James Derek Birchall

The relation between porosity, microstructure and strength, and the approach to advanced cement-based materials

A theory is formulated to connect the strength of cement paste with its porosity. The theory shows that bending strength is largely dictated by the length of the largest pores, as in the Griffith (1920) model, but there is also an influence of the volume of porosity, which affects toughness through changing elastic modulus and fracture energy. Verification of this theory was achieved by observing the large pores in cement, and then relating bending strength to the measured defect length, modulus and fracture energy. The argument was proved by developing processes to remove the large pores from cement pastes, thereby raising the bending strength to 70 MPa. Further removal of colloidal pores gave a bending strength of 150 MPa and compression strength up to 300 MPa with improved toughness. Re-introduction of controlled pores into these macro-defect-free (mdf) cements allowed Feret’s law (1897) to be explained.

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

The physical and mechanical properties of YBa2Cu3O7−° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Youngs modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.

Influence of toughness on Weibull modulus of ceramic bending strength

It is demonstrated both theoretically and experimentally that fracture toughness does not directly influence the Weibull modulus of ceramic bending strength for materials that obey the Griffith criterion for crack propagation. Weibull modulus remains unchanged as toughness is increased. However, toughness variations with crack length do affect the Weibull modulus. Thus materials that display R-curve behavior or Dugdale character give an increased Weibull modulus and appear more reliable.

Elasticity of particle assemblies as a measure of the surface energy of solids

A theory is suggested to explain the elasticity of particle assemblies. It is shown that a regular cubic packing of spheres has an effective Young’s modulus that depends on the contacts between individual particles. In particular, it is noted that the effective modulus depends on the interfacial attractive energy between the spheres, and thus provides a direct method for measuring the surface energy of solids. However, most particle assemblies are neither cubic nor regular. The problem is to describe the properties of these real systems in terms of the packing of the grains. Theoretically, it is shown that the modulus of a powder compact should vary as the fourth power of the particle packing fraction. This result has been verified experimentally and has been used to determine the surface energies of zirconia, titania, alumina, and silica powders. The measured values were sometimes much lower than expected from theoretical calculations of surface energy. Experiment has shown that such discrepancies result from contamination of the solid surfaces.

Toxicity of silicon carbide whiskers

There is growing interest in the use of ceramic materials, such as A1203, Si3N 4 and SiC in the form of whiskers for the reinforcement of ceramics and metals [1 3]. Epidemiological studies have established that exposure to certain fibrous minerals of natural occurrence is associated with increased incidence of mesothelioma a malignant tumour of the pleural or peritoneal cavity. Studies in experimental animals have shown that carcinogenicity (or at least the ability to induce mesothelioma) is restricted to long (> 10 #m), thin ( < 1 ~tm diameter) fibres and is relatively independent of chemical nature, providing the fibres are durable in the body (e.g. [4, 5]). Many whisker products consist of, or contain, fibres in the sub-micrometre diameter range and some have been shown to produce tumours in experimental animals [6]. We have prepared sub-micrometre diameter SiC whiskers by the carbo-thermic reduction of SiO2 and subjected this material to an in vitro cell culture test in which the response correlates well with the results of injection or implantation studies in animals [7]. A comparison has been made with crocidolite (blue) asbestos, which is well established as a causative agent for mesothetioma in human populations. The cellular response to the SiC whiskers is of the same order as crocidolite suggesting that in the absence of any additional information, such whiskers should be considered as potentially carcinogenic with appropriate handling precautions applied. A whisker-containing product of the carbo-thermic reaction (Fig. 1) contained a proportion of nonfibrous materials. The diameter range of the fibrous component is shown in Fig. 2 and no attempt to separate fibrous from non-fibrous material was made. The presence of free silica can lead to confusing results in the in vitro cell tests used and to avoid this the material was extracted with hot aqueous NaOH and washed in distilled water prior to use in the test. X-ray examination showed the SiC preparation to consist of highly crystalline cubic/?-SIC together with a minor amount of hexagonal c~-SiC and traces of mullite and corundum (A1203) from contamination by the crucible. The cell culture test has been previously described [8] and its specificity in terms of fibre size has been established [9]. Survival of V79-4 Chinese hamster lung cells was determined by cloning efficiency from a single-cell suspension. Cell suspension (20ml) was added to the appropriate quantity of test fibre in I ml of physiological saline in a sterile McCartney bottle. Aliquots of this suspension were seeded into 60-mm diameter plastic Petri dishes and the surviving cells allowed to grow and form colonies for 5 to 6 days. Colonies were then washed with physiological saline

On the hydration of Portland cement

When Portland cement is contacted with water, calcium ion is rapidly leached from the solid to form calcium hydroxide solution but only traces of silica are found in the aqueous phase. It is proposed that the hydrated, calcium-depleted surface of grains consists of low molecular mass silicic acids and that these interact with dissolved hydroxylated calcium species (principally Ca(OH)2) to produce a semi-permeable membrane of ‘ calcium silicate hydrate ’ at the hydrated grain surface. Osmotic pressure within this membrane causes its rupture and hence the growth of excrescences from the grain as the contents are extruded into the surrounding calcium hydroxide solution. The interstitial solid material is best regarded as a coagulum resulting from the combination of low molecular mass silicate anions with dissolved calcium hydroxide. It is proposed that the low tensile strength of Portland cement paste results from microstructural features consequent upon this mechanism of hydration and setting.

New phases in the superconducting Y:Ba:Cu:O system

An electron diffraction and microscopy study is presented of a variety of phases in the Y:Ba:Cu:O system in which superconductivity occurs. The superconducting phase is demonstrated by convergent beam electron diffraction to be centrosymmetric with space group Pmmm, in contrast to a previous determination of Pmm2. This discrepancy arises from local symmetry‐breaking defects. In addition to this phase and a cubic BaCuO2 phase, we characterize two other phases. One is the Y‐rich orthorhombic phase: Pnma with a=13.5 A, b=6.3 A, and c=7.6 A. The second occurs by a phase transition of the superconducting Pmmm phase to P4/mmm with a=3.85 A, c=11.7 A. The superconducting phase may now be described as either an ordered array of oxygen vacancies in the perovskite structure, or an ordered array of oxygen interstitials in the new tetragonal phase, which may explain how the material can lose oxygen reversibly.

Processing, properties and devices in high-Tc superconductors

The exploitation of high-Tc superconductors will ultimately depend on successful manufacturing methods. This paper describes methods which have been used to produce a wide range of superconducting devices. These include wire, antennae, microwave resonators, flux transformers, solenoid coils and gradiometers. The properties of the superconducting ceramics are described and the design and manufacture of prototype devices are discussed.

Cement in the context of new materials for an energy-expensive future

Hydraulic cements are energy-cheap relative to other common materials, are manufactured on a large scale and, when mixed with water, form readily mouldable pastes that harden at low temperature. In a technological sense, such pastes can be regarded as inorganic ‘plastics’, but the types of article that can usually be fabricated from the cements has been restricted by the low tensile strength and fracture toughness of hardened cement pastes. Poor mechanical properties are not inherent in inorganic solids formed under mild conditions; mineral structures of biological origin can display relatively high strength and useful toughness as a result of microstructural features determined by biopolymers. Recent studies have shown that the low tensile properties of cement paste result from the presence of macroscopic pores. The elimination of such defects by the use of polymeric rheology modifiers gives unreinforced cement pastes a flexural strength of 150 MPa or more. Such novel materials should considerably extend the range of uses for hydraulic cements.

Critical currents of sintered and melt‐textured YBa2Cu3Ox

High critical‐current densities, Jc, have been observed in single crystals and thin films of YBa2Cu3Ox. Jc in bulk materials is diappointingly low reaching a maximum in untextured material of about 103 A cm−2. The importance of raising Jc in bulk materials is paramount and most applications of bulk materials require high Jc in high magnetic fields. The work of Jin et al. in raising Jc to 17 000 A cm−2 in zero field represents a most important step in the development of bulk materials and gives real hope to bulk applications. In the communication we first describe the properties of untextured wire, and then we describe our process for melt texturing and the difficulties encountered.