Carbon slip systems

It was mentioned in Chapter 4 that the lower atomic density slip systems in bcc materials become inactive at low temperatures and a ductile-to-brittle transition occurs. From Figure 9.6 it is seen that for some high-carbon steels, this transition occurs well above the freezing point of water. A number of the early Liberty ships that operated in the North Atlantic during World War II were apparently made from the wrong kind of steel and broke apart without warning. 

Since the number of slip systems is not usually a function of temperature, the ductility of face-centered cubic metals is relatively insensitive to a decrease in temperature. Metals of other crystal lattice types tend to become brittle at low temperatures. Crystal structure and ductility are related because the face-centered cubic lattice has more slip systems than the other crystal structures. In addition, the slip planes of body-centered cubic and hexagonal close-packed crystals tend to change at low temperature, which is not the case for face-centered cubic metals. Therefore, copper, nickel, all of the copper-nickel alloys, aluminum and its alloys, and the austenitic stainless steels that contain more than approximately 7% nickel, all face-centered cubic, remain ductile down to the low temperatures, if they are ductile at room temperature. Iron, carbon and low-alloy steels, molybdenum, and niobium, all body-centered cubic, become brittle at low temperatures. The hexagonal close-packed metals occupy an intermediate place between fee and bcc behavior. Zinc undergoes a transition to brittle behavior in tension, zirconium and pure titanium remain ductile. 

The electronic character of the stacked aromatic systems makes it possible for atoms or molecules to slip between these layers, either accepting or donating electrons to bond with the carbon system, a process referred to as intercalation. Platinum metal graphite intercalates have been prepared by reducing platinum halide graphite intercalates. These catalysts are particularly useful for the selective hydrogenation of acetylenes to cis alkenes (Eqn. 9.3).22... 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 slip required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availability of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

When we introduced the system at a large copper mine and smelter we decided to do things differently. Our pocket size, perforated and self-carbonized booklets were ideal for underground miners to slip into their pockets where they could stay dry and safe in wet underground conditions. They were also simple, easy to complete forms and the reporter could retain his carbon copy in the booklet. 

Pressure tends to increase the chemical reactivity of nitromethane as well as the rate of thermal decomposition. It was observed, quite accidentally, that a pressure-induced spontaneous explosion of single crystals of nitromethane at room temperature can occur. Further study revealed that single crystals grown from the liquid with the (111) and either the (001) or the (100) crystal faces perpendicular to the applied load direction in the DAG, if pressed rapidly to over 3 GPa, explode instantaneously accompanied by an audible snapping sound. The normally transparent sample becomes opaque instantly. Visual examination of the residue revealed a dark brown solid which was stable when heated to over 300 C. Subsequent x-ray analysis showed the material to be amorphous. Mass spectral analysis of the residue was inconclusive because no well defined spectra were observed. Because most of the sample is recovered as solid residue after the explosion and is stable to over 300°C, the material may be amorphous carbon. This stress-induced explosion occurs only in protonated nitromethane because similar attempts on the deuterated form did not result in explosion. Shock experiments on oriented pentaerythritol (PETN) crystals have shown similar type behavior [25]. In this case it was suggested that the sensitivity of shock pressures to crystal orientation is the result of the availability of slip planes or system of planes in the crystal to absorb the shock, thereby increasing the threshold to explosion. A similar explanation may be applicable to the nitromethane crystals as well. The deuteration effect must play a role in the initiation chemistry. An isotope effect has been observed previously in the sensitivity of HMX and RDX to shock and thermal conditions [23]. 

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