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{MSU-chem 95}}

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{\LARGE Weak gravitation shielding properties of} 

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{\LARGE composite bulk $YBa_2Cu_3O_{7-x}$ superconductor} 

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{\LARGE below 70 $K$ under e.m.\ field.}
	
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{\large E.E.\ Podkletnov}

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{{\em Moscow Chemical Scientific Research Centre} \par
Contact e-mail address: modanese@science.unitn.it}

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\begin{abstract}

A high-temperature $YBa_2Cu_3O_{7-x}$ bulk ceramic superconductor with 
composite structure has revealed weak shielding properties against 
gravitational force in the state of levitation at temperatures below 70 $K$. 
A toroidal disk was prepared using conventional ceramic technology in 
combination with melt-texture growth. Two solenoids were placed around the 
disk in order to initiate the current inside it and also to provide rotation 
about its central axis. Samples placed over the rotating disk demonstrated a 
weight loss of 0.3-0.5\%. When the rotation speed was slowly reduced 
by changing the current in the solenoids, the shielding effect became 
considerably higher and reached 1.9-2.1\% at maximum.

\medskip
\noindent
74.72.-h High-$T_c$ cuprates.

\bigskip 

\end{abstract}

\section{Introduction.}

The behavior of high-temperature ceramic superconductors under 
high-frequency magnetic field is of great interest for practical applications. 
Crystal structure seems to be the key factor determining all physical 
properties of bulk superconductors, and the interaction of this structure 
with external and internal e.m.\ fields might result in 
quite unusual effects. Despite a large number of studies \cite{b1,b2,b3} 
the nature of these interactions still remains unresolved. 

Our recent experimental work \cite{b4} clearly indicated that under certain 
conditions single-phase bulk dense $YBa_2Cu_3O_{7-x}$ revealed a moderate 
shielding effect against gravitational force. In order to obtain more 
information about this unusual phenomenon, a new installation was 
built which allowed to operate with magnetic fields up to 2 $T$ and 
frequencies up to $10^8$ $Hz$ at temperatures from 40 to 70 $K$. A new 
experimental technique was applied to modify the structure of the 
ceramic superconductor. All these efforts yielded a larger value of the 
shielding effect (up to 2\%) and provided good hopes for technological 
applications. 

A gravitational shielding effect of this strength has never been previously
observed and its theoretical explanation presents serious difficulties
(see \cite{epl} for references and an analysis of some hypotheses). 
Thus great attention was devoted to the elimination of any possible
source of systematic errors or of spurious non-gravitational effects. 
The small disturbances due to air flows pointed out by some 
authors \cite{dep,unni} were eliminated weighing the samples in a 
closed glass tube (see Section \ref{ope}). The entire 
cryostat and the solenoids were enclosed in a stainless steel box. 
But probably the best check for the truly gravitational nature of the
effect is the observed independence of the weight reduction (in \%)
of the mass and of the chemical composition of the tested samples
(Section \ref{res}). 

According to public releases, the NASA group in Huntsville, Alabama,
is presently ``cloning" our experiment. This is a difficult task,
especially for the sophisticated technology involved in the construction
of the large ceramic disk and in the control of its rotation. We are
also aware, though still at un-official level, of other groups working
at the experiment with smaller disks.


\section{Experimental.}

\subsection{Construction of the disk.}

The shielding superconducting element was made of dense, bulk, almost 
single-phase $YBa_2Cu_3O_{7-x}$ and had the shape of a toroidal disk with 
the outer diameter of 275 $mm$, the inner diameter of 80 $mm$, and the 
thickness of 10 $mm$. The preparation of the 123-compound consisted of 
mixing the initial oxides, then calcining the powder at 930$^0 \ C$ in 
air, grinding, pressing the disk at 120 $MPa$ and sintering it in oxygen at 
930$^0 \ C$ for 12 hours with slow cooling down to room temperature.  After 
that the disk was kept in a furnace at 600$^0 \ C$ and the upper surface was 
quickly heated to 1200$^0 \ C$ using a planar high-frequency inductor as 
shown in Figure 1. During this last 
heating the gap between the sample and the inductor was chosen 
precisely to provide only the heating of the upper 2 $mm$-thick layer of 
the disk, though the high heat conductivity of the material caused the 
heating of some part of the material below this layer. Finally the disk was 
slowly cooled down to room temperature in a flow of oxygen and 
treated mechanically in order to obtain good balance during rotation. A 
thin (1 $mm$) metal foil of magnetic material was attached to the 
upper surface of the disk using plastic glue
in order to provide the disk rotation as described below.
Table 1 summarizes the disks construction procedure.

The phase and crystal structure of the superconductor were studied 
using X-ray diffraction analysis (XRD) and a scanning electron 
microscope (SEM) equipped with an energy dispersive spectral (EDS) 
analyzer. The samples were cut layer by layer from the bulk ceramic 
disk.

The transition temperature $T_c$ was determined from the resistive 
transition in a variable temperature cryostat, under zero magnetic field, 
using an AC current and sputtered golden contacts. The critical current 
density was measured for samples cut from the top and from the 
bottom of the superconducting disk. Measurements of $J_c$ were carried out 
at 75 $K$ using an AC current, four-probe method and direct transport 
measurements.

The analysis of the cross-section of the ceramic $YBa_2Cu_3O_{7-x}$ disk 
revealed the existence of two zones with different crystal structure. The 
upper part of the disk (6-7 $mm$ of thickness) had an orthorhombic 
structure typical of the quench and melt growth process \cite{b5,b6} and 
consisted mainly of single-phase orthorhombic 123-compound. The 
material was dense and had pure and hardly visible grain boundaries. 
The grains had the size of less than 2 $\mu m$ and were oriented (75\%) 
with c-axis parallel to the surface of the disk. The transition 
temperature for the material as defined by direct measurements was 
94.2 $K$.

The lower part of the disk which was in close contact with the water-cooled 
base during the high-frequency heat treatment had a structure 
with randomly oriented grains and average grain size from 5 to 15 
$\mu m$.  The porosity of this zone varied from 5 to 9\%. The transition 
temperature $T_c$ was equal to 60.5 $K$ and the material contained about 
40\% of the tetragonal phase.

Crystal lattice parameters for these two layers as calculated from XRD 
are listed below, the values being given in $nm$:

\noindent
Upper layer: a=0.381; b=0.387; c=1.165; 

\noindent
Lower layer: a=0.384; b=0.388; c=1.170.

\noindent \hskip 2.4 cm        a=0.387; c=1.183.

\medskip
\noindent
The first (upper) layer was quite homogenous with even distribution of 
elements in the volume of all the samples. EDS analysis showed the 
presence of small inclusions of $Y_2BaCuO_5$ in the lower layer.


\subsection{Operation of the apparatus.}
\label{ope}

Two identical solenoids were placed around the superconductor using 
fibreglass supports as shown in Figures 2, 4, 5. The gaps between these 
solenoids and the disk were large enough for it to easily move about 
20 $mm$ in each direction.  The toroidal disk was placed inside a 
cryostat equipped with a set of three coils (Fig.\ 3) which could 
keep it levitating when it reached the superconducting state. A blocks 
scheme of the electrical connections is shown in Fig.\ 6. High-frequency 
electric current ($10^5$ $Hz$) was first sent to the 
two main solenoids around the toroidal disk, initiating the current  
inside the ceramics at room temperature. Then the system was 
slowly cooled down to 100 $K$ by liquid nitrogen and then quickly 
cooled by liquid helium vapors to the temperature of 65-70 $K$ so that 
the disk became superconducting (Fig.\ 7). The main solenoids were switched 
off. After that the current ($10^5$ $Hz$) was sent to the coils below the 
disk and the superconductor raised up (about 15 $mm$) because of the 
Meissner effect. Then a small current ($10^5$ $Hz$) was sent to the 
main solenoids and the disk began to rotate 
counter-clockwise with increasing speed. The 
rotation speed was increased up to 5000 $rpm$. At this moment the 
measurements of weight for various objects were taken (Fig.\ 8). 

Finally the rotation speed was slowly reduced by changing the current 
in the main solenoids (Fig.\ 9). The rotation speed was controlled  
by means of a laser beam reflected by a small piece of plastic 
light-reflecting foil attached to the disk. The measurements of weight 
were taken constantly during this period, too. 

The frequency of the e.m.\ field was varied from $10^3$ to 
$10^8$ $Hz$. Samples made of various materials were tested,
including metals, glass, plastic, wood and so 
on. All these samples were placed over the cryostat hanging on a thread 
connected to a sensible balance. The distance from the samples to the 
cryostat varied from 25 to 1500 $mm$ in the first run and up 3 meters in 
the second run. The weight of the samples was typically from 10 to 50 
grams. Every precaution was taken to avoid any possible disturbance
including induced magnetic fields and air flows. The samples were placed 
inside a closed vertical glass tube 
in order to eliminate the influence of air flows.


\section{Results.}
\label{res}

The levitating disk revealed a clearly measurable 
shielding effect against the gravitational force even without rotation. 
The values of the weight loss for various samples were within the range 
of 0.05-0.07\% in this case. 
As soon as the main solenoids were switched on and 
the disk began to rotate in the vapors of liquid helium, the shielding 
effect increased, and at the speed of 5000 $rpm$ the air over the cryostat 
began to raise slowly up to the ceiling. The particles of dust or smoke 
made the effect clearly visible. The boundaries of the flow could be 
seen clearly and corresponded exactly to the shape of the toroid. 

The weight of various samples decreased no matter what material they 
were made of. Samples made of the same material but with different 
masses lost the same fraction of their weight. The weight loss depended 
on the shape and the position of the sample. The maximum loss of 
weight could be reached when the sample was 
parallel to the surface of the disk, so that its projection
had the maximum area. The best measurement gave a weight loss of 
0.5\%, while typical values were from 0.3 to 0.5\%. The areas close to 
the inner edge of the toroid (5-7 $mm$ from the edge) gave lower values 
of shielding, from 0.1 to 0.25\% only. 

During the time when the rotation speed was decreased from 5000 to 
3500 $rpm$ using the solenoids as braking tools, the shielding effect 
reached maximum values and the weight loss of the samples was from 
1.9 to 2.1\%, depending on the position of the sample with respect to the 
outer edge of the disk. These peak values were measured during 25-30 
seconds as the speed decreased rather quickly. Because of considerable 
vibration of the disk at the rotation speed of 3000-3300 $rpm$,
further braking was done very quickly in order to avoid unbalanced 
rotation and the weight measurements could not be carried out. 

Remarkably, the effect of the weight loss was the same when the samples 
together with the balance were moved upwards to a distance of 3 $m$,
within the projection of the toroidal disk. No weight loss at all
was observed below the cryostat.

The maximum shielding properties were observed for maximum 
current inside the superconducting disk. According to preliminary 
measurements the upper layer of the disk was able to carry over 15000 
$A/cm^2$. The maximum weight loss of the samples was observed only at high 
frequencies of the magnetic field in the interval from 3.2 to 3.8 $MHz$.

The shield decreases slightly the gravitational force within the vertical
projection of the disk and creates a kind of vertical cylindrical
tunnel in the air with slightly reduced air pressure. (The observed
effect also works in various gases and liquid media.)
The difference between the atmospheric pressure over 
the cryostat and the pressure below it 
was measured with high precision using a mercury barometer. It was equal 
to 8 $mm$ for the maximum shielding effect. Such a pressure difference 
produces a lifting force on the cryostat, which in the present case is 
however of no practical interest (of the order of $10^2$ $Kg/m^2$).

\section{Discussion.}

The interaction of a superconducting ceramic body with the gravitational 
field is a complicated process 
and cannot be characterized by one single law or physical phenomenon. 
Also, an overwhelming explanation of the mechanism of high-temperature 
superconductivity has not been found yet. Still these 
facts do not make the observed phenomenon less interesting.

In our previous work \cite{b4} the loss of weight of the sample over the 
levitating superconductor was smaller and varied from 0.05 to 
0.3\%. At that time it was difficult to exclude entirely any influence of 
the radio-frequency field because the sample was separated from the disk 
and the magnets by a thin plastic film. Now the superconductor was 
situated in a stainless steel cryostat and the influence of 
non-gravitational factors 
causing levitation should be negligible. 

The modification of the 
crystal structure of the superconductor allowed to obtain a composite 
body with a dense and oriented upper layer and a porous lower layer 
with random orientation of the grains. The upper layer is able to carry 
high $J_c$ current under considerable magnetic field while the lower 
layer cannot conduct high currents and is not resistant to the external 
magnetic field. The lower part of the disk with wide intergrain 
boundaries is also a source of a great number of Josephson junctions 
and is responsible for the direct and reverse, primary and secondary 
Josephson effect. The presence of tetragonal non-superconducting phase 
allows interaction with the external magnetic field.

The combination of two different crystal structures with different 
behavior under magnetic field creates a composite ceramic body with 
new properties. According to Faraday law the placement of a normal 
conductor in a magnetic field causes electric current inside it.  Usually 
during levitation
the magnetic field does not penetrate into a superconductor for more 
than a penetration depth, thus the interaction with the field is extremely 
small. But in the described experiment the superconductor also carries 
high frequency electric current modified by Josephson effect. It is 
possible to admit that some interaction between the composite ceramic 
body and the external magnetic field takes place.  This interaction 
depends on the coherence length, the flux pinning, the field frequency 
and the field force, the penetration depth and the parameters of the 
crystal lattice. These characteristics are interrelated in a complex way.
According to the experimental data (compare also \cite{unni}, where only a
static field was applied) 
the ceramic superconductor kept at the temperature below 
70 $K$ does not reveal any unusual shielding if it has no contact with the 
AC magnetic field. 

As analyzed in \cite{b7} pinning centers with different origins may exist 
inside the superconducting disk, and fluxes will be trapped at some of 
them. Fluxes trapped at weak centers will begin to move first while those 
trapped at strong centers will not move until the Lorentz force exceeds the 
pinning force. The overall current will be composed of the 
superposition of flux motions with different speeds. Generally 
speaking, the quantized fluxes move as a bundle locally formed in a flux 
lattice by the magnetic interaction between them.

The temperature is also of great importance as it determines the 
thermodynamic functions and in particular the order parameter and the free
energy inside a superconductor. The shielding effect was observed only 
below 70 $K$, while the ceramic disk became superconducting already at 
94 $K$. 

The electric interactions inside the superconducting body below $T_c$ 
change under the conditions of the experiment and this might alter the 
behavior of the whole atomic structure in such a way that the interaction 
with the gravitational field becomes different. Then, in order to keep a 
stable level of energy and a stable atomic and crystal lattice structure 
the superconductor might exchange some energy with the gravitational field 
and slightly decrease it. There are no grounds to claim that the rotation 
momentum of the disk interacts with gravitation force, but it seems that 
fast rotation is favorable for the stabilization of the shielding effect.

According to BCS theory, in weak bond conditions electrons of 
conductivity and phonons in the crystal lattice interact from time to 
time: particles collide but they still preserve their individual positions and 
properties. If we deal with strong bond the interaction takes place all 
the time and free electrons and phonons exist no longer giving birth to a 
certain mixture called electron-phonon liquid. This liquid has specific 
properties and the behavior of the electron-phonon mixture under 
various conditions is not yet studied. It is possible to admit that this 
liquid has some properties similar to those which are typical for 
magnetic liquids, especially if we take into consideration that magnetic 
hysteresis is characteristic for high $T_c$ compounds. Also the 
experimental equipment described above has much in common with 
magneto-hydro-dynamic (MHD) generators. 

The first attempt at a theoretical explanation of the effect has been done 
by G.\ Modanese \cite{epl,gj}. Further investigations that are in progress, 
will help to prove, change or complete the present understanding of the 
observed phenomenon.

\section{Conclusions.}

A superconducting ceramic levitating disk of $YBa_2Cu_3O_{7-x}$ with 
composite structure demonstrated a stable and clearly measurable weak 
shielding effect against gravitational force below 70 $K$ and under 
high-frequency e.m.\ field. A combination of the high-frequency 
current inside the rotating toroidal disk and the high-frequency external 
magnetic field, together with electronic pairing state and 
superconducting crystal lattice structure apparently changed the 
interaction of the solid body with the gravitational field.  This resulted 
in the ability of the superconductor to modify the energy of the 
gravitational force and yielded a weight loss of various samples up to 
1.9-2.1\% .

Samples made of metals, plastic, ceramic, wood etc. were situated over 
the disk and their weight was measured with high precision. All the 
samples showed the same partial loss of weight, no matter what 
material they were made of. In order to obtain the maximum weight loss 
the samples should be oriented with the flat surface parallel to the 
surface of the disk. The overall maximum shielding effect (2.1\%) was 
obtained when the rotation speed and the corresponding centrifugal 
force were slightly decreased by the magnetic field.

It was found that the shielding effect depended on the temperature, the 
rotation speed, the frequency and the intensity of the magnetic field. At 
present it seems early to discuss the mechanisms and to give a 
detailed analysis of the observed phenomenon as further investigation is 
necessary. The experimentally obtained shielding value might be of 
primary interest for scientific and technological applications.

\subsection{Acknowledgment.}

The author is grateful to the Institute for High Temperatures of the 
Russian Academy of Sciences for the help in the preparation of the 
unique superconducting ceramic disks and for the possibility to use 
their technological equipment.
The effect was first observed 
and studied at Tampere University of Technology.


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\end{thebibliography}

\newpage

\noindent
{\bf FIGURE CAPTIONS.}

\bigskip

\begin{enumerate}

\item Schematic cross-section of the furnace for high-temperature
treatment of the ceramic disk with planar hig-frequency inductors.

\item General magnets and cryostat setup.

\item Typical geometry and position of the disk over
supporting solenoids.

\item Schematic design of rotating solenoids.
a, b: various configurations.

\item Typical configuration of the tested set-up for the
rotating solenoids.

\item Block scheme of the electrical connections.

\item Schematic design of the cryogenic system for the
refrigeration of the superconducting disk.

\item General configuration of the equipment for the
weight loss measurements.

\item Typical design of the three-point disk-braking system.

\end{enumerate}


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