Homogeneous precipitation synthesis of BaCO₃ powders with a needle-like morphology

2.1 Materials

All chemicals utilized for experimentation were analytical-grade reagents that were used without further purification and were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. (China), including barium chloride dihydrate (BaCl₂·2H₂O), sodium hydroxide (NaOH), carbamide ((NH₂)₂CO), ethylene diamine tetra acetic acid (EDTA), sodium triphosphate and citric acid.

2.2 Instrumentation

The experiments were carried out using homogeneous precipitation experimental devices, which are shown in Figure 1. A typical homogeneous precipitation experimental device consists of an iron support, a thermostatic water bath, a condensation pipe, stirring equipment, a two-neck flask and a stirring speed control device.

Figure 1:

Schematic diagram of homogeneous precipitation experimental device.

2.3 Characterization

The phase composition of the BaCO₃ powders which were prepared by the homogeneous precipitation method were characterized by SEM. The SEM was operated at 20 kV in a low vacuum. The particle size distribution data of the particle group was measured using a laser particle size analyzer (Rise 2002, Jinan Science and Technology Co., Ltd). DSC (STA449F3, Netzsch, Germany) was performed to determine the characteristic temperature in the thermal reaction and the heat absorbed and released by the materials. The quality changes and rate changes of materials were measured accurately by TG (STA449F3, Netzsch, Germany).

The Ba²⁺ concentration was calculated using the equation:

where ν is the volume of the reaction solution, and ν₁ is the volume of consumption for the EDTA standard titration solution.

The yield of product was calculated based on the following equation:

where c₀ is the concentration of Ba²⁺ of the initial reaction solution.

2.4 Procedure

The precipitation was performed at temperatures between 75°C and 90°C, holding thermal insulation for a period of time, making the (NH₂)₂CO hydrolyzed and then obtaining the superfine high-purity BaCO₃ powder after filtering, cleaning and drying.

3.1 Effect of Ba²⁺ concentration on particle size and yield of product

The microstructure and properties of BaCO₃ powders are driven by multiple factors, one of which is Ba²⁺ concentration. Experimental conditions are as follows: reaction temperature is 85°C, holding time is 4 h, the amount of NaOH is 2.5 mol, and the amount of urea is 6 mol. The effects of Ba²⁺ concentration on average particle size and yield of product are shown in Figure 2.

Figure 2:

Effect of Ba²⁺ concentration on average particle size (A) and yield of product (B), respectively, in the homogeneous precipitation.

It can be seen from Figure 2A that in the beginning of the reaction, the Ba²⁺ concentration increased, the average particle size decreased, when the Ba²⁺ concentration is 0.8 mol/l, the prepared BaCO₃ powders have the smallest average particle size, then the average particle size increased with the Ba²⁺ concentration increasing. Figure 2B clearly shows the variation tendency in the yield of BaCO₃ powders prepared using homogeneous precipitation at different Ba²⁺ concentrations. It indicates that the Ba²⁺ concentration has positive correlation with yield, which increases with the Ba²⁺ concentration increasing. Therefore, the optimal experimental value of Ba²⁺ concentration is 0.8 mol/l.

3.2 Effect of (NH₂)₂CO concentration on particle size and yield

The effect of (NH₂)₂CO concentration on the microstructure and properties of BaCO₃ powders was investigated, and the average particle size and yield of product, respectively, are shown in Figure 3. Experimental conditions are as follows: reaction temperature is 85°C, holding time is 4 h, the amount of NaOH is 2.5 mol, and the concentration of Ba²⁺ is 0.8 mol/l.

Figure 3:

Effect of (NH₂)₂CO concentration on average particle size (A) and yield of product (B), respectively, in the homogeneous precipitation.

Figure 3A illustrates the average particle size at different (NH₂)₂CO concentrations by homogeneous precipitation. It shows that the average particle size of BaCO₃ powder samples has negative correlation with the amount of urea, with the urea content increasing, the average size of the product continues to decrease. When the molar ratio of urea with barium chloride is between 2 and 6, the average particle size decreased rapidly with an increase of the urea content, while when the molar ratio of urea and barium chloride is more than 6, the average particle size of prepared BaCO₃ powders does not change significantly. Figure 3B illustrates the yield of BaCO₃ powders prepared using homogeneous precipitation at different (NH₂)₂CO concentrations. It can be seen in Figure 3B that the yield of the product has positive correlation with urea content: the yield of the product increases with an increase in urea concentration. When the molar ratio of urea and barium chloride is more than 6, the yield of the product growth urea content change is not obvious. Therefore, the optimal molar quantity of urea is 6.

3.3 Effect of NaOH concentration on particle size and yield

Experimental conditions are as follows: reaction temperature is 85°C, holding time is 4 h, molar concentration of Ba²⁺ is 0.8 mol/l, the amount of urea is 6 mol, the amounts of NaOH are controlled with molar ratios with the barium chloride of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0, respectively. The effect of NaOH concentration on average particle size and yield of product, respectively, are shown in Figure 4.

Figure 4:

Effect of NaOH concentration on average particle size (A) and yield of product (B), respectively, in the homogeneous precipitation.

Figure 4A shows the effect of NaOH concentration on the average particle size of BaCO₃ powder samples prepared by homogeneous precipitation. It indicates that NaOH has little effect on the average particle size of BaCO₃ powder samples, and the average size of samples is between 3.6 μm and 3.9 μm. Figure 4B shows the yield of BaCO₃ powders prepared by homogeneous precipitation at different NaOH concentrations. The yield increases with the increasing amount of NaOH. It indicates that excessive NaOH can promote chemical reactions. The optimal molar quantity of NaOH is 2.5.

3.4 Effect of reaction temperature on particle size and yield

The effect of reaction temperature on average particle size and yield of product, respectively, are shown in Figure 5. Experimental conditions are as follows: time of heat preservation is 4 h, molar concentration of Ba²⁺ is 0.8 mol/l, the amount of NaOH is 2.5 mol, the amount of urea is 6 mol, the controlled reaction temperatures are 75°C, 80°C, 85°C, 90°C and 93°C, respectively.

Figure 5:

Effect of reaction temperature on average particle size (A) and yield of product (B), respectively, in the homogeneous precipitation.

It can be seen from Figure 5A that the reaction temperature has little effect on the average particle size of BaCO₃ powder samples. In different reaction temperatures (75~93°C), the average particle size is 3.6~3.8 μm. Figure 5B illustrates that the yield of BaCO₃ powders prepared by homogeneous precipitation increased in line with an increase in the reaction temperature. Considering the influence of reaction temperature on the role of the abovementioned factor, the optimal reaction temperature is 85°C.

3.5 Effect of reaction time on particle size and yield

The effect of reaction time on average particle size and yield of product, respectively, are shown in Figure 6. Experimental conditions are as follows: the reaction temperature is 85°C, the concentration of Ba²⁺ is 0.8 mol/l, the amount of NaOH is 2.5 mol, the amount of urea is 6 mol, and the controlled reaction time is from 0.5 h to 7 h.

Figure 6:

Effect of reaction time on average particle size (A) and yield of product (B), respectively, in the homogeneous precipitation.

Figure 6A shows the particle size of BaCO₃ powders prepared using homogeneous precipitation at different reaction times. The average size of BaCO₃ powders increased along with the extending of the reaction time as shown in Figure 6A. The yields of BaCO₃ powders synthesized at different reaction times by homogeneous precipitation are shown in Figure 6B. It can be seen from Figure 6A that along with extending the reaction time, the yield of prepared BaCO₃ powders increased. Considering the influence of reaction time on the two parts of the experiment, the optimal reaction time is 4 h.

3.6 The best experimental conditions

BaCO₃ particles prepared by homogeneous precipitation must be considering the single factor experiment research, the optimal process conditions are as follows: the reaction temperature is 85°C, the reaction time is 4 h, the concentration of Ba²⁺ is 0.8 mol/l, the molar ratio of NaOH/BaCl₂·2H₂O is 2.5, and the molar ratio of (NH₂) ₂CO/BaCl₂·2H₂O is 6.

The morphologies of BaCO₃ particles prepared using homogeneous precipitation are observed using SEM techniques and the results are presented in Figure 7. It can be seen in Figure 7 that the typical structure is of a needle-like form, which was caused by stirring during the BaCO₃ crystal growth process. Figure 8 shows the particle size distribution of BaCO₃ particles. The results show that the average particle size is 3.61 μm, and the particle size has a wide range of distribution, which is due to a longer reaction time during which the first crystalline particles develop. The analysis of BaCO₃ powders prepared is performed using TG-DSC) measurement. The results are presented in Figure 9, which shows the three endothermic peaks at 809.7°C, 959.7°C and 1066.0°C, respectively. There are no weight changes in the first two endothermic peaks, which illustrate that it has phase change or chemical reaction. An endothermic peak is observed at 809.7°C on the DSC curve, indicating that γ-type BaCO₃ transform into β-type BaCO₃. A phase transformation reaction of β-type BaCO₃ into α-type BaCO₃ occurs in the second endothermic peak at 959.7°C. The TG curve of the sample shows that weight loss processing is observed from 1066.0°C. When the temperature reaches 1400°C, the weight loss rate is 20.88%, which is close to the theoretical weight loss rate (22.30%). The thermoanalysis results show that the preparation of BaCO₃ powder has high purity.

Figure 7:

SEM photos of BaCO₃ particles prepared under optimal experimental conditions by homogeneous precipitation method. (A) 1000×; (B) 5000×.

Figure 8:

Graph of size distribution of BaCO₃ particles prepared under optimal experimental conditions by homogeneous precipitation method.

Figure 9:

TG-DSC analysis of BaCO₃ particles prepared under optimal experimental conditions by homogeneous precipitation method.