The poling field is 1.5KV/mm to 3.5KV/mm for piezoelectric ceramics. Using silicone oil to cover the surface of the sample. High temperature can help poling. But the temperature should be lower than curie temperature. 

Applying same poling field to all the prepared samples is correct. Because then only one cne able to compare and know whether substitution increases piezoelectric coefficient. But poling above the cercive field is important then only all the domains will be completely oriented. So first find the coercive field of parent compound pole the base compound slightly above that field and maintain the same field for all  the samples.

If the polarization field is saturated，the pieoelectric constant  will not be changed.

As the optimum poling field and poling temperature are known through experimental testing,  the piezoelectric coefficient should  be increased.  If the piezoelectric coefficient is not increased,  the effective method is to modify or change the composition of your materials by doping or fabrication process.

Piezoelectric properties of electroceramics can be improved by optimizing the poling parameters. Poling field will be same for the all samples but poling voltage vary with sample thickness. Please check the following articles

https://www.researchgate.net/publication/273446569_Poling_electric_field_dependent_domain_switching_and_piezoelectric_properties_of_mechanically_activated_Pb092La008Zr060Ti040O3_ceramics

https://www.researchgate.net/publication/283502996_Optimization_of_poling_parameters_of_mechanically_processed_PLZT_86040_ceramics_based_on_dielectric_and_piezoelectric_studies

Article Poling electric field dependent domain switching and piezoel...

Article Optimization of poling parameters of mechanically processed ...

Piezoelectric constant is also affected largely by the processing parameters like sintering/calcination temperature, time and ramp rate. These affect the grain size which in turn is reflected in the hysteresis parameters. 

In terms of electric field, the strength of applied field should be as close as possible to the value necessary for saturation polarization. By keeping it constant for all samples, you should be able to determine the difference (if any) leading to change in piezoelectric constant.

Can anyone recommend a high voltage Equipments for poling of piezoelectric materials? What will be the experimental setup for poling ??

Dear R.M. Toyabur

We have used Stanford DC voltage power supply (20kV) for poling. Please find attched pictures of the poling setup.

Thank you Mr. Ajeet Kumar for your valuable answer. I really appreciate your help.

You can increase the piezoelectric constants by adding the donor dopants such as A-site donors La2O3, Bi2O3, Nd2O3 or B-site Donors Nb2O5, Ta2O5, Sb2O3, or a few amounts of ZnO, CuO etc. In addition, you can try to shift the structure to multi-phase boundary by modifying your composition. These methods should be effective to increase the piezoelectric constants