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[单选题]
The metal powder is compacted by placing in a closed（)cavity under pressure in powder
A.plastic
B.metal
C.wood
D.sand
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题目内容
（请给出正确答案）
如果结果不匹配，请 联系老师 获取答案
题目内容
（请给出正确答案）
A.plastic
B.metal
C.wood
D.sand
如果结果不匹配，请 联系老师 获取答案
更多“The metal powder is compacted …”相关的问题
A.metal powders may deteriorate quickly
B.fixed and setup costs are high
C.non-moldable features are impossible to produce
D.sharp corners can be hard to produce
Fireworks
If you have ever been to an aerial fireworks show at an amusement park, baseball game, Fourth of July celebration or on New Year's Eve, then you know that fireworks have a special and beautiful magic all their own--a good show is absolutely amazing!
Have you ever wondered how this magic works？ What is launched into the sky to make these beautiful displays？ In this article, you will learn all about aerial fireworks.
Basic Components
Just about everyone in the United States has some personal experience with fireworks, either from Fourth of July or New Year's Eve celebrations. For example, you have probably seen both sparklers and firecrackers. It turns out that if you understand these two pyrotechnic (有关烟火制造术的) devices, then you are well on your way to understanding aerial fireworks! The sparkler demonstrates how to get bright, sparkling light from a firework, and the firecracker shows how to create an explosion.
Firecrackers have been around for hundreds of years. They consist of either black powder (also known as gunpowder) or flash powder in a tight paper tube with a fuse to light the powder. Black powder, discussed briefly in How Rocket Engines Work, contains charcoal, sulfur and potassium nitrate. A composition used in a firecracker might have aluminum instead of or in addition to charcoal in order to brighten the explosion.
Sparklers are very different from firecrackers. A sparkler burns over a long period of time (up to a minute) and produces extremely bright and showery light. Sparklers are often referred to as "snowball sparklers" because of the ball of sparks that surrounds the burning portion of the sparkler. If you look at Patent #3,862,865: Sparkler composition, you can see that a sparkler consists of several different compounds:
A fuel
An oxidizer
Iron or steel powder
A binder
The fuel is charcoal and sulfur, as in black powder. The binder can be sugar or starch. Mixed with water, these chemicals form. a slurry that can be coated on a wire (by dipping) or poured into a tube. Once it dries, you have a sparkler. When you light it, the sparkler bums from one end to the other (like a cigarette). The fuel and oxidizer are proportioned, along with the other chemicals, so that the sparkler burns slowly rather than exploding like a firecracker.
It is very common for fireworks to contain aluminum, iron, steel, zinc or magnesium dust in order to create bright, shimmering sparks. The metal flakes heat up until they are incandescent and shine brightly or, at a high enough temperature, actually burn. A variety of chemicals can be added to create colors. See lights and colors for a good explanation of the chemistry and physics of color in fireworks.
Aerial Fireworks
An aerial firework is normally formed as a shell that consists of four parts:
Container----usually pasted paper and string formed into a cylinder
Stars--spheres, cubes or cylinders of a sparkler-like composition
Bursting charge--firecracker-like charge at the center of the shell
Fuse—provides a time delay so the shell explodes at the right altitude
The shell is launched from a mortar. The mortar might be a short, steel pipe with a lifting charge of black powder that explodes in the pipe to launch the shell. When the lifting charge fires to launch the shell, it lights the shell's fuse. The shell's fuse bums while the shell rises to its correct altitude, and then ignites the bursting charge so it explodes.
Simple shells consist of a paper tube filled with stars and black powder. Stars come in all shapes and sizes, but you can imagine a simple star as something like sparkler compound formed into a ball the size of a pea or a dime. The stars are poured into the tube and then surrounded by b
A．Y
B．N
C．NG
Glass ionomer cements are composed of()
A．zinc oxide powder and polycarboxylic
B．zinc oxide powder and phosphoric acid liquid
C．aluminosilicate powder and phosphoric acid liquid
D．aluminosilicate powder and polycarboxylic liquid
A.all the materials
B.elemental
C.pre-alloyed
D.partially alloyed
When mixing alginate impression material()
A．add water to the powder
B．add water to the powder
C．add the powder to the water
D．add them together at the same time
Firecrackers consist of either black powder or flash powder.
A．Y
B．N
C．NG
The best way to mix gypsum products is to()
A．eyeballthe amount ofpowder added to water
B．add water to the powder
C．add powder to the water
D．add powder and water to the bowl simultaneously
A.economic
B.economical
One advantage of nano-armed washing machines is that ______.
A．one wash-load is much larger than before
B．the clean-up is done with an additional benefit
C．cleaning powder is no longer necessary
D．a lot of water could be saved
Resin-modifled GICs()
A．have a powder that is similar in composition to that of traditional GICs
B．release much more fluoride than traditional GICs
C．set only as a result of an acid-base reaction
D．exhibit less wear than composite resins
