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The present study explores the burning of microscale porous silicon channels with sodium perchlorate. These on-chip porous silicon energetics were embedded in crystalline silicon, and therefore surrounded on three sides by an efficient thermal conductor. For slow burning systems, this presents complications as heat loss to the crystalline silicon substrate can result in inconsistent burning or flame extinction. We investigated <100 μm wide porous silicon strips, sparsely filled with sodium perchlorate (NaClO4), to probe the limits of on-chip combustion. Four different etch times were attempted to decrease the dimensions of the porous silicon strips. The smallest size achieved was 12 x 64 µm, and despite the small dimensions, demonstrated the same flame speed as the larger porous silicon strips of 6-7 m/s. We predict that unreacted porous silicon acts as a thermal insulator to aid combustion for slow burning porous silicon channels, and SEM images provide evidence to support this. We also investigated the small scale combustion of a rapidly burning sample (∼1200 m/s). Despite the rapid flame speed, the propagation followed a designed, winding flame path. The use of these small scale porous silicon samples could significantly reduce the energetic material footprint for future microscale applications.
We present the first quantitative assessment of combustion dynamics of on-chip porous silicon (PS) energetic material using sulfur and nitrate-based oxidizers with potential for improved moisture stability and/or minimized environmental impact compared to sodium perchlorate (NaClO4). Material properties of the PS films were characterized using gas adsorption porosimetry, and profilometry to calculate specific surface area, porosity and etch depth. The PS/sulfur energetic composite was formed using three pore loading techniques, where the combustion speeds ranged from 2.9 – 290 m/s. The nitrate-based oxidizers were solution-deposited using different compatible solvents, and depending on the metal-nitrate yielded combustion speeds of 3.1 – 21 m/s. Additionally, the combustion enthalpies from bomb calorimetry experiments are reported for the alternative PS/oxidizer systems in both nitrogen and oxygen environments.
“Covenants, without the Sword, are but Words, and of no strength to secure a man at all,” Thomas Hobbes famously proclaimed. He exaggerated. As I shall point out later, his position is more subtle than that suggested by this famous citation. There is no doubt, however, that he thought the sword to be of central importance to the state. This is one of the very few points about which there is little disagreement today with Hobbes; state power is widely thought to be coercive. The view that governments must wield force or that their power is necessarily coercive is widespread in contemporary political thought. For instance, John Rawls claims that “political power is always coercive power backed up by the government's use of sanctions, for government alone has the authority to use force in upholding its laws.” He is not alone in thinking this, as we shall see. This belief in the centrality of coercion and force plays an important but not well-appreciated role in contemporary political thought. However, it is an idea which is to some extent mistaken and quite misleading.
State Power is Coercive
Rawls's belief in the coercive nature of political power appears to be significant for his liberal theory. It is one of two special features of “constitutional regimes” central to his influential conception of political legitimacy. The other feature is that “[p]olitical society is closed: we come to be within it and we do not, and indeed cannot, enter or leave it voluntarily.” Indeed, it is these two features that he believes give rise to “the question of the legitimacy of the general structure of authority.” Rawls thinks that
our exercise of political power is fully proper only when it is exercised in accordance with a constitution the essentials of which all citizens as free and equal may reasonably be expected to endorse in the light of principles and ideals acceptable to their common human reason. This is the liberal principle of legitimacy.
The electrical heating of Ni/Al laminate foils allows interrogation of phenomena at heating rates as high as 10^12 K/s. In the 2011 Fall MRS meeting, we reported on emission spectra from rapidly heated Ni/Al laminates resolved temporally over 350 ns, which provided qualitative evidence of rapid and exothermic vapor phase mixing of Ni and Al in these experiments which we term electrical explosions. These results were significant, because thermal diffusion processes normally limit Ni/Al reactions to much slower energy release rates, potentially limiting their applications. Here we present further evidence of exothermic Ni/Al mixing, quantified by experimental velocity measurements of encapsulation material and interpreted by numerical calculations of energy partitioning into different processes. These calculations agreed well with experiments from different Al, Cu, and Ni samples, sputter-deposited and lithographically patterned into bow-tie bridge structures. Velocity measurements of up to 5 km/s for 11.5 μm thick parylene encapsulation layers were accurately predicted using a single, empirical fitting parameter which depended on the electrical circuit used. The calculations also agreed with encapsulation layers accelerated by electrically exploded Ni/Al laminates as long as an additional 1.2 kJ/g of energy was included in the model. This value is precisely the enthalpy of mixing between Ni and Al, and therefore quantifies the transduction of energy into encapsulation layer kinetic energy.
This volume contains contributions that consider new approaches to three areas: the documentation of rock art; its interpretation using indigenous knowledge; and the presentation of rock art. Working with Rock Art is the first edited volume to consider each of these areas in a theoretical rather than a technical fashion, and it therefore makes a significant contribution to the discipline. The volume aims to promote the sharing of new experiences between leading researchers in the field. While the geographic focus is truly global, there is a dominant north-south axis with strong representation from researchers in southern Africa and northern Europe, two leading centres for new approaches in rock art research. Working with Rock Art opens up a long overdue dialogue about shared experiences between these two centres, and a number of the chapters are the first published results of new collaborative research. Since this volume covers the recording, interpretation and presentation of rock art, it will attract a wide audience of researchers, heritage managers and students, as well as anyone interested in the field of rock art studies.
Working with Rock Art presents the outcomes of the first ever collaboration between South Africa and Scandinavia in the field of rock art studies. The particular focus was on hunter-gatherer rock arts. Norway and South Africa are two countries that are famous for their ancient rock engravings and rock paintings. Both have rock art of such great international significance that they are registered on the UNESCO World Heritage List. In both countries therefore, rock art has a high public profile and both governments have made rock art research, conservation and rock art tourism national priorities. However, the research traditions in each region have followed vastly different trajectories.
Our collaboration therefore sought to bring together a series of teams from each region to share their experiences on how we work with rock art. We hoped that our different experiences would prove mutually challenging, and they did. It caused a series of profound debates about what constitutes best practice in the field of rock art studies and these have changed the way we work in tangible ways. We challenged all of the collaborators to report their perspectives at a joint conference in Kimberley, South Africa in 2006 and then to engage in further discussions and workshops before writing up these experiences for this volume. This volume therefore represents the consolidated findings of a prolonged engagement of research and debate in rock art practice. It is therefore predominantly a book about method, and this has great importance in itself, because rock art studies is a growing discipline but one in which we have no internationally agreed upon methods or standards of practice.
We divide the book into three parts, each reflecting one of the core foci of our collaboration:
METHODS OF DOCUMENTING AND RECORDING ROCK ART
All of us face a common problem in our rock art data capture: how do we reduce rock art on a three dimensional surface to a two-dimensional recording in a manner that does not damage the art in any way and without losing any three-dimensional features that may prove vital to interpretation? In both Norway and South Africa research has shown that natural rock features such as cracks and hollows played a fundamental role in determining the placement of rock art imagery.
State power is widely thought to be coercive. The view that governments must wield force or that their power is necessarily coercive is widespread in contemporary political thought. John Rawls is representative in claiming that (political power is always coercive power backed up by the government(s use of sanctions, for government alone has the authority to use force in upholding its laws.( This belief in the centrality of coercion and force plays an important but not well appreciated role in contemporary political thought. I wish to challenge this belief and the considerations that motivate it. States are not necessarily coercive or coercive (by definition.( Their claimed authority is prior to the force they wield. Legitimate states should need to resort to coercion and force much less than other states, and that fact seems unappreciated in contemporary political thought.