is occupied by the other enantiomer. When second-closest group interactions are relaxed, the ground states of the homochiral and the associated racemic phase have the same energy; in fact, the total number of ground state configurations becomes infinite and yields a residual entropy. This residual entropy is calculated exactly. In a third range of interactions, an unusual ordered racemic phase occurs in which the two enantiomers are assembled in alternating infinite rows, thus forming a pattern independent of the sublattice structure. In order to prove these results, we apply the Pirogov–Sinai theory, a powerful generalization of the Peierls-type approach of statistical mechanics.

UHPFRC at high temperatures – Simultaneous thermal analysis and thermodilatometry

Simultaneous Thermal Analysis (STA) and Thermodilatometry Analysis (TDA) are done to reveal the structural and chemical changes in UHPFRC during its high-temperature load. Based on the measured results, several physical and chemical processes that studied material underwent at high-temperatures are recognized. In the temperature interval from 25 to 300 °C, the liberation of physically bound water from pores and the dehydration reaction of C-S-H take place. Additionally, AFt and AFm phases dehydrate at 110 – 156 °C. Endothermic peat at 460 °C corresponds to the portlandite decomposition. At 575 °C, the α → β transformation of quartz is found. This reaction is accompanied by a sharp endothermic heat flow peak and a volume expansion, whereas no change of mass is measured. In the temperature interval 580-800 °C, the calcite and C-S-H gels decomposition is monitored. At the temperature above 800 °C, there is one significant exothermal peak corresponding to a crystallization of wollastonite. In summary, STA and...

Modelling of phase transitions: do it yourself

We present the basics of a powerful contemporary statistical mechanical technique that can be used by students to explore first-order phase transitions by themselves and for models of their own construction. The technique is a generalization of the well-known Peierls argument and is applicable to various models on a lattice. We illustrate the technique with the help of two simple models that were recently used to simulate phase transitions on surfaces.

Many-particle surface diffusion coefficients near first-order phase transitions at low temperatures.

We analyze the chemical and jump surface diffusion coefficients, D(c) and D(J), near a first-order phase transition at which two phases coexist and the surface coverage, θ, jumps between single-phase values θ(-)(*) and θ(+)(*). Contrary to other studies, we consider temperatures that are sufficiently subcritical. Using the local equilibrium approximation, we obtain approximate analytical formulas for the dependences of D(c) and D(J) on the coverage and system size, N, near such a transition. In the two-phase regime, when θ ranges between θ-* and θ+*, the diffusion coefficients behave as the sums of two hyperbolas, D(c) ≈ A-/N|θ-θ(-)(*)| + A+/N|θ-θ(+)(*)| and D(J) ≈ A(-)|θ-θ(+)(*)|/θ+A(+)|θ-θ(-)(*)|/θ. This behavior rapidly changes as the system goes from the two-phase regime to either of the single-phase regimes (when θ goes below θ(-)(*) or above θ(+)(*)). The crossover behavior of D(c)(θ) and D(J)(θ) between the two-phase and single-phase regimes is described by rather complex formulas involving the Lambert function. We consider a lattice-gas model on a triangular lattice to illustrate these general results, applying them to four specific examples of transitions exhibited by the model.

Surface diffusion near first-order phase transitions for a model on a triangular lattice

We rigorously study the thermodynamic factor of surface diffusion for a two-dimensional one-component lattice-gas system on a regular triangular lattice. First, we prove that four low-temperature phases exist in the system, their surface coverages being 0, 1/3, 2/3 and 1, and that, depending on the interparticle interactions, a phase transition between any two of them can occur. Three of these transitions are shown to be of first order at low temperatures, while in the other three cases there is an infinite number of ground states, yielding in fact a residual entropy. Then, for the three first-order phase transitions, we provide explicit finite-size formulae for the free energy, surface coverage and isothermal susceptibility, showing their low-temperature dependence on the chemical potential and the total number of adsorption sites. Using these formulae, we numerically obtain the coverage dependence of the thermodynamic factor and compare it with analytical results.

Residual Mechanical Properties of Hybrid Fiber Reinforced HPC Exposed to High Temperatures

The effect of high temperature load on mechanical properties and porosity of a newly designed Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is studied. The hybrid reinforcement of UHPFRC is based on a mixture of polypropylene and steel fibers. In order to identify influence of high temperature exposure on UHPFRC, its residual mechanical parameters such as compressive strength, flexural strength and Young’s modulus of elasticity are accessed. Moreover, residual bulk density, matrix density and total open porosity are examined and related to the monitored structural changes. Simultaneous Thermal Analysis (STA) is employed in order to describe transformation processes during high temperature loading. The conducted tests provide practical information for controlled regulation of water vapor transport in a low permeable cementitious composite in order to decrease risk of spalling.