Spherulites and phase separation in plasma-dissociated zircon

Plasma-dissociated zircon consists of a mixture of monoclinic and tetragonal ZrO₂ in SiO₂ glass. The proportion of tetragonal ZrO₂ increases with decreasing initial zircon particle size and increasing cooling rate; zircon sprayed onto a cold substrate consists entirely of tetragonal ZrO₂ in glass. Tetragonal ZrO₂ is nucleated at large undercooling during cooling of molten zircon particles and grows in a spherulitic manner because of a low-diffusivity boundary layer at the growing crystal-liquid interface. At smaller particle sizes and higher cooling rates the thermal history of liquid droplets is such that an alternative process of phase separation into ZrO₂-rich and SiO₂-rich liquids becomes possible in the liquid remaining between spherulites. The particle size distribution of the ZrO₂ crystallites which form in this way shifts towards smaller particle sizes with increasing cooling rate and those smaller than 20 nm diameter do not transform to the equilibrium monoclinic form on cooling to room temperature because of a surface/ matrix restraint effect.     

The influence of quenching rates on the microstructure and properties of plasma-dissociated zircon

The physical nature of what is usually described as plasma-dissociated zircon is very much dependent on the rate of quenching of the molten material after it has passed through or around the plasma. A product composed of small particles is rapidly cooled and the dominant microstructure is that of tiny crystallites of monoclinic zirconia intimately intergrown with a matrix of amorphous silica. If, as a consequence of the layout of the device itself, the product is in a more massive form, then the cooling rates in the interior are slower and considerable grain growth occurs. The zirconia forms segregations of comparatively large grains which, though still surrounded by silica, lack the intimate intergrowth we now find to be necessary to impart certain unique properties recently shown to be of interest to the ceramic colour, refractory and metallurgical industries. There is a continous relationship between microstructure and quenching rate, whether the samples come from the same or different plasma furnaces.     

Resistance parameters and water quench test as criteria of thermal shock behaviour of alumina based refractories

Abstract

The materials investigated were five alumina based commercial refractories with different contents of Al₂ O₃ +TiO₂ from 28 to 78%. Thermal (k thermal conductivity, α coefficient of thermal expansion) and mechanical (E Young's modulus of elasticity, ν Poisson's ratio, σ_f flexural strength, σ_m mean strength, γ fracture surface energy) properties of the chosen materials were measured. The thermal shock stability of specimens was determined using a water quench test (JUS B. D.8.319). Fracture and damage resistance parameters were calculated using average values of the measured thermal and mechanical properties. Calculated resistance parameter values were compared with the results of water quench testing. Linear regression analysis showed that fracture (R and R′) and damage (R?) parameters could be correlated with the results of the water quench test, with high values of the correlation coefficient.     

Some numerical approaches of creep, thermal shock, damage and delayed failure of ceramics and refractories

Numerical simulation is now very often used to predict the behaviour of components in service conditions. This paper is interested in specific approaches concerning ceramic materials and refractories. Creep can be satisfactorily described by a kinematic hardening, and exhibits different creep rates in tension and compression. Concerning the thermal shock of materials, the numerical approach depends whether or not the material is able to develop a sprayed out damage, leading to micro or macro-cracking. Finally, delayed failure at high temperature can be considered as a consequence of creep, but the random aspect of failure seriously complicates the numerical models. The lack of experimental data presently limits the calibration and the validation of the numerical models.     

The zircon thermal behaviour: effect of impurities

The effect of impurities on zircon thermal behaviour is discussed and explained from a thermodynamic point of view. Based on the data obtained from the ZrO₂-Al₂O₃-SiO₂-TiO₂ system, previously studied by the authors, the influence of the impurities, normally present in zircon sands, on the dissociation and initial melting temperatures in natural zircon, has been studied.     

Improved crack resistant liquid epoxy resins

Recent trends in the electrical industry indicate a growing demand for liquid epoxy castings that combine a high usage temperature (i.e. high glass transition temperature of the cured resin) with good crack resistance.It is well-known that the use of liquid instead of solid epoxy resin results in an increase in the glass transition temperature, but this is usually accompanied by a reduction in crack resistance. Frequently, such liquid epoxy resin based castings have to be flexibilized to improve crack resistance, but then the glass transition temperature is inherently lowered.The present paper describes some approaches to a combination of a high glass transition temperature with excellent crack resistance. A study of a number of criteria showed that this can be achieved by careful control of the crosslink density.The proper ratio of epoxy resin to curing agent, in combination with a suitable catalyst ensures castings with a glass transition temperature ranging from 95 to 125°C, and which pass standard crack tests even at temperatures below -80°C and temperature cycling from +150°C to -75°C.     

Preparation of a tantalum-based MoSi2-Mo coating resistant to ultra-high-temperature thermal shock by a new two-step process

To develop an ultra-high-temperature resistant coating for a reusable thermal protection system,the preparation of a tantalum-based MoSi2-Mo coating by a new two-step process of multi-arc ion plating and halide activated pack cementation is presented.The coating has a dense structure and is well compatible with the tantalum substrate,which can be thermally shocked from room temperature to 1750℃ for 360 cycles without failure.The mechanism of the coating's excellent resistance to high-temperature thermal shocks is that a strong-binding gradient interface and a dense SiO2 oxide scale with good oxygen resistance are formed by the high-temperature self-diffusion of Si.     

A model of crack extension from thermal shock by surface contact

The growth of cracks in glass arising from thermal shock on contact with a second material is an important problem in the volume production of glass articles. The thermal properties of the contacting material are controlling factors in producing cracking and a simple model of crack extension from thermal shock was developed to investigate the process. Given a pre-existing microscopic crack at the surface of the glass, it is shown that one dimensionless parameter, termed the thermal index of the materials combination, controls the process of crack extension and crack arrest. If the index is very much less than unity, the probability of crack extension can be made very small. The consequence for practical applications of new materials in handling hot glass is discussed.     

Relationship of gas phase mass transfer to thermal conductivity of refractories

The problem of change in thermal resistance of pores and microcracks in refractories with occurrence of heterogeneous reactions and formation of gas with diffusion product transfer in the temperature gradient field is analyzed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 3, pp. 452–455, March, 1978.     

Contact thermal shock test of ceramics

A novel quantitative thermal shock test of ceramics is described. The technique employs contact between a metal cooling rod and hot disc-shaped specimen. In contrast with traditional techniques, the well-defined thermal boundary condition allows for accurate analyses of heat transfer, stress, and fracture. Uniform equi-biaxial tensile stresses are induced in the centre of the test specimen. Transient specimen temperature and acoustic emission are monitored continuously during the thermal stress cycle. The technique is demonstrated with soda-lime glass specimens. Experimental results are compared with theoretical predictions based on a finite element method thermal stress analysis combined with a statistical model of fracture. Material strength parameters are determined using concentric ring flexure tests. Good agreement is found between experimental results and theoretical predictions of failure probability as a function of time and initial specimen temperature.     

Damage-tolerant laminated composites in thermal shock

Alumina is very susceptible to thermal shock which often leads to catastrophic failure. The addition of tape-cast nickel layers to laminated alumina can increase its tolerance to damage induced during thermal shock. The fracture behaviour after thermal shock of laminated composites showed non-catastrophic failure during loading at room temperature. The retained strength of the laminates was determined for a wide range of quenching-temperature differences (T= 150–1200C). The retained strength and critical quenching-temperature difference, T _c, of the laminated composites were a significant improvement over the values for the respective monolithic alumina. This improvement in behaviour can be related to the compressive residual stress in the alumina layers and ductile-layer crack blunting.     

The thermal shock of ß-alumina

The thermal shock of sodiumβ-alumina with relative densities from 60 to 98% theoretical has been investigated over the temperature range 150 to 700° C by quenching into water. The samples were ring segments cut from electrolyte tubes and were subsequently tested in both compression and tension. For relative densities of 75% and below the thermal shock damage was typical of stable crack growth and a steady decline in strength with sintering temperature was observed. For relative densities of 95% and above, thermal shock causes unstable crack growth and a critical value of ΔT was observed in the range 170 to 250° C depending on initial strength. From the linear relationship between observed ΔT _c and the thermal shock resistance parameter,R, it was concluded that the rapid heat transfer during quenching was nucleate water boiling and that cooling from ∼110° C to 0° C was not responsible for damage. The fracture stress after thermal shock above ΔT _c was consistent and showed little dependence on initial strength for relative densities ⩾95%. However, the fractional reduction in strength was related to the damage resistance parameterR‴. An estimate of the energy expended in fracture has been made, based on microscopic observation and compared with estimates of the stored strain energy due to thermal stresses.     

Acoustic emission from crack growth in an advanced zirconia refractory under thermal shock

Crack initiation and growth during the thermal shock tests of a partially stabilized zirconia advanced refractory were investigated by the analysis of acoustic emission (AE) amplitudes. The growth of cracks that were detected by AE was systematically monitored by SEM observations as increasingly severe thermal shocks were applied. The measurements of strength loss after thermal cycling in the ribbon test with various applied temperature differentials correlated with continuous monitoring by acoustic emission and confirmed the effects of microcrack growth on the resistance to thermal shock damage.     

Bi-layer molybdenum disulfide obtains from molybdenum disulfide-melamine cyanurate superlattice with a thermal shock

To investigate thermal shock properties of melamine cyanurate (MCA) for molybdenum disulfide (MoS₂) exfoliation, the MoS₂-MCA superlattice is prepared through self-assembly of pristine MoS₂, cyanuric acid (CA) and melamine (MA) in water solution. The prepared MoS₂-MCA is heated in tubular furnace with controlled temperature to obtain two-dimensional (2D)-MoS₂ precursor. Their morphologies are characterized by using Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The obtained 2D-MoS₂ precursor shows a high specific surface area at 336.90 m²/g due to bi-layer MoS₂. TEM and atomic force microscope (AFM) images of 2D-MoS₂ N-methyl-2-pyrrolidone (NMP) solution further confirm the bi-layer MoS₂ exfoliation. The above results prove that MCA is a good thermal shock agent for MoS₂ exfoliation. And the prepared 2D-MoS₂ precursor could also be a good solid reservoir.     

Thermal shock and thermal fatigue study of ceramic materials on a newly developed ascending thermal shock test equipment

A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples.     

Deformation of cylindrical shells by thermal shock

In a plane two-dimensional statement the article investigates numerically the processes of deformation and rupture of cylindrical shells under asymmetric thermal shock. It studies the effect of the thickness of the shell on the nature of deformation and rupture. Computing difficulties in the solution of similar problems are pointed out.Translated from Problemy Prochnosti, No. 6, pp. 64–69, June, 1990.