Explosives

on

What is explosion?
An explosion is a type of spontaneous chemical reaction that, once initiated, is driven by both a large
exothermic change (great release of heat) and a large positive entropy change (great quantities of gases are released) in going from reactants to products.
Definition of explosive
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material.
This potential energy stored in an explosive material may be
Categories of explosives
a-      By velocity
Explosive materials may be categorized by the speed at which they expand.
1-      High explosives
Materials that detonate (explode faster than the speed of sound) are said to be "high explosives" Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it.
Example Aircraft application. Pulse detonation engines utilize the detonation wave for aerospace propulsion. The first flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on January 31, 2008.
Mining & military applications
2-      Low explosives
Materials that deflagrate are said to be "low explosives".
Deflagration is a term describing subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration. Deflagration is different from detonation, which is supersonic and propagates through shock.
Deflagration is a rapid high energy release combustion event that propagates through a gas or an explosive material at subsonic speeds, driven by the transfer of heat.
Examples
In engineering applications, deflagrations are easier to control than detonations. Consequently, they are better suited when the goal is to move an object (a bullet in a gun, or a piston in an internal combustion engine) with the force of the expanding gas. Typical examples of deflagrations are the combustion of a gas-air mixture in a gas stove or a fuel-air mixture in an internal combustion engine.
b-     By sensitivity
1-      Primary explosive
A primary explosive is an explosive that is extremely sensitive to stimuli such as impact, friction, heat, static electricity, or electromagnetic radiation. A relatively small amount of energy is required for initiation.
Examples
Ø  Ethyl azide (C2H5N3) is an explosive compound sensitive to rapid heating, shock or impact. It has exploded when heated to room temperature. When heated to decomposition it emits toxic fumes of NOx.
Ø  Silver nitride is an explosive chemical compound with symbol Ag3N. It is a black, poorly soluble in water, but decomposes in mineral acids.
Ø  Nitrogen trichloride, also known as trichloramine, is the chemical compound with the formula NCl3. This is yellow, oily, pungent-smelling liquid.

2-      Secondary explosive
A secondary explosive is less sensitive than a primary explosive and require substantially more energy to be initiated. Because they are less sensitive they are usable in a wider variety of applications and are safer to handle and store. Secondary explosives are used in larger quantities in an explosive train and are usually initiated by a smaller quantity of a primary explosive.
Examples of secondary explosives include TNT Trinitrotoluene C6H2 (NO2)3CH3.
RDX, It was developed as an explosive which was more powerful than TNT, and it saw wide use in World War II. RDX is also known as cyclonite, hexogen, cyclotrimethylenetrinitramine C3H6N6O6

3-      Tertiary explosive

Tertiary explosives, also called blasting agents, are so insensitive to shock that they cannot be reliably detonated by practical quantities of primary explosive, and instead require an intermediate explosive booster of secondary explosive.
ANFO is an example of a tertiary explosive.
ANFO (or AN/FO, for ammonium nitrate/fuel oil) is a widely used bulk industrial explosive mixture.
Dynamite is an explosive material based on nitroglycerin or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp.

Properties of explosive materials

To determine the suitability of an explosive substance for a particular use, its physical properties must first be known. The usefulness of an explosive can only be appreciated when the properties and the factors affecting them are fully understood. Some of the more important characteristics are listed below:

1-      Availability and cost

The availability and cost of explosives are determined by the availability of the raw materials and the cost, complexity, and safety of the manufacturing operations.

2-      Sensitivity

Sensitivity refers to the ease with which an explosive can be ignited or detonated, i.e., the amount and intensity of shock, friction, or heat that is required. When the term sensitivity is used, care must be taken to clarify what kind of sensitivity is under discussion. The relative sensitivity of a given explosive to impact may vary greatly from its sensitivity to friction or heat. Some of the test methods used to determine sensitivity relate to:
  • Impact — Sensitivity is expressed in terms of the distance through which a standard weight must be dropped onto the material to cause it to explode.
  • Friction — Sensitivity is expressed in terms of what occurs when a weighted pendulum scrapes across the material (it may snap, crackle, ignite, and/or explode).
  • Heat — Sensitivity is expressed in terms of the temperature at which flashing or explosion of the material occurs.

3-      Velocity of detonation

The velocity with which the reaction process propagates in the mass of the explosive. Most commercial mining explosives have detonation velocities ranging from 1800 m/s to 8000 m/s. Today, velocity of detonation can be measured with accuracy.

4-     Volatility

Volatility is the readiness with which a substance vaporizes. Excessive volatility often results in the development of pressure within rounds of ammunition and separation of mixtures into their constituents. Volatility affects the chemical composition of the explosive such that a marked reduction in stability may occur, which results in an increase in the danger of handling.
5-      Stability
Stability is the ability of an explosive to be stored without deterioration.
The following factors affect the stability of an explosive:
Ø  Temperature of storage.
Ø  Exposure to sunlight.

6-      Power, performance, and strength

The term power or performance as applied to an explosive refers to its ability to do work.

7-      Toxicity

There are many types of explosives which are toxic to some extent. Manufacturing inputs can also be organic compounds or hazardous materials that require special handing due to risks (such as carcinogens). The decomposition products, residual solids or gases of some explosives can be toxic, whereas others are harmless, such as carbon dioxide and water. Examples of harmful by-products are:
  • Heavy metals, such as lead, mercury and barium from primers (observed in high volume firing ranges).
  • Nitric oxides from TNT.
  • Perchlorates when used in large quantities.


Rocket fuel
A propellant is a chemical used in the production of energy or pressurized gas that is subsequently used to create movement of a fluid or to generate propulsion of a vehicle, projectile, or other object.
Rocket propellant is a material used by a rocket as, or to produce in a chemical reaction, the reaction mass (propulsive mass) that is ejected, typically with very high speed, from a rocket engine to produce thrust, and thus provide spacecraft propulsion.
Classification of propellant
1-      Solid propellant
Two general types of solid propellants are in use. The first, the so called double-base propellant, consists of nitrocellulose and nitroglycerine, plus additives in small quantity. There is no separate fuel and oxidizer. The molecules are unstable, and upon ignition break apart and rearrange themselves, liberating large quantities of heat. These propellants lend themselves well to smaller rocket motors.
The other type of solid propellant is the composite. Here, separate fuel and oxidized chemicals are used, intimately mixed in the solid grain.
The oxidizer is usually ammonium nitrate, potassium chlorate, or ammonium chlorate.
The fuels used are hydrocarbons, such as asphaltic-type compounds.
Advantages
Ø  Minimum maintenance and instant readiness.
Ø  Less smoke, low cost
Disadvantages
Ø  More energetic solids may require carefully controlled storage conditions, and may offer handling problems in the very large sizes, since the rocket must always be carried about fully loaded.
Ø  Hazardous to manufacture

2-      Liquid propellants

The highest specific impulse chemical rockets use liquid propellants. Approximately 170 different liquid propellants have undergone lab testing.
The most common liquid propellants in use today:
1)      LOX and kerosene
2)      LOX and liquid hydrogen,
3)      Nitrogen tetra oxide (N2O4) and hydrazine (N2H4),
4)      Monopropellants such as hydrogen peroxide, hydrazine, and nitrous oxide
Advantages
Ø  Liquid-fueled rockets have higher specific impulse
Ø  liquid propellants are cheaper than solid propellants
Disadvantages
Ø  The main difficulties with liquid propellants are also with storage & handling.

3-      Gas propellants

A gas propellant usually involves some sort of compressed gas. However, due to the low density and high weight of the pressure vessel, gases see little current use; GOX (gaseous oxygen) was used as gas propellants.

4-      Gel propellant

Some work has been done on gelling liquid propellants to give a propellant with low vapor pressure to reduce the risk of an accidental fireball. Gelled propellant behaves like a solid propellant in storage and like a liquid propellant in use.
Characteristic of a good propellant
A good propellant:
1)      Should have high specific impulse that is the propellant should produce greater thrust (downward force or push) per second for 1 kg of the fuel burnt.
2)      Should produce high  temperatures on combustion
3)      Should produce low molecular weight products during combustion and should not leave any solid residue after ignition.
4)      Should burn at a slow and steady rate (that is predictable rate of combustion).
5)      Should possess low ignition delay (that is it should burn as soon as it is lighted up).
6)      Should possess high density to minimize container space.
7)      Should be stable at a wide range of temperatures.
8)      Should be safe for handling and storage.
9)      Should not be corrosive and hygroscopic (ability to attract and hold water molecules).
10)  Should not produce toxic gases or corrosive gases during combustion.


What is chemical formula of dynamite?
             Dynamite is a high explosive, which means its power comes from detonation rather than deflagration. It is based on nitroglycerin, using diatomaceous earth (A light soil consisting of siliceous diatom remains and often used as a filtering material) or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp. Dynamites using organic materials such as sawdust are less stable and such use has been generally discontinued.  Dynamite is mainly used in the mining, quarrying, construction, demolition industries and it has had some historical usage in warfare. Its chemical formula is C3H5 (ONO2)3 or (C3H5N3O9).



Write 4 examples of solid lubricants & their daily uses i-e write name of equipments or machine or part of machine where solid lubricants are used?

SOLID LUBRICANTS

  • CERIUM TRI-FLUORIDE (CeF3) in powder form is used as a lubricant for die casting components.
  • TALC (H2Mg3(SiO3)4 or Mg3Si4O10(OH)2) in powder form is used as a lubricant in pharmaceuticals as well as in the transport of dry materials.
  • GRAPHITE Used in air compressors, food industry, railway track joints, open gear, ball bearings, machine-shop works etc.
  • HEXAGONAL BORON NITRIDE. Used in space vehicles. Also called "white graphite".

Explain the factors affecting the stability of explosives?

FACTORS AFFECTING THE STABILITY OF EXPLOSIVES

Stability is the ability of an explosive to be stored without deterioration. The following factors affect the stability of an explosive.
·         Chemical constitution.
·         Temperature of storage.
·         Exposure to sunlight.
·         Electrical discharge.
CHEMICAL CONSTITUTION:
           In the context of explosives, stability commonly refers to ease of detonation, which is concerned with kinetics (i.e., rate of decomposition). It is perhaps best, then, to differentiate between the terms thermodynamically stable and kinetically stable by referring to the former as "inert." Contrarily, a kinetically unstable substance is said to be "labile." It is generally recognized that certain groups like nitro (–NO2), nitrate (–ONO2), and azide (–N3), are intrinsically labile. They generally have positive enthalpies of formation and there is little mechanistic hindrance to internal molecular rearrangement to yield the more thermodynamically stable (more strongly bonded) decomposition products. For example, in lead azide, Pb(N3)2, the nitrogen atoms are already bonded to one another, so decomposition into Pb and N2 is relatively easy.

TEMPERATURE OF STORAGE: 
         The rate of decomposition of explosives increases at higher temperatures. All standard military explosives may be considered to have a high degree of stability at temperatures from –10 to +35 °C, but each has a high temperature at which its rate of decomposition rapidly accelerates and stability is reduced. Most explosives become dangerously unstable at temperatures above 70 °C.
EXPOSURE TO SUNLIGHT:
                     When exposed to the ultraviolent  rays of sunlight, many explosive compounds containing nitrogen groups rapidly decompose, affecting their stability.
 ELECTRICAL DISCHARGE:
Electrostatic sensitivity to initiation is common in a number of explosives. Static or other electrical discharge may be sufficient to cause a reaction, even detonation, under some circumstances. As a result, safe handling of explosives and pyrotechnics usually requires proper electrical grounding of the operator.




Comments

Post a Comment