Abstract: The decomposition curves of TiH2 at different temperatures were measured, the decomposition laws of TiH2 were studied from the thermodynamic and kinetic perspectives, and the application modes of preparing foam aluminum with TiH2 were analyzed, The decomposition of TiH2 is intense in the first 10 minutes, and the decomposition rate of TiH2 becomes slow in 10~20 minutes. After 20 minutes, the decomposition of TiH2 tends to stagnate gradually, between 700~720 ℃. The decomposition rate of TiH2 is very fast in the first 6 minutes, and decreases in 6~10 minutes. After 10 minutes, the Chemical decomposition stagnates·

Key words: Titanium hydride; Decomposition curve; Melt foaming; Foam aluminum; Preparation

Chinese Library Classification Number: T G 146 2. Literature identification code: A Article number: 1005-3026 (2007) 01-0087-04

Composition Behavior of Ti H2 and Its Application to Fabricating Al uminum Foam

L U O Hong jie, JI Hai bi n, YA NG Guo j u n, YA O Guang ch u n

(School of Materials&Metallurgy, Nort Western University, Shenyang 110004, China. Response: L UO Hong jie, Email: luohj @ smm. neu. edu. cn)

Abstract: The decomposition curves of Ti H2 were determined at different temperatures, and how to decompose Ti H2 was studied t hermodynamically and kinetically The way to use Ti H2 as focusing agent to prepare aluminum foam was al so discussed The results showed t hat t he decomposition rate of Ti H2 increases wit h increasing temperature ure, and t he decomposition process of Ti H2 can be divided into t hree phases in a cert ain temperature ure range In t he range from 620 ° C to 680 ° C, t he decomposition of Ti H2 i s violet for 10 minutes after starting, t hen t he decomposition speed benefits slow during t he next 10 minutes and t he decomposition tens to st agnate after 20 minutes In t he range from 700 ° C to 720 ° C, t he decomposing speed of Ti H2 benefits very fast for 6 minutes after starting, t he decelerations during t he next 4 minutes and st agnates later

Key words: Ti H2; Decomposition curve; Foaming in melt; Aluminum foam; Preparation

Titanium hydride (Ti H2) is a metal hydride, which can be used as a hydrogen storage material. Its brittleness and dehydrogenation behavior at high temperature in vacuum can also be used to prepare high-purity titanium powder and hydrogen. With the rise of foam metal research, Titanium hydride, as an efficient foaming agent, has begun to attract people's attention, especially in the process of preparing foam aluminum, The foaming effect of Titanium hydride can not be replaced by other foaming agents until now·

The power of foam process of aluminum melt comes from the thermal decomposition of TiH2. The decomposition behavior of TiH2 is directly related to the formation and growth of bubbles in the melt, as well as the process and performance parameters for preparing foam aluminum, such as the diameter and wall thickness of bubbles. Therefore, the first step in the research of foam aluminum is to conduct the thermodynamic and kinetic research on the Chemical decomposition of TiH2, The research on Chemical decomposition of TiH2 can be summarized as follows: First, the weight loss curve [1-2] of TiH2 is measured by thermogravimetry (TG), and then the change trend of the process of Chemical decomposition of TiH2 and the starting and ending temperatures of the reaction are obtained; The second method is to measure the decomposition curve [3-4] of TiH2 by Differential thermal analysis (DTA) or Differential scanning calorimetry (DSC), and quantitatively determine the temperature range and extreme point of the rapid reaction of TiH2 according to the appearance of the peak heat absorption (release) of the curve; The third method is to use the thermal desorption spectroscopy (TDS) to make the decomposition diagram of TiH2 [5-6], from which the decomposition temperature of TiH2 when the decomposition rate reaches the maximum value is obtained. The measurement results obtained by the above method reveal the general rule of TiH2 decomposition, which has important guiding significance in theory for the study of the preparation process of foam aluminum. However, these methods for measuring the thermal Chemical decomposition of TiH2 belong to a dynamic analysis method, That is, the decomposition rate of TiH2 changes with temperature and time. However, the temperature of aluminum melt is basically unchanged during the actual preparation of foam aluminum, so the static analysis method is used to measure the decomposition rate of TiH2 more closely to the actual situation of preparing foam aluminum. This study first measured the relationship curve between the decomposition rate of TiH2 and time at a specific temperature, Then further analyze the decomposition mode and process control link of TiH2, aiming to guide the foam behavior of aluminum melt·

1 Experimental Method

The experiment used the weightlessness method to measure the decomposition curve of Ti H2 and conducted it in a self-made temperature controlled tube heating device. The experimental process was as follows: first, the tube furnace was heated up, and when the temperature of the reaction zone in the furnace reached the set decomposition temperature, high-purity argon gas (purity of 99.999%) was introduced, with a flow rate of 50 mL/min; Then add 2 g of Ti H2 powder (61 μ m) Place the specially designed burning boat suspended under the electronic balance and preheat it in the preheating zone of the tube furnace (temperature<400 ℃); After the temperature of the reaction zone in the furnace remains constant again at the set decomposition temperature, quickly move the burning boat from the preheating zone to the reaction zone; Clear the electronic balance, and then read the mass lost in the reaction every 1 min, and the data acquisition card will send the recorded data to the computer and draw the weight loss curve·

In order to reduce the impact caused by errors in the experiment, it is necessary to determine the specific positions of the reaction zone and preheating zone in advance. The position of the reaction zone is the position of the thermocouple temperature measuring point. The position of the preheating zone needs to be measured with thermocouples in the preliminary experiment. In the decomposition experiment, the rise and fall of the tubular furnace are controlled to ensure that the boat burning is in the correct position. The boat burning and hanging wire also need to be pretreated at the Chemical decomposition temperature, In order to ensure that the mass of the burning boat and suspension wire does not change during the decomposition experiment, it can be observed from the experiment that the temperature fluctuation in the reaction zone caused by moving the burning boat from the preheating zone to the reaction zone is ± 1 ℃·

2 Experimental Results and Discussion

The decomposition curves of TiH2 at 620, 640, 660, 680, 700 and 720 ℃ were measured respectively, and the relationship curve between the decomposition rate of TiH2 Chemical decomposition and time was obtained as shown in Figure 1·

As can be clearly seen from Figure 1, the decomposition rate of Ti H2 gradually increases with the increase of reaction temperature. Within the range of 620-680 ℃, the increase in the decomposition rate of Ti H2 is relatively small, but there is a significant jump in the range of 680-700 ℃, and the increase in the decomposition rate between 700-720 ℃ decreases again·

It can be seen from Figure 1 that the Chemical decomposition of TiH2 can be roughly divided into three stages. At 620~680 ℃, TiH2 decomposes violently in the first 10 minutes, and the curve slope is large; The decomposition rate of Ti H2 becomes slow and the slope of the curve decreases within 10-20 minutes; After 20 min, the decomposition of TiH2 tends to stagnate gradually. At 700~720 ℃, the decomposition rate of TiH2 is very fast in the first 6 min, decreases in the first 6~10 min, and the Chemical decomposition stagnates after 10 min·

Fig . 1    Relationship between decompo sition

rat e of TiH2  and time

2 Thermodynamic analysis of Chemical decomposition of Titanium hydride

The characteristics exhibited by the decomposition of Ti H2 at different temperatures can be explained from a thermodynamic perspective. Figure 2 shows the Ti H2 powder (61 μm) Differential thermal analysis diagram · Figure 2 shows that there is an endothermic peak in the curve near 600℃, indicating that Ti H2 decomposes violently near this temperature · As the temperature selected in the experiment is higher than 600 ℃, Ti H2 shows intense decomposition behavior at the beginning of the reaction stage, and Chemical decomposition gradually tends to be stable in the middle and later stages · 620, 640, 660 in Figure 1, The decomposition behavior of TiH2 at four temperatures of 680 ℃ is basically consistent with the results obtained by Differential thermal analysis, but the obvious increase in the decomposition rate of TiH2 at two temperatures of 700 and 720℃ is difficult to give the corresponding explanation from Figure 2. The Differential thermal analysis diagram of Li Guangming and others in the study of the preparation and decomposition of TiH2 shows that, Ti H2 exhibits an endothermic peak at 680 ℃ [4]. The existence of this endothermic peak well explains the significant difference in the decomposition rate of Ti H2 at 680 and 700 ℃, that is, the second endothermic peak of Ti H2 at around 680 ℃ causes its decomposition at 700 and 720 ℃

Fig . 2    Diferential thermal analysi s of TiH2

It can be seen from reaction formula (4) that after Chemical decomposition of TiH2 occurs, a simple substance Ti is formed, and TiH2 particles will be surrounded by the gradually formed Ti layer. The decomposed H2 needs to diffuse outward through the Ti layer, so the decomposition of TiH2 is a multiphase reaction with a phase interface. According to the gas-solid reaction mechanism, the Chemical decomposition of TiH2 includes the following three steps:

(1) The crystallization chemical reaction that occurs at the interface between reactants and products (Ti H2 Ti);

(2) The internal diffusion of gas product H2 through the Ti layer of the product;

(3) External diffusion of H2 through Boundary layer on Ti surface·

Although each link of the gas Dry media reaction is completed continuously, the speed of each link is unequal, and the total reaction speed depends on the slowest link, that is, the restrictive link. In the decomposition experiment of TiH2, due to the high reaction temperature and the presence of updraft (Ar gas protection flow), the movement speed of H2 is higher than the critical flow rate of the Boundary layer on the surface of Ti, and the external diffusion of H2 will not become a restrictive link, Therefore, the limiting links mainly exist in two stages: internal diffusion and crystallization chemical reaction· 
 
Figure 3 is a scanning electron microscope photo of Ti H2 particles. It can be seen that Ti H2 is a polyhedron with a relatively dense structure and a regular shape

Fig . 3    SEM image of TiH2 particles

As the Chemical decomposition proceeds, the TiH2 particles will gradually shrink, the phase interface will gradually shrink inward with time, and the thickness of the resultant Ti layer will increase. Therefore, this decomposition mode of TiH2 conforms to the typical characteristics of the shrinking core model·

The shrinking core model of Ti H2 can be represented in Figure 4 (for simplification, TiH2 particles are approximated as spherical)

Fig . 4    Schematic s of nuclear model of TiH2 particle

The small circle in Figure 4 represents the Ti H2 core; The ring between the large and small circles represents the generated Ti layer; R0 is the original radius of Ti H2 particles; R is the core radius after contraction; A is the reaction interface area (A=4 π r2) · At the initial stage of the Chemical decomposition of TiH2, the product Ti layer is very thin · Because H2 is precipitated from the Chemical decomposition, the product Ti layer is also relatively loose, at this time, the internal diffusion rate is fast, and the reaction is in the crystallization chemical reaction control stage · At the late stage of the Chemical decomposition of TiH2, as the product Ti layer thickens and becomes dense, the reaction generated H2 diffusion rate becomes smaller, and the internal diffusion becomes the control link·

Further analysis of the decomposition curve of TiH2 in Figure 1 from the perspective of kinetics shows that temperature plays a decisive role in the length of the chemical reaction time and the decomposition rate. In the range of 620~680 ℃, due to the relatively low temperature, the chemical reaction speed is slow, and the control time of crystallization chemical reaction and internal diffusion is relatively long, the sum of the time of the first two stages of Chemical decomposition is 20 minutes, The decomposition rate of TiH2 is lower than 45%. When the decomposition temperature is between 700~720 ℃, the chemical reaction speed is accelerated, and the control time of crystallization chemical reaction and internal diffusion is shortened. It is shown that the time sum of the first two stages of Chemical decomposition is 10 min, but the decomposition rate of TiH2 can be as high as 80%·

3 Application Analysis

The thermal decomposition curves of TiH2 at different temperatures have certain guiding significance for the preparation of foam metal with TiH2 as foaming agent, especially for the preparation of foam aluminum by melt foaming method. When the melt foaming method is used to prepare foam aluminum, the mixing and foaming temperatures of the melt are generally controlled within the range of 630~720 ℃ [7-10], Based on this, the decomposition temperature of TiH2 was selected in the experiment. According to the decomposition curve of TiH2 obtained in the experiment, the decomposition rate of TiH2 can be controlled by changing the process conditions during the foam process of aluminum or aluminum alloy melt, thus controlling the cell structure of foam aluminum. When preparing low-density foam aluminum materials, the stirring temperature of aluminum based melt can be appropriately reduced, because the decomposition rate of TiH2 is low at low temperature, At this time, the stirring mainly plays the role of uniformly dispersing the foaming agent. After the stirring is completed, the low-density foam aluminum material can be obtained by correspondingly increasing the foaming temperature of the melt; On the contrary, when preparing high-density foam aluminum materials, it is necessary to increase the stirring and foaming temperature of the melt, control the foam degree of the melt, and obtain high-density foam aluminum materials. After the melt stirring and foaming temperature are determined, it is also necessary to control other factors that affect the foam behavior of the melt. Taking melt stirring as an example, selecting a stirring paddle with a specific shape can obtain good turbulence effect, The appropriate size of the stirring paddle can ensure that the melt has a certain volume flow rate, no mixing dead angles, and a large shear force, which can disrupt the aggregation of the foaming agent and ultimately achieve the goal of fully dispersing and suspending the foaming agent in the melt. After selecting the shape and size of the stirring paddle correctly, the determination of stirring time and stirring speed is related to the crystallization chemical reaction during the decomposition process of Ti H2 There is a close relationship between internal diffusion. The stirring time affects both the crystallization chemical reaction and internal diffusion, while the stirring speed directly affects the speed of internal diffusion. In the case of high-speed stirring, the stirring time can be shortened. Although there are many factors that need to be controlled, corresponding adjustments and determinations can be made around the key condition of Ti H2 decomposition·

4 Conclusion

(1) The decomposition rate of Ti H2 gradually increases with the increase of temperature. Within the range of 620-680 ℃, the increase in the decomposition rate of Ti H2 is relatively small, but there is a significant jump in the range of 680-700 ℃·

(2) The Chemical decomposition of TiH2 can be roughly divided into three stages. In the range of 620~680 ℃, TiH2 decomposes violently in the first 10 minutes, and the decomposition rate of TiH2 becomes slow in 10~20 minutes. After 20 minutes, the decomposition of TiH2 gradually tends to stagnate. In the range of 700~720 ℃, the decomposition rate of TiH2 in the first 6 minutes is very fast, and the decomposition rate decreases between 6~10 minutes. After 10 minutes, the Chemical decomposition stagnates.