From melts objectives

Flame spraying is no longer the most widely used melt-spraying process. In the power-feed method, powders of relatively uniform size (<44 fim (325 mesh)) are fed at a controlled rate into the flame. The torch, which can be held by hand, is aimed a few cm from the surface. The particles remain in the flame envelope until impingement. Particle velocity is typically 46 m/s, and the particles become at least partially molten. Upon impingement, the particles cool rapidly and soHdify to form a relatively porous, but coherent, polycrystalline layer. In the rod-feed system, the flame impinges on the tip of a rod made of the material to be sprayed. As the rod becomes molten, droplets of material leave the rod with the flame. The rod is fed into the flame at a rate commensurate with melt removal. The torch is held at a distance of ca 8 cm from the object to be coated particle velocities are ca 185 m/s. 

From the abundances of igneous rims, compound chondrules, and relict grains and the dearth of unmelted, chondrule-sized dustballs, we infer that collisions between partly melted objects and remelting of aggregates from such collisions were important processes that enabled chondrules to form from fine particles. 

Chondrites—meteorites from parent bodies in the asteroid belt that never melted—represent the most primitive samples available of material that formed in the protoplanetary nebula. Chondrites are mainly composed of chondrules, with smaller amounts of refractory inclusions and a fine grained matrix of silicate, metal, and sulfide. Chondrules are roughly spherical objects, typically —1 mm in size, and largely composed of olivine and pyroxene (Taylor, 2001). They appear to have formed from melt droplets that cooled on timescales of... 

Entropy changes the state of an object noticeably. If matter, for example, a piece of wax or a stone, contains little entropy, it is felt to be cold. If, however, the same object contains more or a lot of entropy, it can feel warm or even hot. If the amount of entropy in it is continuously increased, it will begin to glow, firstly dark red, then bright white, subsequently melt, and finally vaporize like a block of iron would, or it may transform and decompose in another way, as a block of wood might. Entropy can also be removed from one object and put into another. When this is done, the first object becomes cooler and the second, warmer. To put it succinctly Entropy plays a role in all thermal effects and can be considered their actual cause. Without entropy, there is no warm and cold and no temperature. The obvious effects of entropy allow us to observe its existence and behavior quite well even without measurement devices. 

Atomization. This technique involves the melting at high temperatures (i.e., above 1800°C) of raw material particles together to obtain a molten bath of bulk liquid. Usually, the bulk liquid contains more than thousands of times the amount of raw material required to make a single product particle. A thin stream of molten material is atomized by dropping it into a disruptive air jet, subdividing the stream into fine, molten droplets. The droplets are then kept away from one another and from other objects until they have been cooled and solidified. Then they can be recovered as substantially discrete ellipsoidal glassy (i.e., amorphous) particles. 

Melt crystallization is an important separation, purification, and concentration technique used in the chemical, pharmaceutical, and food industries. Crystallization from melt is a very powerful separation process for the purification of organic compounds up to very high purities of 99.99%. Hence, the objectives of melt crystallization (purity, separation, or concentration) are quite often different compared to crystallization from solution (purity and defined crystal size distribution). 

Temperature also determines the direction of thermal energy transfer, or what we commonly call heat. Thermal energy transfers from hot objects to cold ones. For example, when you touch another person s warm hand (and yours is cold), thermal energy flows from his or her hand to yours, making your hand feel warmer. However, if you touch an ice cube, thermal energy flows out of your hand to the ice, cooling your hand (and possibly melting some of the ice cube). 

The PLM can be used in a reflection or a transmission mode. With either mode, light of various wavelengths from ultraviolet to infrared, polarized or unpolarized, is used to yield a wide variety of physical measurements. With just ordinary white light, a particle or any object detail down to about 0.5 p.m (500 nm) in diameter can be observed to detect shape, size, color, refractive index, melting point, and solubiUty in a group of solvents, all nondestmetively. Somewhat larger particles yield UV, visible, or IR absorption spectra. 

---