The Effect of Calcination Temperature on Synthesis of B4C-Nano Tib2 Composite by Co-Precipitation Method

Introduction

As one of the hardest materials known, boron carbide ranks third behind diamond and cubic boron nitride. Boron carbide (B₄C) ceramics possesses excellent mechanical and physical properties of good impact and wears resistance, hardness and high melting, excellent resistance to chemical materials as well as high capability for neutron absorption [1-6]. Moreover the low density of B₄C and its high Young’s modulus recommend this material for the construction of light-weight armor such as in the helicopter and similar aero-application [7] However, the widespread application of B₄C ceramics has been restricted because of the poor sinterability due to a low self-diffusion coefficient and the low fracture toughness (< 2.2 MPa m^1/2) [8, 9].

Titanium diboride with high melting point (3498 K), high hardness (25 GPa), low density (4.5 g cm^-3), high electrical conductivity (22× 10⁶ Ω cm), good thermal conductivity (96 W/mK) and considerable chemical stability is one of the candidate materials for high temperature structural and wear application [10, 11]. Since both B₄C and TiB₂ have high hardness and high melting points as well as chemical stability at elevated temperature, the B₄C-TiB₂ composites were expected to be used for advanced structural materials.

TiB₂ can be prepared by carbothermic reduction of mixed oxides of titanium and boron, reduction of mixed oxides by metals like aluminum, magnesium and silicon, reduction of titanium oxide by boron carbide and carbon or synthesized from the elements by heating, mechanical alloying or self-propagating high temperature synthesis [12-14], pulsed electric current sintering [15], mechanochemical synthesis [16], dc magnetron sputtering [17] and milling assisted sol-gel [18-21].

Being intrinsically brittle, B₄C often requires some additive to improve its mechanically properties and sintering behavior. Numerous researchers have shown that the addition of TiB₂ to B₄C can decrease the porosity level and improve the fracture toughness as well as flexural strength [22]. Recently, some researchers of B₄C based-composites such as B₄C/CrB₂, B₄C/Al, B₄C/TiC and B₄C/ZrB₂ have been carried out [23]. It has been considered that the additions of secondary phases to B₄C matrix can improve its mechanical properties [24].

In the present study, B₄C-nano TiB₂ composites with 10 ww% TiB₂ were obtained by in situ synthesis from boron carbide, titanium tetraisopropoxide and isopropanol. The effect of calcination temperature on the size, morphology and phase were discussed. Titanium diboride was synthesized by co-precipitation method. Also, thermogravimetric study on the formation of TiB₂ by boron carbide reduction of TiO₂ was carried out in our laboratory.

Experimental

Boron carbide (95%, B₄C, Merck), titanium tetraisopropoxide (97%, TTIP, Alfa Aesar), isopropanol (99.6%, Merck) were used to synthesis. All materials were used without further purification, but were dried in an oven at 423K to remove the moisture content before use. Boron carbide contains 5.0 ww% phenolic resin which used as carbon source. Deionized water was used for all experiments. The precursor powders were obtained by using co-precipitation method. TiB₂ was prepared by the reaction given below:

2TiO₂ + B₄C+ 3C   →   2TiB₂ + 4CO                                                                           (1)

Titanium tetraisopropoxide (1.15 mol) and boron carbide (18.1 mol) were dissolved in isopropanol (solution A). Also, boron carbide was dissolved in deionized water and isopropanol (solution B). Solution B was gradually added to solution A. The prepared mixture was stirred and heated at 298K for 4h. In this work, B₄C-nano TiB₂ composites are contained 10 ww% TiB₂.

The X-ray diffraction pattern (XRD, X’Pert MPD, Philips, Holand) and scanning electron microscopy (SEM, XL-30, Philips, Holand) of initial B₄C is shown in Figures 1 and 2, respectively. The crystalline phase during the reaction was investigated by an X-ray diffractometer using Cu-K_α radiation (40 kV, 40 mA). The mixture of TTIP, B₄C and isopropanol was placed at 423-433K.

Figure 1: XRD pattern of initial B4C. Click here to View table

 

Figure 2: SEM image of initial B4C. Click here to View table

 

Then, the powder were milled and after that, the XRD pattern and SEM were used to determined of particles size and to study of morphology that are illustrated in Figures 3 and 4, respectively. The prepared powder were placed to determine calcination temperature at 973, 1273 and 1523K for 60 minutes with 150 sccm (denotes cubic centimeter per minute at STP) argon flow.

Figure 3: XRD pattern of the mixture of TTIP, B4C and isopropanol after milling at about 430K.   Click here to View table

 

Figure 4: SEM image of the mixture of TTIP, B4C and isopropanol after milling at about 430K. Click here to View table

 

Figure 5 presents the DTA/DDTA and TG/DTG plots of the formation of titanium diboride.

Figure 5: DTA/DDTA and TG/DTG curves of the formation of titanium diboride for a temperature up to 1700K.   Click here to View table

 

The composition and nanostructure of the   B₄C-TiB₂ composites in 1273 and 1523K were shown by SEM in Figure 6.

Figure 6: SEM image of the B4C-nano TiB2/TiO2 composite at: (a) 1273 and (b) 1523K. Click here to View table

 

Also, XRD patterns of B₄C-TiB₂ composites at 937, 1273 and 1523K for 60 minutes with 10 ww% of TiB₂ are shown in Figure 7 that is used to determined for composites phases.

Figure 7: Diffractographe showing the comparative effects associated at different temperature of introducing the mixture: (a) 973,(b) 1273 and (c) 1523K. Click here to View table

 

Results and discussion

The prepared powder for formation of B₄C-nanoTiB₂ composites with 10 ww% TiB₂ are placed to determine calcination temperature at 973, 1273 and 1523K with argon flow. The DTA/DDTA and TG/DTG plots in Figure 5 show that the first exothermic reaction is associated with many weight loss leading to dehydration and evaporation of water from Ti(OH)₄ phase and the formation of TiO₂ phase by a decrease in mass of about 13.2 wt% that is given by below reaction:

Ti(OH)₄  →  TiO₂ + 2H₂O                                                                                   (2)

second exothermic reaction takes place at 1523K which also in this reaction is associated with weight loss about 4.39 wt% that leading to the formation of TiB₂ nanopowder from TiO₂ particles that is given by

TiO₂ + 0.5 B₄C + 1.5 C  →  TiB₂ + 2CO                                                              (3)

Figure 6(a) shows the SEM image of B₄C-nano TiO₂/TiB₂ composites at 1273K. Particles size at 1273K is between 10-70 nm. FE-SEM image indicate the determination of nanoparticle sizes and morphology of TiB₂ on B₄C at 1523K are shown in Figure 6(b). In the SEM of samples, the distribution of the B₄C and nano TiB₂ was not uniform. The dark elongated grains are B₄C, whereas the bright irregular or equiaxed grains are TiB₂. TiB₂ nanoparticles size at 1523K is between 10-40 nm. Figure 7(a) shows the XRD pattern of composite at 937K. In this calcination temperature exist B₄C and TiO₂ phases that is meaning this temperature is not enough to convert TiO₂ to TiB₂. The XRD pattern of composite at 1273K shows that three phases of B₄C, TiO₂ and TiB₂ exist but calcinations temperature should be increased. Figure 7(c) confirms that only TiB₂ and B₄C phases exist at 1523K that is enough to convert TiO₂ to TiB₂. Pure B₄C-nano TiB₂ phases were detected with no peaks of unreacted TiO₂ and Ti(OH)₄, thus indicating the full conversion of reagents into products was achieved according to reaction (3). Therefore, results indicate that synthesis of B₄C-nano TiB₂ from calcinations process of mixed powder is achieved at temperature higher than 1523 K. The XRD patterns of the fractured and the ground surfaces of the composites at 973, 1273 and 1523K are compared in Figure 7. The JCPDS card matching the spectra is 86-1129 for rhombohedral B₄C, 84-1286 for anataze TiO₂ and 85-2083 for hexagonal TiB₂. Since the C source is amorphous carbon black only TiO₂ and B₄C phases were found in the starting powder. The XRD pattern reveals full conversion of TiO₂ to hexagonal TiB₂ after 60 minutes at 1523K. Calcinations process at lower temperature than 1523K contain B₄C, TiO₂ and TiB₂ powders. Decreasing of particles size take place because of suitable selection of optimum calcinations and time for growth of TiB₂ nano particles.

Conclusions

Titanium diboride was synthesized successfully by co-precipitation method using TTIP, B₄C and isopropanol. In this study, nano TiB₂ particles in three steps produce on microstructure surfaces B₄C at three calcination temperature. In the first reaction, TTIP converted to Ti(OH)₄. Then, TiO₂ formed from Ti(OH)₄ at 553K. Finally, TiO₂ was transferred TiB₂. The fracture surfaces of the powder were observed by FESEM. The micrographs, reported in Figure 6 confirmed the complete densification of the nanostructured powders. The XRD and SEM results are proved which two phases of B₄C and nano TiB₂ with 10 ww% at 1523K. The nanoparticle sizes of the synthesized nano TiB₂ on surface of B₄C microstructures were found between 10-40 nm.

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