Engineering Physics of High-Temperature Materials. Nirmal K. Sinha

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СКАЧАТЬ geologic time (Bates and Jackson 1980; Lapidus 1987).

      Trinity of Fire

      PRODUCTION

      CONTROL

      MAINTENANCE

      In the early Holocene age, the primary human needs arose from requiring protection from the elements and broadening food sources. These necessities led to the quest for controlled fire and with it the realization of the need for techniques for producing (P) and controlling (C) fire at will, rather than depending on its natural sources, as well as maintaining (M) fire as the need arose.

      The trinity of fire (PCM) – i.e. generating fire whenever the need arises, using it, and preserving the source – was probably the most important aspect of human development. PCM provided hominines with the opportunity to extend their habitats to colder regions of the earth where caves could be used as heated (air‐conditioned!) and protected shelters. It also started a new era – the taste for cooked food and the beginnings of the kitchen for such food items anywhere, any time. The concept of “home cooking” and the advent of kitchens, marked, albeit debatable, the departure point from the cohabiting animal, although many animals and birds have developed the taste for burnt food produced during natural fires in forests or grassy lands.

      Producing fire under dry environmental conditions was perhaps the beginning of this phase of human development and that of “kitchen” and “home cooking.” PCM heralded a transition from nomadic, constantly moving communities of hunters and gatherers and cave dwellers, to relatively stable societies with fabricated housing, making tools for various applications, and eventually to the development of agriculture and thus creating new social structures, to name a few.

      Trinity of Civilization

      TECHNOLOGY

      FIRE

      LANGUAGE

      No doubt, PCM was responsible for opening an unlimited number of opportunities for the development of wide‐ranging technologies. As suggested by Gowlett (2016), the combination of technology (T), fire (F), and language (L), “can be seen as ‘the big three’, deeply connected [concepts] in the end and perhaps at earlier stages” of civilization. We may refer to this as the trinity of civilization.

      The latest cycle of long‐term global warming started around 18 000 years ago, which is the estimated peak period of the last glaciation. Of course, there were mini cycles of freezing and thawing within the Holocene age. The current stage of post‐glacial period also helped the human race, equipped with PCM and TFL knowhow, to spread its wings to northern countries of the globe. As the ice cover melted and the flora and the fauna took over the arid landmass, migration of human race occurred in Scandinavia and Siberia in Eurasia and the continents of North America, as well as South America and the south‐Pacific islands, and probably caused migration from the north and the center of the continent of Australia to its southern areas. The cyclic nature of the global‐scale warming and deglaciation also forced the human race to relocate from time to time due to the rise of the oceans. There are legends that tell the stories about the First Nations of Australia moving inland as the GBR developed. The GBR did not exist during the Pleistocene (meaning most new or newer) epoch or more than about 15 000 years ago. The Holocene era is marked as the current age of global warming and deglaciation. During the latter part of the Holocene, repeated and “sustained” flooding due (most probably) to meltwaters from the Himalayan ice caps forced the Harappans of the Indus Valley Culture (IVC) to abandon their network of planned cities built with “modern‐day” fired bricks along the Indus River.

Figure 1.1 Harappan style of brick production from clay. (a) Process of sun drying of clay bricks (in Kolkata)

      Source: Courtesy of Srewoshi Sinha;

      (b) and (c) Fired products at a construction site in New Delhi in 2010.

      Source: Nirmal Sinha.

The spread of knowledge for producing fire under widely different environmental conditions to various communities living under diverse environmental conditions evolved over thousands of years. All along the development of settlements of humankind – whether in the well‐known cradle of civilization or cultures in Egypt, Mesopotamia, or the not so known, but still living, IVC of Harappa and Mohenjo‐daro in ancient India (National Geographic Society 1979; Keay 2004) – kitchens with fireplaces provided the place for innovative thoughts on making “ceramic” pots and pans and building materials from baked clay. Interestingly, Harappan bricks were used for making railway lines even a hundred years ago, and the IVC technology in its most rudimentary stage (handmade, sun‐dried, and fired in stacks) is still practiced and it provides millions of jobs all over the Indian subcontinent, making bricks and constructing even high‐rise living quarters (Figure 1.1).

      Innovation naturally led toward the use of various silicate materials for making sun‐dried bricks, as well as pots and pans for storing dried food and grains. It is strongly believed that cooking over fires using sun‐dried pots led to the invention of durable fired clay or terra‐cotta pots for storage of water as well. In ancient kitchens, the containment of fire by sun‐dried clay blocks, made from fine‐grained clay materials, probably led to the observations on high‐temperature sintering of clay particles. The next stage was the development of fired bricks for making shelters. The challenge was to make bricks without cracks.

      Building materials, whether they are rocks or metals, are mostly polycrystalline materials. Glass, unlike crystalline materials, is amorphous and lacks any periodicity in arrangements of its atomic structure. The world of natural silicates is vast, and molten silicates are part of the magma deep inside Earth's surface. If a mixture of natural silicates melts and then cools rapidly below its melting point, a silicate glass is formed. Naturally, on top of Earth's surface, there are localized glassy phases within volcanic rocks. No wonder glass became one of the oldest and familiar materials known to mankind and found use in many forms.

Glass as we know it – generally in the form of containers, sheets, mirrors, etc. and in buildings as cladding and window materials – is primarily the form of silicate glasses, particularly soda‐lime–silica glasses. There is a wide variety of optical, cookware, and specialty glasses. Very rarely, unless laminated with layers of polymers, glass is used for any load‐bearing components of buildings. Glass plates or sheets develop microscopic surface flaws after manufacturing. Brittle propagation of these surface cracks is the limiting factor for the use of glass. Creep properties of window glass, even for laminated products, limits its use at high temperatures. Understanding the rheological properties of glass, however, is crucial for manufacturing glass articles and strengthening them for use as safety glasses and tempered windows of automobiles and buildings. We will broaden the exploration of the structural aspects of glass in Chapter 2 and experimental techniques for microstructural analysis of ice (especially dislocations as line defects and pileups) in Chapter 3. Rheology, failure of glass, and thermal tempering toughening are presented in detail in Chapters 4 and 5. The mechanisms for the nucleation of grain‐facet‐sized intergranular cracks, kinetics of crack generations, СКАЧАТЬ