Peritectic temperature

Praseodymium di-iodide, Prl2, can essentially be made in the same way. If sufficient care is taken to exclude air and moisture, oxidic impurities can be avoided. To avoid the formation of Pr2ls, praseodymium metal is used in excess as chunks to easily remove the unreacted metal after the reaction is completed. The pure compound Prl2 is thus obtained, with a reaction temperature well above the peritectic temperature, around 800 °C. Reaction times seem not to matter much, a few days are usually sufficient, perhaps even less. The cooling procedure, however, is crucial as it determines the phases (I through V) that are formed and their relative quantities. Section 4.3 will deal with this issue. 

Still, the question has to be addressed as to which of the many modifications of Prl2 is thermodynamically stable under standard conditions. So far, it is clear that Prl2-IV must be a high-temperature phase as it is produced in pure and single-phase by annealing just below the peritectic temperature (with an excess of praseodymium metal in order to avoid the formation of Pr2ls) and rapid cooling to ambient temperature. 

PERITECTIC TEMPERATURE. Temperature in a peritectic system at which there is equilibrium between the solid and the remaining melt, the composition of which conforms to the peritectic point, at which the temperature line meets the liquids curve. 

The principle of the method can be seen by referring to Fig. 23, which is a partial phase diagram of the cerium hydrogen system. Molten cerium metal is placed within a thermal gradient such that the lowest temperature is above the peritectic temperature (ca. 1010°C., from the diagram), and hydrogen is slowly dissolved in... 

CH30H-H20(s), H20(s) and liquid solution are in equilibrium. In the warming process, the temperature will stay at the peritectic temperature until all of the hydrate has melted (decomposed). Continued warming melts ice, with the composition of the liquid changing along line pb until point b is reached, at which temperature the last of the solid melts. Continued heating increases the temperature of the liquid (x2 = 0.5) mixture to a temperature represented by point a. 

This kind of reaction is called a peritectic reaction and the point p is called the peritectic point. The temperature TP is called the peritectic temperature. 

Just below the peritectic temperature, the coexistence of two solid phases, A at q mid AmB at r, is resulted in. Thus the microstructure of die system will consist of large primaiy crystals of A and small crystallites of AmB . 

Just below the peritectic temperature, the coexistence of the liquid phase at p and AmB at r, is resulted in. 

This reaction is called the peritectic reaction, and the point p is known as the peritectic point and Tp the peritectic temperature. 

When liquid x is cooled maintaining equilibrium conditions, calculate wt% a that exists just above the peritectic temeperature Tp When liquid y is cooled maintaining equilibrium conditions, calculate wt% P that exists just below the peritectic temperature Tp. 

Above the peritectic temperature, solids C and AC continue to dissolve into the liquid... 

When solid NaOH 2H20 is heated to 10°C, it converts to a mixture of NaOH H20 and solution of composition a. During melting, this solution exists at equilibrium with the two solid phases, p = 3, and the system is invariant (at the pressure of the diagram). This is similar to the behavior of a eutectic solution, but because the solution of composition a is of lower concentration than both of the solids, it is known as a peritectic solution, with a corresponding peritectic temperature and peritectic composition. 

Aluminium, gallium and indium nitride belong to a family of materials described as peritectic compounds. A peritectic compound is defined as a compound that transforms, at the peritectic temperature and atmospheric pressure, according to the reaction ... 

The results on liquidus, solidus and peritectic temperatures, measured for the three cooling rates, are shown as a function of the carbon content in figures 6.1 -6.4 for carbon, low alloy and chromium steels. The thermal data for the stainless and heat resistant alloys are plotted as a function of alloy content, expressed as equivalents of chromium and nickel, in figure 6.6. 

The tendency was for the liquidus and the peritectic temperatures to be independent of cooling rate, whereas the solidus was markedly lower at a higher cooling rate. The reason for this is the higher degree of backdiffusion and homogenization possible at a low cooling rate. 

The points Tfus, a and Tfus, b are again the melting points of components A and B. The lines originating from the individual melting points represent the curves of their primary crystallization. The curve of primary crystallization of the component A stops at the peritectic temperature at the peritectic point P. Below the peritectic temperature, the curve... 

When the system attains the peritectic temperature the peritectic reaction A + L = A4B starts and crystals of the compound A4B appear. Since now there are three phases in the system (component A, compound A4B, and melt L), the system has no degree of freedom ( = 3 - 1 = 2, / = 3, V = 0), which means that its cooling due to the evolution of the reaction heat of the peritectic reaction must halt, even when the surrounding cools further. The system stays at the peritectic temperature until the melt disappear and the system solidifies. Below the peritectic temperature, there is again a mechanical mixture of the crystals of component A and the crystals of compound A4B. 

Phase diagrams are the roadmaps from which the number of phases, their compositions, and their fractions can determined as a function of temperature. In general, binary-phase diagrams can be characterized as exhibiting complete or partial solid solubility between the end members. In case of the latter, they will contain one or both of the following reactions depending on the species present. The first is the eutectic reaction is which a liquid becomes saturated with respect to the end members such that at the eutectic temperature two solids precipitate out of the liquid simultaneously. The second reaction is known as the peritectic reaction in which a solid dissociates into a liquid and a second solid of a different composition at the peritectic temperature. The eutectic and peritectic transformations also have their solid state analogues, which are called eutectoid and peritectoid reactions, respectively. 

If an unsaturated solution of composition g is heated, anhydrous salt will crystallize at/ if it is cooled, the decahydrate will crystallize at h. It is possible to supercool the solution to a temperature below /i then the heptahydrate will crystallize at i Fig. 15-15(b). The curve e b is the solubility curve for the heptahydrate, Na2S04 7H20. The peritectic temperature for anhydrous salt-heptahydrate-saturated solution is at 24.2 °C. In Fig. 15.15(b), the dashed lines are the curves for the decahydrate. The solubility curve for the heptahydrate lies for the most part in the region of stability of solid decahydrate-saturated solution. Therefore the equilibrium between solid heptahydrate and its saturated solution is a metastable one the system in such a state can precipitate the less soluble decahydrate spontaneously. 

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